HUMAN BODY.

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The mechanism of the human body is so wonderfully, so curiously, and so wisely contrived, that the more we examine it, the more we must with admiration acknowledge, that nothing but an Almighty Being could be its author. This incomprehensible Being let us ever remember to reverence and adore, when we examine into the stupendous contrivance of our fabric; HE is the fountain, the benevolent upholder of nature, and all the creation; in whom we live, breathe, and have our being.

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Of the Composition and close Connection of Parts in the Human Body.

The human body is composed of solids and fluids; these are so closely, and so intimately combined with each other, that we find upon a physical enquiry and examination, they are scarce separable. They aid and give action to each other, and life depends not on each singly, but on the united, and on the separate spur, and on the soul of activity each gives to the other: Their substances form the body, and their action produces the operation of the mind.

Whilst every thing moves free and agreeable to the talk prescribed by nature, the automaton is in a state of health; body and mind enjoy peace and tranquility. So closely are body and mind in the most perfect bond of friendship, that they share pleasure and pain sympathetically with each other; and reciprocally contribute to each other’s welfare.

With what satisfaction is the mind continually engaged to give pleasure to the body! and with what alacrity are for ever the Members ready to obey the will! Happy harmony, by which we so wonderfully exist!

The solids are the canals and springs of life, and the fluids, which circulate in those canals, nourish them, and set the springs in action.

By the solids we understand,

1st The bones; the frame and support of our wonderful structure.

2dly. Gristles or cartilages; which cover the ends of the bones, in order to make them move with ease and agility.

3dly. Muscles; the fleshy parts, which by a nervous sensation, are induced, either voluntarily or involuntarily, to contract or dilate themselves, in order to give motion to the various parts of our body, to which they are fastened or destined to move.

4thly. Tendons; the chords as it were, by which the muscles are fastened to the bones, so as to make them follow the contraction of the muscles.

5thly. Ligaments; the chords by which the bones are connected with each other, and which give a firmness to their contact and articulation.

6thly. Arteries; the canals which transport the blood from the heart, to all the parts of the body.

7thly. Veins; the canals which carry the blood from the remotest arteries (in the most minute parts of the body) back again to the heart, for a new rotation of the blood.

8thly. Lymphatic Vessels; which are small cellular canals, that convey the lymph in different parts of the body.

9thly. Glands; which are kernels formed to secrete or separate various humours from the blood.

10thly. Nerves; these are medullary branches springing from the brain, and distributed in the most minute parts of the body; ordained to give the act of sensation, as the soul of animal activity.

11thly. Hair and nails; these are a horny substance, and appropriated either for defence or warmth of the different parts where they are placed.

The fluids; are, 1st. Blood; which is circulated in the arteries and veins, and is the nutriment and source of life.

2dly. The different humours separated by the glands from the blood; of which there are many, as the lymph, serum, spittle, seed, chyle, urine, gall, &c. appropriated to different uses, as hereafter will be described.

3dly. Fat, and marrow in the bones; which are designed not only as a store of nourishment, but to lubricate and warm the parts where they are lodged.

These are the principal materials which compose our wonderful machine. There are various other parts mentioned by Anatomists, both solids and fluids; but as this is designed only to give a universal idea of the most material parts of the structure of this stupendous fabric, we will let this definition suffice, and now consider with what order the principal organs of life act, either in consort, or in opposition to each other.

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SECT. II.

Of the Brains and Nerves.

The brains are by all Physiologists esteemed the most mysterious organs of life, being the fountain of the nerves, the organs of sensation, and hence allowed by all to be the seat of the soul, or the sanctum sanctorum of the human understanding.

They are distinguished into the common or great brain called cerebrum; the small part called cerebellum; and the spinal marrow, which projects down the back-bone. The great brain is divided into two great lobes, seated in the upper and fore part of the skull, of which it takes in the greatest cavity. It is so exceedingly sensible, that the least pressure puts an immediate stop to the sensation of the animal; and which is generally termed a apoplexy.^[2]

This part of the brain, though it is so exquisitely sensible, seems to be designed by nature as preparatory to the cerebellum, the immediate root of the nerves.

The cerebellum is the lesser, but the more material part of the brains. It is seated in the hinder cavity of the skull, (which for the sake of protecting this exquisite sensible organ, is remarkably strong and firm) and from thence as the fountain of life itself, the nerves, the instruments of sensation in the whole animal frame, take their commencement: For whereas the other is so sensible, as that the least pressure will cause a stupor; this is so far superior in point of exquisite sensibility, that a pressure on that part, not only occasions an apoplexy, but even stops life itself.

From this cerebellum prolongates the marrow in the back-bone, called medulla spinalis, which is but a continuation of the same, and is equally sensible, and of the same substance with the former.

The whole brain, that is, the great brain, the small brain, spinal marrow, together with all the nerves which spring from thence, are covered with two coats, called the external stronger coat, dura mater; and the thinner and inner coat, pia mater; which continue with each nerve, even to its smallest ramification. The whole in a living subject is in a continual motion peculiar to itself.^[3]

From the cerebellum, and medulla spinalis, all the nerves take their commencement and origin. Ten pair spring forth from the part in the skull; the principal are those which form the various senses, tasting, smelling, hearing, and seeing, and that common to all, feeling. From the medulla spinalis, or marrow in the back-bone, proceed thirty pair of nerves, which spread themselves variously over the whole system.

All nerves have this in common with each other, namely, exquisite sensibility; but in other respects they differ however widely; particularly the nerves destined for some of the external senses.

It is from that common quality feeling, that they become the subtile springs of life; and give an edge to all the animal functions.

There is nothing so plentifully, and nothing more curiously interwoven with every other substance, as the nerves; and so amazingly quick is their sensation, that the very instant a single nerve, even the smallest and remotest in the system, is affected, the soul, or the common sensorium, is sensible of the identical part so affected.

It has been asserted, that this amazing quick conveyance of the sensation of the nerves, is owing to a subtile nervous fluid; but on a rational reflection, and a just enquiry into the animal oeconomy, it must appear, that such conveyance of a fluid is inconsistent with probability and experience: For, neither are the nerves tubulous, nor has there ever been discovered any circulating humour in them. And secondly, is it consistent with reason, that a conveyance of a fluid from the toe to the finger, should be so momentarily quick, as is sometimes experienced in the gout, and other the like ailments? That the quick transposition of sensation of the nerves, is rather effected on a principle of vibration, appears more probable, and will admit of a more convincing experiment; which may be exemplified in the following manner: Suppose (in the language of a seaman) a rope runs from the top-mast-head, or any other part of the rigging, to the deck, and is kept tought and free from any interception; then it will be evident, that the least touch at one end will instantaneously be conveyed to the other end; in like manner the nervous sensation may be communicated, perhaps not unlike electricity; which similarly will in an instant be conveyed to any part, by a direct communication, but equally subject to be intercepted.

Hence, a paralytic limb will lose, together with its irritable sensibility, its circulation of fluids, and its nourishment; and recover again when the pressure or interruption is ceased. Hence also, spasmodic contractions will be affected at the irritation of some principal parts, and from this instantaneous irritability, the motion of every part is thought to obey the will.

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SECT. III.

Of the Heart and Lungs.

The heart is a muscular body, situated in the breast or chest, somewhat between the lobes of the lungs, and formed by nature to be the principal organ for the circulation of the blood and other fluids.

Its structure is wonderful, and of a most curious piece of mechanism. It has two principal cavities, called ventricles, which are separated length-ways by a wall, and distinguished into the right and left.^[4]

Each of these cavities or ventricles, have particular valves, call’d ears, or auricles; which perform the function somewhat similar to the suckers of a pump: By these contrivances the heart receives the blood from the veins, and expels it again by the arteries, to every part of the human body; in a manner that I shall hereafter describe.

The lungs are organs immediately concerned in the circulation of the blood also; their function is to receive the air, to purify it, to circulate, and to distribute it in due proportion in the sanguineous mass, and to extract and expel that part of air already made use of; and thus act the part of ventilators in the animal oeconomy.

They are divided into two lobes. These lobes are a continuation of little bladders, that have an immediate continuation with the wind-pipe, aspera arteria. The wind-pipe from the larynx down to the very lungs, is protected externally with semicircular gristles, in order to keep it always open, and so shelter it from external injury; when it enters the lungs it spreads itself into numberless branches like a tree, and sends ramifications into every little air-bladder in the lungs.

All along these ramifications and the air-bladders, are arteries, veins, and lymphatic vessels, which have an immediate communication with the heart, by means of the pulmonary arteries and veins.

Now at the instant of inspiration, these little bladders in the lungs are filled with air, which infuses itself by the small arteries into the whole mass of blood; and at the expiration again, the air which has been made use of, and drawn from the neighbouring little veins, is again expelled by the wind-pipe through the mouth: And thus is respiration performed by alternate motions of the lungs; receiving the fresh, and alternately expelling the foul air again; thence justly may they be deemed the ventilators of the human automaton, and a principal organ of life, in the animal creation in general.

The upper part of the wind-pipe, which is in the fauces of the mouth, is called the larynx. This is the organ of voice.

In this animal mechanism is contained, all the variety of formation requisite to every kind of sound that can be performed upon any instrument whatever. Its parts are chiefly of a gristly substance, at the same time exquisitely delicate, with nerves and glands; from whence it is subject to diseases on the slightest indisposition.

In men it is larger than in women; hence their voice is rougher; which generally increases by exercise of the voice also. Its formation is very complex, and too tedious here to enter upon; we will therefore pass it over, and hereafter say as much as relates to its preservation, and giving it relief when disordered.

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SECT. IV.

Of the Circulation of the Blood.

We shall now consider the circulation of the blood; the grand source of our existence. At the contemplation of which we must be lost in admiration, at the wonderful wisdom of its contrivance; and with uplifted eyes adore the cause of its motion!

The Heart, as I before observed, has two chambers or ventricles, distinguished by the right and left; each of these chambers have also a valvular cavity call’d auricle, or ear. Each ventricle opens itself into an artery and auricle; and each auricle opens itself into a ventricle and vein. These openings have valves, which open and shut alternately, and by this, prevent any confusion or hindrance in the influx and efflux of the blood.

The right ventricle is the longest and greatest, but weakest: From this ventricle springs forth the pulmonary artery, which spreads itself throughout the Lungs.

The right auricle is also the largest; in which opens itself the large vein, called vena cava; as the common trunk that receives the blood from all its branches spread over the whole human body.

The left ventricle is shorter, but the walls are considerably stronger than that of the right ventricle. Here begins the aorta, or the great artery, by which the blood is sent to all the parts of the human body.

The left auricle is also somewhat smaller than the right; and in that opens itself the pulmonary vein, which returns the blood from the lungs, after it has received its necessary assistance from the vesicles of air in the lungs.

This premised and understood, the circulation is performed in the following manner:

The right ventricle being full of blood, contracts itself. This contraction is called the systole, by which it forces the blood through the pulmonary artery into every part of the lungs, even into every vesicle; where it receives a portion of the air, drawn in by the wind-pipe.

At the end of these arteries join the ramifications of the pulmonary vein, which receives the blood from the ramifications of the arteries, by anastomosation; which, by little quantities, these veins gather gradually into little branches, and at last become one common trunk, which empties the blood into the left auricle of the heart.

From the left auricle it is let, by means of a peculiar constructed valve, into the left ventricle; which, by a dilation called the diastole, receives it from its auricle.

Immediately upon this reception, the systole or contraction takes place, and the blood is forced into the great universal artery, aorta; by which it is farther transported into every part of the human body.

This artery spreads as it goes, and continually decreases in magnitude, as it becomes numerous in branches; until it terminates into the smallest ramification in the remotest part of the body.

The systolic contraction, and the diastolic dilation, which alternately take place in the heart, continue throughout the arteries from the great trunk to the remotest capillary ramifications,^[5] by which means the blood is transported with more certainty and facility to the minutest part.

At the extremity of these arteries the veins take their commencement, in ramifications equally small with the former; these take up the blood from the arteries, by many infinite small quantities, and carry it gradually back from those extremities to the heart again, for a new rotation.

These veins commencing infinitely small, but gradually gathering, become branches, and at length form one general trunk, called the vena cava, (or the great universal vein); and this empties itself again into the right auricle; from whence it is let again into the right ventricle, whence it came; then again forced into the lungs; thence back again into the left ventricle; from thence all over the whole body, and continues the Circulation.

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SECT. V.

Of the Joint Connection of the Brain, Heart, Lungs, &c.

These are the principal actions in the grand movement of the human automaton.

The nerves are the subtile springs of the sensation, by which the whole becomes sensible of irritation, and agitated to its functions. Their ramifications are extended ad infinitum, and so curiously distributed, that though there is not a part in the whole system exempt from their spreading, yet they are distributed with such regularity, that they don’t interfere with each other. A regularity absolutely requisite to the order in the animal oeconomy: For whereever the vibration of the nerves is obstructed, life or motion must in consequence cease.

The lungs are the ventilators in the system: they receive and let in the air; they sift it from all its gross particles, and extract the useful parts from this universal element of life; thus they give from every little vesicle, a due proportion of extracted air to the blood by the adjacent veins; and by the neighbouring arteries, again extract the foul air from the circulated blood, which by the mouth and nostrils is expelled again; at that instant a fresh quantity of air is drawn in again for a new supply.

Thus by intervals, the lungs, like a perfect ventilatory machine, repeatedly, and without intermission, keep time with systolic and diastolic motions of the heart, in exchanging fresh and foul air for the support of this wonderful animal structure. This is the function of breathing respiration, namely inspiration, or the drawing in the fresh air; and expiration, the expelling the foul air.

The heart may be considered as the grand perpetual pumping engine, constructed on principles, so as to have the least friction possible.

This curious pump, the heart, sends the fluid of life, the blood, to all the parts, even the remotest in the whole machine. The arteries are the canals of the conveyance; they are strong elastic tubes, whose fibres are of spiral direction, and receive from the heart a power of contraction; which is continued from the heart in the moment of the systole, even to the remotest and most minute ramifications; by which the blood is pushed along to the minutest extremities. This is immediately succeeded with a diastolic reception of a fresh quantity of blood; so that on succession, the diastole and systole is perpetuated not only in the heart, but throughout all the arteries, in a friendly correspondence over the whole system: And this constitutes the pulse.

The veins, however, are somewhat of a different texture from the arteries; whilst the arteries transport the blood by an elastic and pulse-like contraction, the veins on the contrary, receive it from them at the extremities, in an easy and imperceptible gradation; for whereas the fibres in the arteries are in a spiral direction, elastic and strong, the coat of the veins are tender, and the fibres more longitudinal.

Hence, the wounds in the arteries and veins differ greatly; for whilst the arteries are for ever in a strong agitation, the veins perform their functions with less exercise; and therefore the wounds in arteries are always attended with danger, whilst the veins heal easy. From the same reason also, the arterial blood differs from the venal; for as the arterial blood is sent to all the Parts, as the grand nutriment, from whence the glands secrete their peculiar humours; the veins only transport it back again, to prepare it for a new rotation. Whence the blood in the arteries is of a high colour, and of a more alkalascent nature, than that in the veins.

Collateral with the minute ramifications of the veins and arteries, over the whole system, are the lymphatic vessels, which separate from the blood as it passes along, a transparent lymphatic humour, adapted for the different parts, which they are connected with.^[6] These vessels are materially different, from either arteries and veins in their structure, and justly to be compared to a continuation of little cells; so constructed, as to transport the humours, without admitting a return.

From hence we see plainly how these three organs, the brains, the lungs, and the heart, are concerned, and mutually assist each other in the actions, and circulation of the fluid of life; nothing could move without the sensation of the nerves; these could not exist without the blood, from which they receive their support; the blood could not circulate without the heart, arteries, and veins; and in them it could not move with a necessary freedom, except its being supplied with a sufficiency of air from the lungs: So that evidently all three are so interested in this grand movement, that the stopping of the one, must needs be the stopping of the whole.

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SECT. VI.

Of Chylifaction and Nutrition.

In the preceding we have briefly considered the state of the fluid of life, and mechanical instruments of their motion in the human machine. In this we will consider how the whole is supported, and the vital lamp, from time to time, becomes supplied.

Under this consideration we will examine the canal of food, and the various changes the morsel undergoes from the time we take it in our mouths, till the nutriment is reduced to blood, and the remaining dross is expelled the body as useless.

The animal functions towards chylifaction and nutrition, are mastication, or chewing the food; deglutition, or swallowing; digestion; chylifaction; nutrition; and the excretion of the fÆces.

In order therefore to understand this clearly, it becomes necessary to describe the organs which nature has formed for the requisite performances thereof.

In the first place then, smelling is that sensation which nature has given to every animal, that has a choice of food, as the first safe-guard to inform it of any thing agreeable or disagreeable, useful or pernicious to its body; its seat is in the nose chiefly, but we find by experience, that it has a friendly connection with our palate and stomach; for the effluvia of any thing will either create a desire, or give us the greatest aversion to every substance that throws out a flavour.

It is performed by means of a subtile spreading of nerves, peculiarly delicate; which continues through the membrane of the nose, the roof of the mouth, gullet, and the very stomach.

Taste is the next sensation, which nature has given us, not only as a distinguisher of proper food, but a sensation from which we receive many luxurious pleasures; and to the indulgence of this sensation, most evils and plagues to mankind, take their origin.

The tongue is the principal instrument of that peculiar quality; but if we examine somewhat closer into this affair, we shall find that the soul of pleasure and pain of that sensation, as well as that of smelling, has its seat in the stomach; for that which will taste pleasing and good at the first approach, will soon lose its relish when the stomach is gratified: and, if any thing tastes disagreeable, the stomach receives it with reluctance, and will ever incline to discharge it again.

The tongue is an instrument (if I may be allowed the term) very curiously constructed; it is moved by a variety of muscles, and serves not only for tasting, but also as a labourer, to shovel and to turn our meat between our grinders; so that nothing may escape being well masticated, and intermixed with that fine digesting balsam, the spittle, in order that it may be easily swallowed.

Besides this, it makes the most requisite instrument for the noble and excellent faculty of speaking; which forms one of the principal characteristics that distinguishes man from the brute creation.

The gullet or oesophagus, is the canal which conveys drink and food from the mouth to the stomach; this canal is a muscular, tendinous, and vascular tunic.

The commencement of it is in the mouth, and is called the pharynx; a curious structure, that receives the food, and by its contractive motion, and the help of the tongue, forces the aliment into the stomach.

The stomach is much like the bag of a Scotch bag-pipe; it lies immediately under the diaphragm or midriff, covered partly on the right side with the liver, and on the left side with the spleen. The left and superior part, is continued with the oesophagus; and the right and inferior part, or orifice, commences the intestines.

The first orifice is called the mouth of the stomach; and the second the pylorus, or porter: At the porter there is a curious valve which lets the aliment out by small parcels into the intestines, where it undergoes its various other changes.

The stomach has three teguments, a muscular, a tendinous, and nervous coat; this nervous coat has another slimy one, but this in reality, is a delicate lining, interwoven with nerves, and the ramifications of fine blood vessels.

The intestines or guts, are a continuation of the stomach, they are a canal which is generally reckoned six times as long as the subject it is taken from; it is distinguished in small tenuia, and wide crassa.

Each again is divided into three parts: the tenuia, or small narrow intestines, are the duodenum, or twelve finger-gut; the jejunum; the ileum: The wide or crassa, is divided into the cÆcum; the colon; and the rectum.

Throughout the whole canal of intestines are numbers of little vessels, called lacteals, which lead the chyle, extracted from the aliment, into a receptacle, which is lodged in the mesentery, and from thence, by another duct call’d the thoracic duct, is carried along the back-bone upwards, and joins to the left subclavian vein, where the chyle gradually commences to be blood.

This short description we will let suffice, and now enter upon the action itself.

The morsel now, which is designed for food, is taken into the mouth, masticated with the teeth, turned about with the tongue; and as the mouth is at work, the saliva or spittle is squeezed from the salival glands, and thus intermixed with the aliment; when enough chewed and moistened with this saliva, it is conveyed to the pharynx, or swallow, which receives it, and, by its contraction, forces it into the oesophagus; and by a repeated contraction, is carried down into the stomach.

There it is again moistened with saponaceous liquid, or pancreatic juice; by which and by a perpetual motion of the stomach, it is brought into a state of digestion; then by small degrees entered through the pylorus or porter, into the first division of the gut, the duodenum.

This gut is about twelve fingers long; and whilst the aliment is there, it is intermixed with the gall, which is a liquor separated by the liver, and contained in the gall bladder; this liquor, the gall, is carried into the duodenum, by a small duct, called the ductus cysticus; where also enters another kind of liquor called the pancreatic juice.

When the aliment is thus prepared, and fit for a particular state of dissolution, it is carried into, and through the jejunum. This gut is in length about twelve or thirteen hands breadth, and its motion somewhat brisk; through which the aliment passes pretty quick, and hence, generally is somewhat empty.

As it passes through this part, the chyle is separated from it by the lacteals, which are small vessels that separate the chyle from the aliment, and abound there more than in any other part of the gut.

From thence it comes into the ileum; that is the longest of all the divisions of the guts, and is in length about twenty-one hands breadth; it has a great many circumvolutions, and next to the jejunum, has many lacteals to separate the chyle.

Now the aliment comes into the wide gut, and gradually becomes fÆces; and first, the cÆcum: This part is rather an appendix only, and hangs from the main part like a finger to a glove. The use of this gut has been much controverted by anatomists; it seems however, very probable, that this appendix is designed to keep the aliment in for further digestion, as it now begins to putrefy, and becomes fÆces or excrement.

From thence it enters the colon, which is a long, and very winding intestine; it runs up along and about the liver, touches the gall bladder, and the spleen; from thence it descends again to the os sacrum. It has but few lacteals, and is, as it were, the last drainer of the fÆces: It is this intestine which is the seat of the Colic, and of most other complaints of the belly.

Next to this comes the last and straightest, the rectum; this gut is closely adherent to the sacrum, and ends in the fundament; which is provided with muscles to open and shut the anus, in order to contain the fÆces, and discharge it.

The mechanism of chylifaction in the human body differs from the brute creation in general, except that most contemptible of the whole, the hog; to which it bears a very near resemblance, insomuch that there is very little distinction.

Both have that advantage over the generality of terrestrial animals, that they are confined to no particular food; which favours greatly the luxury of the one, and the beastiality of the other.

The brute creation are generally distinguished into carnivorous and granivorous: The first is that kind which feeds upon flesh; and the latter upon grain and vegetables. Upon examination however, we find, that the stomach and guts are peculiarly adapted to their food; and that grass agrees no more with the dog, than mutton does with the horse.

But man is so happily made, that any thing which is food, is proper for him, and he may become used to it; and thus is either carnivorous or granivorous.

The whole canals, from the stomach to the anus, is in a continual vermicular motion, which is called the peristaltic motion; by this the aliments are dissolved, and disunited; and as they pass along, are drained by the lacteals, of their nutriment or chyle.

These lacteals are, by means of a membrane (with which they are surprisingly interwoven, and connected to the whole canal) called the mesentery, lead regularly into one common cistern, lodged almost in the middle of the intestines, in that membrane, called the receptacle of chyle; and from thence the chyle is carried by a duct up along the back-bone, called the ductus thoracicus, into the left subclavian vein, where it gradually commences to be blood.

By this mechanism we are nourished, and the substance of our food converted into blood, and transported through the whole animal machine, for the support of every part of its wonderful composition.

As all animals which feed upon flesh, are more subject to diseases, nature has provided them with these advantages: that when any thing is obnoxious to their nature, and received into their stomach, or their being over loaded, it can discharge itself of so troublesome a burthen, by vomiting, which is effected thus: when the inner coat of the stomach, which is irritable and nervous, is stimulated by whatever is obnoxious, it will cause in the whole stomach, a contraction; and by that, force its contents to the shortest direction of evacuation, namely, by the canal of the oesophagus, through the mouth. This expulsion is peculiar to carnivorous animals only.

Purging or discharging by the fundament is common to all animals of whatever kind; and is performed by an irritation in the intestines, by which the peristaltic motion is increased; to this I must add a reversion of the secretion of the lacteal vessels, by which the humours are increased, the motion accelerated, and the fÆces discharged, without giving any nourishment to the body, and consequently the system diminished.

Digestion is that act by which the aliment or food is prepared, so as to produce a good chyle, and consequently good blood, for the nourishment of the body. Though no animal has a more delicate stomach than man, yet it must be observed, that none has a stomach better adapted for all kinds of food.

Whence therefore in the common course of life, temperance and gentle exercise is what nature requires to maintain health. But nothing becomes more obnoxious to that blessing, than gluttony, voluptuousness, and idleness.

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SECT. VII.

Of the Bones, Muscles, Ligaments, Tendons, and their Connections.

The bones may be considered as the timber-work of the human frame; by which this wonderful fabric is supported, and kept in its due form, that the whole may be brought into its various movements, without confusion or obstruction to each other.

The bones are the most solid parts of the human body, composed of hard and indurated fibres, striated over each other, in a manner peculiar to that substance. The bony fibres are in themselves insensible, yet as the parts are variously distributed with arteries and veins, and that the nerves must necessarily have a share in their formation, they have a peculiar sensibility, which is perceptible in some parts more than others; they cannot strictly speaking, be deemed quite insensible.

The whole bony frame is covered with a tendinous and nervous tegument, called periosteum; except such parts of the teeth as are designed for mastication, which are provided with a peculiar enamel, that is harder than the rest of the bony substance.

The periosteum is exquisitely sensible, and is the safe-guard to the substance of the bone, which is delicately tender notwithstanding it is not so sensible as the skin that covers it; insomuch that it will become carious on the least exposure to the air, or the attack of any foreign body of matter whatever; whence in wounds and fractures in general, great attention should be paid to the substance of the bone, being very subject to become carious and to exfoliation, which is of the greatest consequence; but of this I shall say more in another place.

The marrow is principally designed for the nourishment of the bones; which is evident from its being plentiest in young people, when the bones are strongest; and that when it is deficient, they become brittle, and lose their tenacity.

The marrow is contained in a cellular substance, partly in vesicles of a nervous texture, and partly bony cells. At the ends of the long bones the texture is more spungy than in the middle, where the cavity is less, but the substance is most compact. Though anatomically there is no perceptible circulation in the bones, yet, that a circulation is actually existing, is evident, from a liquor oozing out from the ends of a fractured bone in the living animal; by which a fractured bone again unites, and this is called the callus; and whilst in its liquid state, resembles the white of an egg, which gradually ossifies, and becomes as hard as the main substance of the bone.

The number of bones differ somewhat in various subjects; ordinarily they amount to two hundred and fifty-two: In the head sixty-three; in the trunk seventy; in the arms and hands sixty; and in the legs and feet sixty.

As the bones are the support of the animal fabric, I have inserted in the next page, a catalogue of the human skeleton, which occasionally may be referred to:

Os Frontis

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Os Occipitis

1

The connection of the bones are in various ways; those connections that are designed for rest, are by close contact of parts, and are called sutures or seams; such are the bones of the skull with themselves and the face. Those connections which are designed for motion are called articulation. Some of the articulations have but an obscure motion, as the ribs with the back-bone, and the back with itself, &c. Others have an angular motion, as the elbows and knees; and others again have an universal motion, as the arm-bone with the shoulder, and the thigh bone with the hip. The sutures, and such connections as have no motion, are merely dovetailed into one another in close connection; but those articulations that are designed for motion are connected by cartilages, either in close contact, or so as to move slippery over one another.

The ligaments are those tough tendinous parts, by which the articulations designed for motion are joined together; at some parts they cover the joints only, and at others, they are immediately fastened to each other, besides the external coverings; those ligaments are very strong and elastic, and have a close connection with the nerves that pass by them.

The surrounding ligaments of all moveable joints, form a capsular-bag, which contains a slippery liquor, called synovia, that lubricates the ends of the bones covered with cartilages, that they may move with ease and agility over one another.

Muscles are the fleshy parts on the human body, appointed for motion. They are a composition of arteries, veins, nerves, and tendons.

Muscles are generally divided into two kinds; those for necessary or involuntary motion, and those for voluntary. Some again are fastened at each end to some of the bones at their extremity, and contract themselves in a straight direction; others again are annular, or in the form of a ring; and by this manner open and shut. The first kind are generally divided into three parts, the head, belly, and tail. The whole muscle begins and ends in a tendon, by which it is fastened to the part it is designed to move; which, when the belly of the muscle contracts itself, must consequently draw both bones nearer together, fastened to the head and tail; and thus performs the motion. Some of these muscles act in consort, and others in opposition to each other.

The tendons are the principal parts of the muscle; and that part thereof, as before observed, which is fastened in the bones, namely, at the head and tail of the muscle, and is the chord, as it were, by which the limb is brought into motion.

A tendon is a hard, substantial, flexible, and elastic body, which, at the head of the muscle, is very compact; but as it enters the muscle, spreads its fibres over the whole body, gathers again at the tail, in as compact a manner as it began, and continues in this chord-like texture till it fastens itself at its appropriated place.^[7]

The tendons and ligaments are of equal substance, and differ in nothing but their various uses; namely, the ligaments to connect the articulation; and the tendons, or the ends of the muscles, to give motion. The muscles are a composition of the tendinous fibres, plentifully intermixed with small ramifications of blood vessels; all which compose the fleshy part thereof. Hence, the tendons being white and hard, the muscles gradually grow softer and redder in the middle of them; and they verge gradually to become tendinous at the tail, the same as the head.

Hence, from the close texture of the tendons, they are very painful when wounded, or otherwise hurt; but as the muscles are softer and more pliable, their wounds and other accidents are of less consequence, and not so dangerous as the former.

The number of muscles are undetermined, for though the principal muscles are so distinct that they cannot be mistaken as to their form and use, yet there is so great a number of small ones, or so many of the great ones capable of being subdivided again, that anatomists have not, as yet, agreed about their number.

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SECT. VIII.

Of the Urinary Organs and Genital Parts.

The kidnies, the ureters, the bladder, and the urethra, are the principal urinary organs. The kidnies receive two arteries called the emulgent arteries, from the great arterial trunk, called aorta, before described; they each send also two veins back again, accompanying the arteries to the vena cava. Whilst the blood is circulated in the kidnies, the urine is secreted in many small quantities, and carried from each kidney by a tendinous tube, the bigness generally of a small goose quill, called the ureter to the bladder. The ureters enter the bladder between its lamillas, so as not to admit of a return. Hence, a bladder in its natural position is both wind and water tight, and if reversed, is neither.

The bladder of urine is of a tendinous and nervous texture, and capable of great extension, but at the same time exquisitely sensible. The neck of the bladder is very muscular, and by this forms a sphincter, or annular muscle, by which the urine is retained; and as it is of a very sensible nature, this part is subject to inflammatory constrictions; great pain, and many evils have there been occasioned, either from venereal cases, or gravelous complaints.

From the neck of the bladder to the end of the penis, is the urethra; which canal serves for the emission of the urine, as well as that of the seed.

The whole is lined with a number of very little glans, to lubricate and protect the sensitive lining from the sharpness of the urine.

These are the urinary organs. We will in the next place, proceed to the organs of generation.

The testicles are the principal instruments for forming the human seed; they are generally two in number. The testicles receive the blood, of which they secrete the seed, from the aorta, about the same parts, whence the emulgent arteries project. These arteries run down from the aorta to the testicles, in a very contorted and winding manner, accompanied by the veins which return back again from the testicles, and fix themselves in the ascending vena cava. These contorted and interwoven vessels are called the spermatic vessels, which carry the blood from the arteries in little quantities, and slowly return it again into the great vein.

Whilst the blood is circulated in the testicles, the seed is secreted in them; but as it is secreted, it is transported back again from thence by tubes, called vasa deferentia, towards the neck of the bladder, where there are two irregular bladder-like vessels, wherein the seed is contained, and kept for use, called the vesiculÆ seminales.

In these seed bladders the seed is kept for use, till it is wanted. They are remarkably delicate and nervous; and when they are filled with seed, they stimulate all the organs of generation with a desire for venery.

The penis, or the manly member of generation, is partly covered with the common skin; the foremost part of which, that covers the glans, is called the prÆpuce. This prÆpuce is tied underneath to the substance of the penis, by what is called the frÆnum.

The inner part of the penis is composed of two kinds of bodies, the cavernous bodies of the penis, and the cavernous body of the urethra. The first are bodies enveloped in their peculiar teguments, and make the greatest part of the penis. Each of these bodies takes its origin from the erecting muscle of the penis, which is near the pubis; through each of them goes an artery and a nerve, which spread themselves through these bodies.

These bodies are full of cavities, which have all a communication with one another. The cavernous body of the urethra continues from the bulb, near the neck of the bladder all along to the end of the penis, and includes the glans, or the nut of the yard; and is much of the same substance with the former. At the back of the penis, goes along the large vein, called the vena penis, which spreads itself all along into numberless branches that connect themselves with the ramifications of the arteries throughout all the cavernous bodies; and from thence is caused the erection, as I shall presently describe. The glans, or the nut of the yard, is remarkably delicate and sensible, as being the seat of pleasure in coition; it is also of a cellular substance, and in one continuation with the cavernous body of the urethra.

Its sensation is greatly heightened by the frÆnum being tied underneath, from the prÆpuce to that part which keeps it, as it were, in an agreeable constriction when erect.

The penis is allowed two pair, and a single muscle. The first pair is the erectores, which take their origin from the fleshy protuberances of the ischium, and lose themselves in the cavernous body of the penis. The second pair of muscles is the transversales; these take their rise near the forementioned, and fasten themselves at the root of the penis. The fifth is the accelerator, or the odd muscle; this muscle takes its commencement from the cavernous body of the urethra, and grasps the whole penis, and adheres to the sphincter of the anus; the muscles aid and assist each other in the erection of the penis, which happens in the following manner:

At the influx of the animal spirits, the vena penis, or the great vein in the penis is somewhat contracted, whence the blood is forced into the cavernous bodies of the penis and that of the urethra, as the reflux of the circulating blood from the arteries is hindered; hence, the cellulÆ of the cavernous bodies are obliged to swell up and distend all the parts, and thus make the penis turgid and erect; when the penis is further titillated by the friction of coition, the parts become generally irritated to an agreeable constriction, whereby the seed rushes from the seed-bladders into the urethra, and is thus forcibly ejected in the time of ejaculation; then the whole fabric slackens, the constriction of the great vein relaxes, the blood regains its free circulation, and the penis becomes slack again.

These are the faint out-lines of the genital parts; but to have a full description and comprehension of the peculiar properties of the genitals of both sexes, and also the sacred act of generation, I recommend to a serious perusal of the anatomical and physiological description of the genital parts of both sexes, in my Treatise on the Venereal Disease, where they will find a full and satisfactory account.

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SECT. VIII.

Of the Skin.

The whole human automaton is covered with a skin, the mechanism of which is not less curious than what we have already described.

It is a texture curiously interwoven with an infinite number of tendons, arteries, veins, nerves, and lymphatic vessels. The tendons form the net-work of the whole, and give it an elasticity, from whence it requires a muscular motion; and the rest add each their parts, as I shall hereafter describe.

The skin, universally over the whole body, is divided into three parts; but in most parts it is found to have four.

The external, is the cuticula, or scarf skin; this is a horny and insensible substance, formed by nature as a shelter for the next part of the skin, which is exceedingly sensible; on examination it appears to be a horny substance, and as it were, little scales continued from the immediate skin, which on any external injury, such as repeated friction, fire, scalding, or blistering, separates from the main skin. The next is the real skin, called cutis; This is strongly interwoven with tendinous and nervous fibres, interspersed with numberless ramifications of arteries, veins, and lymphatic vessels; whence it is not only exquisitely sensible, but elastic, and on the least injury, subject to bleed. On the external surface is a very thin plexus, called the reticulare; which has the tinge of the natural hue of the skin, and distinguishes the complexion of the negro, the tawny, and white, from each other.

The structure of the cutis is peculiarly curious, and may justly be considered as a continued secretory and excretory gland; or more properly, like the bark of a tree; for whilst there is continual transpiration from the body, it, at the same time, is capable to absorb the subtile particles of whatever surrounds it, and exclude the grosser. Over the whole surface are innumerable little pores, which are nothing but the little openings of the various lymphatics, and of the blood vessels corresponding with their peculiar glandules, called the milliary glans; between those on the hairy parts, the hair is planted as it were, with their distinct roots, corresponding with the hue of the subject they belong to. These pores widen or contract according to the internal or external warmth of the body; which when they contract, cause a kind of roughness, or continuation of little warts; and on the contrary, when dilated, smooth the skin. Next to this follows the fat skin, or membrana adiposa; which is a continuation of cells, wherein the fat is contained, and has an immediate communication with the neighbouring blood and lymphatic vessels. This part is not universal, for some parts of the face, the genitals, &c. are without it. The last of all is the membranosa, a thin parchment-like texture, which adheres spontaneously to whatever part it covers, either periosteum, the tendons, ligaments, or muscles.

These are the four principal divisions of the skin; but that which is the principal, is the second mentioned, namely, the cutis; on which I shall have occasion to offer something hereafter, that may prove of the greatest advantage to the sea-faring people, for whose benefit I have particularly intended these Lectures.

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Of the External Senses.

Feeling is the universal sensation of the nerves, on which all other senses depend, as the subtile spring of life itself. What it is, and whence it proceeds, has already in the foregoing been explained; namely, a sensation of the nerves, by which we perceive an idea of hard or soft, wet or dry, hot or cold, or in general terms, pleasure or pain. The peculiar seat thereof seems to be placed at the ends of our fingers; whether this is the absolute seat ordained by nature, or rendered so by custom, is not my business here to enquire; perhaps, both conjoin to make it there most perfect, as best suited for that purpose.

Taste is the next sensation of the nerves, regarding an immediate approach and action of substances themselves, in order to distinguish betwixt the nature of foods, by which we are enabled to form an idea of salt, sweet, sour, bitter, sharp, or mild; which according to their mixture and proportion, render taste agreeable or disagreeable. Its seat is principally in the tongue; but that it extends to the very stomach itself, is evident, from the immediate consent of parts; namely, that what is disagreeable to the palate, will ever be nauseous to the stomach: Of which I shall say more hereafter.

Smelling is that sensation given to the animal creation, as a friendly messenger of taste, in order to enable us the better to chuse our food. It is actuated by means of the olfactory nerves spread in the pituitary membrane of the nose, and has an immediate communication with the brain; which nervous plexus is actuated by odorous particles, exhaling from substances, by which the olfactory nerves are stimulated agreeably, or disagreeably, as the first intelligence to the palate, or to the whole consent of the nervous system.

Hearing is that sensation of the nerves which is occasioned by the vibration of the air, and forms the sound, which enters the cavity of the ear, and tremulates and puts in action, the organs of hearing. The ear is perhaps as curious a piece of mechanism to our conception, as any part whatever we shall meet with in anatomy.

To the organs of hearing we must reckon the external and internal auditory, divided by a thin membrane, which is a thin spreading of nerves, called the drum; within this, or in the internal cavity, we observe, 1st. Four little bones called, from their forms, the hammer, malleus; the anvil, ancus; the stirrup, stapes; and roundling, orbicularis. 2dly. Their peculiar muscles. And, 3dly. Two considerable cavities, the labyrinth and aqueduct. How hearing is actually brought about, Anatomists have not as yet agreed; and which would be too tedious here to enquire into. That the little hammer is perpetually moving on the tympanum, according to the different sounds, may be one part; which, however, is greatly assisted by the other organs.

The labyrinth, with many turnings and windings, penetrates through the skull and forms part of the aqueduct, by which the sound is modulated and magnified, so as to cause the distinction between them, and convey the idea of their sonorousness to the common receptacle of sensation.

Seeing is that great faculty by which the form, colour, and motion of objects is represented to our ideas. The principal conveyer of this wonderful sensation, is the optic nerve; the eye is a telescope finished to the greatest perfection; or like a camera obscura, where objects are pictured to the utmost nicety, and a due proportion observed in their nature. It is a globe enrolled with a strong tendinous coat, and filled with three kinds of humours perfectly clear and transparent, which differ in nothing but their various degrees of liquidity and form. Each of these humours is enclosed in a delicate thin transparent membrane. The external coat of the eye is called the cornea, from its resemblance to horn, being a continuation of the dura mater and pia mater; and is every where opaque, except at the entrance of the light, which is called pupilla, and projects somewhat beyond the spherical form of the eye, similar to the crystal of a watch. Inside the cornea it is lined with a spreading of the optic nerves, called the retina, where objects are reflected; by which the ideas are immediately conveyed to the brain.

The humours are the following: The aqueous humour, the most external, which is designed for collecting the rays of light; the second is the crystaline humour, which is in the form of a magnifying lens, and invested with a subtile transparent tegument, and so curiously fixed, as to be contracted and dilated, according as the objects are near, or removed from the eye; and accordingly the crystaline humour requires to be flattened or thickened; the last is the vitrous humour, which fills the remaining cavity of the eye; this humour serves to spread the objects again on the retina, and arrange them in their natural order, so that the picture of the objects viewed, may be preserved in its due proportion and regularity.

Now the act of vision is performed thus: The convex transparent part of the cornea faces at once every object which comes within the direct lines thereof, and this is termed a full view.^[8] This view enters through the aqueous humour, where the whole is contracted according to its convex and concave form, so as to make it fit to enter the pupilla, which is a muscular continuation of the sclerotis, and called the uvea; and from thence the whole view passes through the crystaline lens, by which the view is again magnified, and passed through the vitrous, and the whole depicted on the retina, in the same concave curve, and at equal angles in which it was first received, and apparently the same magnitude; thence representing the objects in their natural perfection, figure, colour, magnitude, and motion.

When, however, an object is singled out of the whole view for particular attention, the uvea then contracts or dilates, which, together with the muscular action of the whole ball, proportions all the humours suitable to the magnitude and distance of the object under examination, and the quantity of light; and to make the eye more capable, the lids are frequently moving, in order to moisten and varnish, as it were, with a liquid pressed from the innermost gland, called canthus major, to keep the surface smooth, and assist its transparency. If this intense looking is continued beyond the strength of this noble organ, it relaxes from its proper destined expansion, and the objects are rendered obscure.

Darkness is ease and rest to the eye, because it is then out of action; whilst light always keeps it in exercise; And in diseases of the eyes, light should be kept away as much as possible.

Here then I shall beg leave to conclude this short description of the wonderful structure of our frame; which when we duly consider, will never fail of filling our breasts with a sense of admiration; and induce us to lift up our souls with reverence and gratitude, to that incomprehensible GREAT CAUSE, by whose wisdom we are formed; and by whose goodness we exist.