The animal, like the vegetable, is also composed of chemical elements, and by chemical analysis has been found to contain eighteen, as follows:

1. Of primary or vital importance: Oxygen, Hydrogen, Carbon, Nitrogen.

2. Of secondary importance, entering into the more solid structures: sulphur, phosphorus, calcium, sodium, chlorine, silicon, potassium, fluorine, magnesia, iron.

3. Accidental constituents: Magnesium, alumina, copper, and lead.

The compounds found in the body are recognized as being derived from organic and inorganic substances.

Organic substances are obtained:

1. From plants and vegetables, and are termed carbohydrates or non-nitrogenous substances, being composed of Oxygen, Carbon, and Hydrogen—as starch, sugar, etc.

2. From animals: nitrogenous substances; these compounds contain Oxygen, Hydrogen, Carbon, and Nitrogen—as meat, white of eggs; these are also termed albuminous.

3. Mineral, elements of inorganic origin, as soda, potassium, phosphorus, etc.

The more highly organized tissues found in the animal are composed of five elements, as muscle, brain, blood; these are Oxygen, Carbon, Hydrogen, Nitrogen, and sulphur.

Albumen, for example, exists in most of the tissues of the body, but especially in the nervous tissue, lymph, chyle, blood, etc.

Fibrine is found most abundantly in the blood and the more perfect portions of the lymph and chyle.

Gelatinous substances are contained in the cellular or fibro-cellular tissues in all parts of the body, as tendons, ligaments, cartilages, bone, skin, mucous membranes, etc.

Chondrine is obtained from cartilages, etc.

The general chemical composition of these substances is as follows:

It will be observed that in the composition of these tissues, more than half of their constituent elements is Carbon. There is but a very small quantity of Hydrogen.

The most abundant inorganic substance in the body is water, which is composed of Oxygen one and Hydrogen two (OH₂). More than two-thirds of the body is made up of water.

The body is composed of various structures. Of the chief tissues of the human body, the weight is as follows:

Let us examine, briefly, each of these.

The skeleton.—The skeleton, or solid framework of the body, is mainly formed of bones, but is completed in some parts by the addition of cartilages. The bones are bound together by means of ligaments, and are so disposed as to support the softer parts, protect delicate organs, and give attachment to the muscles by which the different movements are executed.

There are two hundred and four bones in the body:

The organic constituents form about 33.3 per cent of the composition of bone, while the remainder, 66.7 per cent, is inorganic matter; as follows:

The mineral or earthy matter enters very largely into the composition of bone.

A fibrous membrane covers bone externally, and is called periosteum. The hollow bones contain marrow, composed of fat, 96 parts; water, 3; connecting tissue, 1. Bones are supplied with blood-vessels, which carry the nutritious fluid to them.

1. The master tissues. Primarily, it is the tissue, and not the blood, that gets loaded with carbonic acid, the latter simply receiving the gas from the former by diffusion, and the oxygen which passes from the blood into the tissues being at once taken up in some combination.

2. Nearly one-half of the weight of the body consists of the skeletal muscles, and about one-quarter of the total blood in the body is contained in them.

3. The muscles are always producing carbonic acid (CO₂), and when they contract there is a sudden and extensive increase of the normal production.

4. Oxygen is necessary for the life of the muscle; it is for the nervous tissue, but for muscular tissue especially.

5. When venous blood, instead of arterial, is sent through the blood-vessel of a muscle, the irritability speedily disappears, and unless fresh oxygen is administered the muscle soon ceases to act and dies.

6. The oxidation power is determined by the tissue and by the tissue only.

7. All the available evidence goes to show that oxidation takes place in all the tissues and not in the adjoining blood.

The master tissues of the body are the muscular and nervous tissues. All other tissues may be regarded as the servants to these.

These tissues are the all-important tissues in the body. The muscular tissues constitute and carry out the power, force, or energy of the body. They set the body in motion. They do the work. They regulate the delicate movements of the organs of special sense or function, as the eye, the ear, the tongue, the nose, larynx, thorax, abdomen; and fighting, defending, building, destroying, labor and mechanical skill of whatever nature, depend on them. Of exercise, sport, pain and pleasure, sensation, emotion, expression of the face, in fact all in all in every act of life, the muscles, the voluntary muscles, must perform the work.

They are called the muscles of Animal life. They are Voluntary; they may be set in action at will.

For guidance, control, coÖrdination, sensation, and motion, the muscular tissues are dependent on the nervous tissues.

It is not difficult to understand, I think, as will be explained later on, that all muscular movements are perfectly natural, purely physical and mechanical.

The nervous tissue will be a little more difficult to comprehend, for causes that are reasonable and plain.

All animals are provided with two distinct sets of organs: 1. The master tissues, the nervous and muscular tissues, the voluntary muscular tissues, which are the organs of animal life, the voluntary, the active organs that do the work, consume the food, and throw off the waste material; and 2. The servants to these, the involuntary tissues, the organs of organic life that prepare the food, carry it to the master tissues, and bring away the waste material.

The inherent qualities of both these sets of organs are instinctive, with this difference—the former, the voluntary, the controlling and working master tissues, are capable of development, progressively, acquiring intelligence, maturing into educatedness, etc.

The latter, the involuntary, are simply servants to these, and they perform their functions in the same manner instinctively all through life.

The muscular and nervous tissues are the educable tissue. By repetition, practice, and exercise they improve and at length exhibit certain degrees of skill in the performance of their work.

On the other hand, the organs of mastication, deglutition, digestion, absorption, excretion, circulation, and respiration simply perform their functions instinctively, without possessing the capacity of improvement, and without regard to volition. These act involuntarily throughout life, as preparers and carriers of nourishment to the master tissues, and removers of waste material.

The work of the muscular tissues is comparatively easy to understand. We can see the work done, can account for it, can demonstrate it. The performances are capable of absolute proof, and controversy therefore is out of question.

The nervous tissues present quite another state of things. The great mass of cerebral matter, with all its complicated organs and their appendages, are hid within the cranium of the skull. We have no ocular proof of anything that is done by that structure, or of the manner in which the tissue acts. That we can see, hear, taste, and smell we know. We recognize the organs that perform these functions. Sensation, feeling, memory, thinking, cannot so easily be accounted for. Among the masses it is a mystery to-day. The doctrine of a dual existence in man is old, still it is held on to with remarkable tenacity. The church still teaches and preaches that soul or spirit is a part of some great personality or individuality not at all connected with nature—supernatural, divine, godly. This supernatural part, it is said, is placed in man some time during the process of birth. This subject will be more fully discussed farther on, in order to show what queer views theologians formerly held on some scientific subjects. I beg to quote from a Talmudistic scholar and philosopher some thousand years ago:

Jehuda ha-Levi ben Samuel, whose Arabic name was Abulhassan, considered an authority and philosopher of repute, was born in Castile 1085 A.D. He adopted medicine for his profession, but was also a traveler, philosopher, and student, and a Talmudistic scholar and writer. He wrote a book called “Sepher Hakusir: Book Kusari.” It is a philosophico-theologico-scientific treatise, conducted in dialogue between himself and the king of Kusar, who became convinced of the truth of his argument and was converted to the Hebrew faith.

In the fourth part, section 25, page 246, Jehuda ha-Levi is explaining the harmonious working of the whole universe, and in evidence thereof he cites the world, soul, and year, very learnedly setting forth the mysterious working of creation, the supernatural origin and significance of the Hebrew letters, the secret and hidden meaning of their number, etc. This is based upon the principle that one rests on three, three on seven, and seven on twelve, as follows:

World: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon.

Man: Wisdom, riches, dominion, life, kindness, posterity, peace.

Year: The seven days in the week.

The twelve single one letters not mentioned—Man: Organs of hearing, seeing, smelling, speaking, tasting, begetting, dealing, walking, thinking, anger, laughing, sleeping.

World: The twelve Zodiacs.

“One on three and three on seven and seven on twelve. And these numbers have their functions in common one with another. For example, ‘the kidneys counsel,’ ‘the spleen laughs,’ ‘the stomach sleeps,’ ‘the liver gets angry.’ It is not to be wondered at that the kidneys have power to give counsel; we observe something similar when the testicles have been removed; one that has been castrated is weaker than a woman; the beard does not grow, and, what is more significant, the person can no longer give advice, counsel. The spleen laughs because of her natural functions, by reason of the blood being protected against the black gall and thickening and turbidity, and from this clearness, purity, nothing but brightness and joy comes. The liver is angry because of the bitterness she forms. The stomach sleeps by reason that it stands in relation with the organs of nourishment. The heart is not thought of, because it is the king. No more do they take in consideration the lungs and diaphragm, because they are necessarily so constituted to be of service to the heart; accidentally only do they serve the rest of the body, and are originally not designed to serve. The brain is under the senses, which emanate from that organ, and are thence distributed. Moreover, as to the organs that are situated below the diaphragm, therein lies a deep meaning. These are the primary vegetations, the primary generatives. The diaphragm separates the vegetative from the animal life, as the throat separates the animal from the rational (Plato in his TimÆus explains). Out of the primary generatives, out of the world vegetative, there where the root of being exists, the seed comes, and there the embryo is fashioned out of four elements. God has selected certain parts for his sacrifices—fat, blood, the peritoneum on the liver, the kidneys. On the contrary, he did not select the heart, or the brain, or the lungs, or the diaphragm. This is a deep mystery; the explanation is forbidden. Therefore the prescript, that the Jezisa is permitted to be studied only after undergoing some preparation, by few persons, and only under certain formalities,” etc.

Maimonides, or Moses ben Maimon (Rambam), 1131–1205 A.D., wrote God Hazaker, the Strong Hand, a very celebrated commentary on the Talmud. He held similar views, and is also considered a very learned authority.

The voluntary muscles are for the most part placed in close relation with the skeleton, being attached to the hard parts, and moving these in different directions by their contraction.

The muscles are all symmetrical, and with the exception of the sphincters and one or two others are in pairs. Each muscle constitutes a separate organ, composed chiefly of contractile fibrous tissue, which is called muscular, and of other tissues and parts which may be regarded as accessory. Thus muscular fibers are connected together in bundles or fasciculi, and these fasciculi are again embedded in and united together by a quantity of connective tissue, forming the perimysium; and the whole is usually inclosed in an external sheath of the same material. Many of the muscles are connected at their more or less tapering extremities with tendons by which they are attached to the bones or hard parts; and the tendinous bands frequently run to a considerable length either on the surface of the muscle or between its fibers.

There are two chief kinds of muscular tissue, the striped, and the plain or unstriped, and they are distinguished by structural peculiarities and mode of action. The striped form of muscular fibers is sometimes called voluntary muscle, because all muscles under the control of the will are constructed of it. The plain or unstriped variety is often termed involuntary, because it alone is found in the greater number of muscles over which the will has no power.

The involuntary or unstriped muscles are made up of elongated, spindle-shaped fiber cells, which in their most perfect form are flat, from about 1/4500? to 1/3500? of an inch broad, and about 1/600? to 1/300? of an inch in length; very clear, and granular and brittle so that when they break they often have abruptly rounded or square extremities.

The fibers of involuntary muscles form the proper muscular coats of the digestive canal, Æsophagus, urinary bladder, trachea, bronchi, gall-bladder, blood-vessels, lymphatics, etc. To this kind of fiber, muscular fiber, the term organic is often applied. The sympathetic or ganglionic portion of the nervous system, which consists of a chain of ganglia connected by nervous cords, extends from the cranium to the pelvis, along each side of the vertebral column, and from which nerves with ganglia proceed to the viscera in the thoracic, abdominal, and pelvic cavities. By its distribution, as well as by its peculiar mode of action, this system is less immediately connected with the mind, as conducting either sensation or the impulses of the will; it is more closely connected than the cerebro-spinal system is with the processes of organic life.

The muscles of animal life, or striped muscles, include the whole class of voluntary muscles, the heart, and those muscles neither completely voluntary nor completely involuntary, etc. All these muscles are composed of fleshy bundles called fasciculi, inclosed in coverings of fibro-cellular tissue, by which each is at once connected with, and isolated from, those adjacent to it. Each bundle is again divided into smaller ones similarly ensheathed and similarly divisible; and so on, through an uncertain number of gradations, till we arrive at the primitive fasciculi or the muscular fibers peculiarly so called.

Muscular fibers consist each of a tube or sheath of delicate structureless membrane, inclosing a number of filaments or fibrils. They are of cylindrical form, or of prismatic with one or more sides, according to the manner in which they are compressed by adjacent tissues. Their average diameter is about 1/500? of an inch, and their length never exceeds an inch and a half.

The arrangement of the elementary substances in a muscular fiber (the sarcos element or protoplasm inclosed in the sarcolemna, the sheath) composing a muscular fiber may be compared to Volta’s pile or an electric battery. In fact, both muscle and nerve are made up of electrical molecules, each of the two ends of which is negative—though the development of the electrical current is at present very imperfectly known.

Besides, there is every reason to believe that the ground substance is similar in nature to ordinary protoplasm, but without the granular character commonly but not always exhibited.

Blood-vessels are largely distributed in the substance of a muscle, carrying the materials necessary for its nourishment and chemico-vital changes, and there are also lymphatic vessels as in other vascular parts of the body.

Nerves run through every muscle, by which the muscular contractions are called forth, and a low degree of muscular sensibility is conferred upon the muscular substance.

The blood-vessels of the muscular tissues are extremely abundant, so that when they are successfully filled with a colored injection the fleshy parts of the muscle contrast strongly with its tendons. The arteries, accompanied by their veins, enter the muscle at various points and divide into branches, etc.

The nerves of a voluntary muscle are of considerable size. Their branches pass between the fasciculi and repeatedly unite with each other in form of a plexus, which is for the most part confined to a small part of the length of the muscle, or muscular division, in which it lies.

The voluntary muscles to which distinct names have been given in the system amount to about 240, and they naturally fall under the following four great divisions (the muscles are symmetrical and with few exceptions are in pairs):

Flesh and blood have nearly the same ultimate composition. On evaporating 1000 parts of blood it yields 790 parts of water and 210 parts solid residue. The elements that enter into the composition of the solid matter are as follows:

The general composition of a human muscle is shown by the following table:

The muscles of the flesh form a large proportion of the weight of the whole body. Calculated for a man of 150 pounds’ weight:

The property of muscular tissue by which its peculiar functions are exercised, is its contractility—contraction or shortening. This is excited by all kinds of stimuli, applied either directly to the muscles, or indirectly to them through the medium of their nerves.

The muscular tissues perform all the physical work—as locomotion, every kind of action and exertion—of the body.

The quantity of blood circulated through the body is estimated to be from about 1/10? to about 1/13? part of the body’s weight, and about ¼ of that is distributed in the muscles.

As regards the action of the muscles the following general principles ought to be kept in view:

1. That the force exerted by any muscle during its contraction is in proportion to the number of muscular elements or fibers composing the muscle.

2. That the extent of motion, in so far as it merely depends on the shortening of the fibers of the muscle, is in proportion to the length of the fibers.

3. That the direction of the force produced by a contracting muscle is in the line of the axis of the whole muscle if it runs straight between its opposite points of attachment, but in the line of the portion attached to the moving part of the muscle, or its tendon, if it be bent in its course, etc.

The Nervous Tissue.

The nervous system consists of the cerebrum, pons varolii, cerebellum, medulla oblongata, the spinal cord with its nerves and the sympathetic ganglia, etc.

The cerebrum or brain proper constitutes the highest and much the largest portion of the encephalon. The cerebrum consists of two halves, that are connected with each other by the corpus callosum, and with the peduncular masses of the cruri cerebri, the processus a cerebello ad cerebrum; the series of eminences, or cerebral centers or ganglia, concealed from view, named corpora quadrigemina, optic thalamus and corpora striata, etc.

The cerebral hemispheres are by far the most bulky part of the cerebrum. Various commissural structures unite the two hemispheres, including the corpus callosum and fornix; and some smaller structures, viz., the pineal gland, the petuitary bodies, and the olfactory bulb.

The cerebral hemispheres together form an ovoid mass, in contact with the vault of the cranium, and with its smaller end forward, its greatest width being opposite to the parietal eminences. They are separated in the greater part of their extent by the great longitudinal fissure.

The surface of the hemisphere is composed of gray matter, and is molded into numerous smooth tortuous eminences, named convolutions, or gyri, which are marked off from one another by deep furrows, called sulci.

The cerebrum is divided into lobes for convenience of study, five in number, called frontal, parietal, occipital, temporal, sphenoidal, and central.

The internal structure of the cerebrum is composed of white matter. It consists of tubular fibers varying in size in various parts, but in general still smaller than those in the cord, their average diameter being 1/10000? of an inch. The fibers of white substance present no division. They are arranged in bundles, separated by a network of delicate connective tissue, consisting of cells, etc. The cells are of various forms and sizes—spheroidal, angular, fusiform, etc. The fibers radiate from the white center of each convolution in all directions into the gray cortex, having a course for the most part perpendicular to the free surface. In passing through the gray substance they are arranged in bundles about 1/1500? of an inch in diameter, thus separating some of the nerve cells, etc.

The olfactory tract and bulb, the corpora quadrigemina, corpora genicolate, optic thalamus, corpora striata, are all more or less mixed. They possess gray matter.

The nerves immediately connected with the brain are of several kinds. And there are twelve pairs of them. They are called cerebral nerves.

There are four kinds.

1. Nerves of special sense.

2. Nerves of common sensation.

3. Nerves of motion.

4. Mixed nerves of sensation and motion.

The nerves of special sense may with great propriety be termed the nerves of observation, perception—the gateways of intelligence and education.

I.—Nerves of special sense:

1. The olfactory supplies the nose, special sense of smell.

2. The optic supplies the eye, special sense of sight.

3. The auditory supplies the ear, special sense of hearing.

4. Part of the glosso-pharyngeal supplies the tongue and pharynx.

5. The gustatory, lingual branch of the fifth, supplies the tongue, sense of taste.

II.—Nerves of common sensation:

1. The ophthalmic supplies the eye.

2. The superior maxillary supplies the upper jaw and teeth.

3. The inferior maxillary supplies the lower jaw and teeth.

III.—Nerves of motion:

5. The facial nerve supplies the muscles of the face.

6. The hyperglossal supplies the muscles of the tongue.

IV.—Mixed nerves:

1. The pneumogastric supplies lungs, heart, stomach, larynx, etc.

2. The spinal accessory supplies some muscles of the back.

The average weight of the brain in the adult male is about 49½ ounces, a little more than three pounds avoirdupois; in the female 44 ounces; the average difference between the two being from 5 to 6 ounces.

The spinal cord has a length of about 16 to 17 inches, and weighs about 1½ ounces.

The spinal cord is a continuation of the medulla oblongata, is lodged in the spinal canal, and gives off 31 pairs of nerves, that supply all the muscles of the body with sensitive and motor nerves.

The medulla oblongata is pyramidal in form, having its broad extremity upwards. It is expanded laterally at its upper part. Its length from the pons varolii to the lower extremity of the pyramid is about an inch and a quarter; its greatest breadth is nearly an inch; and its thickness from before backwards is about three-quarters of an inch.

The medulla is the link between the brain and the spinal cord. The majority of centers for various organic functions are situated in it; as follows:

1. The respiratory center, with its neighboring convulsive center (venous blood excites convulsive centers, etc.). 2. The vaso-motor center. 3. The cardiac-inhibitory center. 4. The diabetic center, or center for producing artificial diabetes. 5. The center for deglutition. 6. The center for the movements of the Æsophagus, with its vomiting center. 7. The center for reflex excitation of the secretion of saliva, with which may be associated the center through which the vÆjus (pneumogastric) influences the secretions of pancreatic juice, and possibly of the other digestive juices. 8. The center for the dilation of the pupil by means of the cervical sympathetic.

From the surface of the medulla certain of the cranial nerves arise, namely the sixth (abducens), glosso-pharyngeal, pneumogastric, spinal accessory, etc.

The fibers from the spinal cord pass upwards through the medulla oblongata and various other structures and finally reach the cerebrum.

The cerebellum, or hinder brain, consists of a body, and of three pairs of crura or peduncles, by which it is connected with the rest of the cerebro-spinal axis. The cerebellum is covered with a gray cortical substance, rather darker than that of the cerebrum. Its greatest diameter is transverse, and extends to about three and a half or four inches; its width from before backwards is about two or two and a half inches; and its greatest depth is about two inches, but it is much thinner round its outer border. It consists of two lateral hemispheres joined by a median portion called the vermiform process, and other structures therewith connected, etc.

Minute structure: The cortical gray substance is composed of an external clear gray layer, an inner grayish-red “granule” layer, and between the two a single layer of large cells with long processes, termed the corpuscles of Porkinge (after the man who first described them). Outside all is the layer of fibers and vessels of the pia mater. The external layer consists of a delicate matrix, probably of the nature of connective tissue, consisting of cells and fibers, etc.

The cerebellum is probably concerned in the coÖrdination of movements. Its functions seem especially connected with afferent impulses proceeding from the semicircular coats.

The spinal cord is a cylindriform column of nerve substance connected above with the brain, through the medium of the medulla oblongata, terminating below, about the lower border of the first lumbar vertebra, in a slender filament of gray or vesicular substance, the filum terminale, which lies in the midst of knots of many nerves forming the codÆ equina. Through the center of the cord, running in a longitudinal direction, is a minute canal, which is continuous through the whole length of the cord, and opens above into the space at the back of the medulla oblongata and pons varolii, called the fourth ventricle; the aqueduct of silvius connects it with the third ventricle, lateral and fifth ventricles, near the base of the brain. The cerebro-spinal fluid circulates in the interior of these ventricles and spinal cord. What precise mechanical function it subserves is only surmised, not known.

The cerebro-spinal axis is protected by three membranes, named also meninges. They are: 1. An external fibrous membrane, named dura mater, which closely lines the interior of the skull, and forms a loose sheath in the spinal canal; 2. An internal areolo-vascular tunic, the pia mater, which accurately covers the brain and spinal cord; and, 3. An intermediate membrane, the arachnoid, which lies over the pia mater, the two being in some places in close connection, and in others separated by a considerable space.

The sympathetic nerves are distributed in general to all the internal viscera, and to the coats of the blood-vessels. Some organs, however, receive their nerves also from the cerebro-spinal system, as the lungs, the heart, and the upper and lower parts of the alimentary canal.

The great gangliated cords consist of two series, in each of which the ganglia are connected by intervening cords. These cords are placed symmetrically in front of the vertebral column and extend from the base of the skull to the coccyx.

With respect to the functions of the sympathetic nervous system, it may be stated generally that the sympathetic nerve fibers are simple conductors of impressions as those of the cerebro-spinal system are, and that the ganglionic centers have (each in its appropriate sphere) the like powers of conducting and of communicating impressions.

The general processes which the sympathetic appears to influence, are those of involuntary motion, secretion, and nutrition.

Nerve centers. This term is applied to all those parts of the nervous system which contain ganglion corpuscles, or vesicular nerve-substance—i.e., the brain, spinal cord, and the several ganglia which belong to the cerebro-spinal and the sympathetic system. Each of these nervous centers has a proper range of functions, the extent of which bears a direct proportion to the number of nerve fibers that connect it with the various organs of the body, and with other nervous centers; but they all have certain general properties and modes of action common to them as nervous centers. The brain does not issue any force, except when itself impressed by some force from within, or stimulated by an impression from without; neither do the other nerve centers without such previous impressions produce or issue motor impulses.

The more certain and general office of all the nervous centers is that of variously disposing and transferring the impressions that reach them through the several centripetal fibers. In nerve fibers impressions are conducted only in the simple isolated course of the fiber; in all the nervous centers an impression may not only be conducted, but also communicated; in the brain alone it may be perceived.

In all cases in which the mind either has cognizance of, or exercises influence on, the process carried on in any part supplied with the sympathetic nerve, there must be conduction of impressions through all the nervous centers between the brain and the part. But instead of, or as well as, being conducted, impressions made on nervous centers may be communicated from the fibers that brought them to others, and in this communication may be either transferred, diffused, or reflected. Along nerve fibers impressions or conditions of excitement are simply conducted; in nerve centers they may be made to deviate from their course, and may be variously diffused, reflected, or otherwise disposed of.

Function of nerves. The office of nerves as simple conveyors or conductors of nervous impressions is of a twofold kind: 1. They serve to convey to the nervous centers the impressions made upon the peripheral extremities or parts of their course; 2. They serve to transmit impressions from the brain and other nervous centers to the parts to which they are distributed. For this twofold office of the nerves two distinct sets of nerve fibers are provided, in both the cerebro-spinal and sympathetic systems. Those which convey impressions from the periphery to the center are classed together as centripetal or afferent nerves, or nerves of sensation—sensitive nerves. Those, on the other hand, which are employed to transmit central impulses to the periphery are classed as centrifugal or afferent nerves or motor nerves, conveying impulses to the voluntary and involuntary muscles, etc.

Nerves are constructed of minute fibers or tubules full of nervous matter, arranged in parallel or interlacing bundles, which bundles are connected by intervening connective tissue in which their principal blood-vessels ramify.

The size of nerve fibers varies, and the same fibers do not preserve the same diameter through their whole length, being largest in their course within their trunk and branches of nerves, in which the majority measure from 1/2000? to 1/3000? of an inch in diameter. As they approach the brain or spinal cord, and generally also in the tissue in which they are distributed, they gradually become smaller. In the gray or vesicular substance of the brain or spinal cord they generally do not measure more than from 1/10000? to 1/14000? of an inch.

The chemical composition of nervous matter. Like most of the other tissues of the body, the nervous substance contains a large proportion of water (from three-fourths to four-fifths of its weight). Of the residue which remains after the removal of this by evaporation or other means, the larger part consists of a phosphuretted fat, which may be obtained crystallized, and in this condition was termed protagon. The crystalline substance, however, is in reality a mixture of two other substances, lecithin and neurin. Cerebrin is also described as being frequently met with in conjunction with lecithin.