Physiology is a science, because it embodies a collection of general principles and ascertained truths relating to a particular subject, and is called a natural science because these truths are founded on observation. The word “physiology” is derived from two Greek words meaning a discourse about nature; but it is used in a restricted sense, and is the science of the functions of the different parts of any living body. Thus we have animal and vegetable physiology, while the former is divided into human and comparative. The first of these divisions relates only to man, while the other reviews the entire animal kingdom.

Our object is to teach some of the simple truths of human physiology; such as may be intelligible without any extended knowledge of other sciences. It must be remembered, however, that a more thorough and complete study of physiology can not be undertaken without a considerable acquaintance with such sciences as mechanics, hydraulics, optics, etc., without which the action of the muscles, the circulation of fluids, and vision can not be properly and fully comprehended.

Whenever a piece of mechanism, designed for some particular use, is brought under our notice, and we wish to understand its manner of working, we naturally inquire about its structure; for without some knowledge of how its component parts are put together, and by what means it is put in motion, we can not hope to understand how it performs the part which we see it do. Such is the case, for example, with a watch or steam engine; their parts must be carefully studied in order that their workings may be fully understood.

Hence, it will be observed from what has been said, that it is impossible to study the uses of various parts of the body without some knowledge of anatomy, this being that branch of knowledge which treats of structure.

In order that anatomy may be studied the organs must be dissected.

The words “anatomy” and “dissection” have the same literal meaning, the former being derived from the Greek language, and the latter from the Latin, meaning to cut apart or separate. But anatomy is employed to signify the science of structure in living bodies, and dissection is used to denote the unravelling or laying bare the parts of the body, by means of which anatomy is studied.

The words “organ,” “organization,” and “organize,” are so convenient and necessary that we must know what they mean. The word “organ” signifies some part or parts of the body that have a particular use or function: thus, muscles are organs of motion, and nerves of sensation; the eye is the organ of sight, and the heart and blood vessels are the organs for circulating the blood. Now, any structure is said to be organized, or to have organization, that possesses the properties which distinguish a living body from one that never had life. Therefore we speak of the organic world as distinguished from the inorganic; the former includes plants and animals, the latter minerals, etc.

The nature of that mysterious principle which we call life is unknown to scientists. Yet we may know and understand many of those things which are believed by physiologists concerning life. Certain of them believe that life is a phenomenon that follows organization; or, in other words, that organization is the cause, life the result; while others contend that organization is the result of life. In the former life is produced by changes that take place in matter, under the influence of those forces of nature called heat, light, electricity, and chemism; by the latter theory all these forces are present and act under the influence of a potent force called life. This will be our belief: Life is a distinct endowment, capable of propagation, and superior to all other forces by which it is attended. Let us see by what means living bodies are distinguished from unorganized. Living bodies increase in size; so do minerals; the former by the addition of material throughout the tissue, the latter by outside additions. Organized bodies have a limited existence. All are subject to constant change, and to final dissolution. They all spring from a parent, and only originate in this method. The opposite characteristics belong to unorganized matter.

It has been the business of chemistry to determine the ingredients of the earth and atmosphere, by resolving them into what are called elements. By the word element we mean that which can not be resolved into any simpler form. For example, take a piece of chalk; by chemical action it can be divided into a gas called carbonic acid, and a solid called lime; therefore, it is known that chalk is a compound body. If we take a piece of iron or gold there is no process known by which we can resolve it into simpler form, and these are therefore elements. There are over sixty elements and many of them are found in the human body. All these elements are derived from unorganized matter, and the special conditions under which they are formed in organisms is due to the principle of life. The predominant elements that make up the human body are carbon, hydrogen, oxygen, and nitrogen, while iron, potassium, sodium, phosphorous, calcium, exist in smaller quantity. These different elements are variously combined to form compounds, of which water is the most abundant, for it forms more than two-thirds of the entire weight of the body: water is a compound of hydrogen and oxygen. The principal organic compounds are albuminous, of which the white of the egg is a typical example; the gelatinous, or jelly-like compounds, including cartilage, oleaginous, or fatty compounds, and saccharine, or sugary compounds, such as starch. The first two contain nitrogen, hydrogen, oxygen, and carbon, while fat and starch are rich in carbon, but lack nitrogen. In tracing back the development of living matter to its simplest form, it is found to exist as specks without any definite shape or structure, or as granules of extreme minuteness, perhaps the ten thousandth part of an inch in diameter.

This elemental living matter is called protoplasm. Its simplest form is termed a cell, and the word is applied to little bodies varying much in form. Thus some cells are really little bags filled with fluid. Such are those in which the fat is deposited; others are disk like, others lengthened, while some so-called cells are simply masses of jelly.

Out of these organic compounds all the softer tissue of the body is formed, such as fibrous, muscular, cellular, and adipose tissue.

Fibrous tissue consists of fine threads, arranged in various ways, to adapt it to some special use; some of these threads are elastic, others are wholly inelastic. These tissues of mixed character form what is called connective tissue, which is found throughout the body.

Fat is deposited in little cells situated in the connective tissue.

Muscular tissue, which forms the great bulk of the body, is easily recognized by its ruddy color, and it constitutes what is known as “flesh.” It is fibrous, and if these fibers be examined under the microscope there will appear, in those under the influence of the will, transverse markings, while the muscles not under the will-power lack these markings.

Nervous tissue is that which superintends all the actions of the living body. It is accumulated in nerve centers of which the principal is the brain. It consists of minute cells in these nerve centers, and of delicate tubes filled with nervous matter throughout the organs of the body. Such tubes are called nerves.

The skeleton is the frame-work of the body, and serves to support all the softer organs as well as to protect them. It is composed of bones which serve as attachments for the muscles.

Bone is the firm tissue of the body, and to a certain degree is hard and brittle. If bone be burned its brittleness is very much increased, but if it be placed in acid it loses its brittle properties. Thus we see that bone is made of two entirely different materials. The one called “animal matter” is easily burned out, while the other called “mineral matter” resists the action of heat, but is quickly dissolved by acid. The mineral matter is a compound of lime. This it will be seen gives to the bones their rigidity. Bones are of various shapes and sizes, adapted to the work which they have to perform.

The most remarkable part of the skeleton is the spinal column, commonly called the back-bone. It is made up of twenty-six bones, or spines, which are united together in a marvelous manner, combining strength and freedom of motion. Each spine has a central aperture which communicates with that of the adjoining spines, thus forming a long canal in which the spinal cord lies. This cord connects the brain with the various parts of the body. There is a soft cushion of cartilage between each spine, which adds much to the elasticity of the back-bone, protecting the brain from shocks. This protection is further accomplished by the curvature of the spine. These bones are not placed directly over each other, but are so arranged as to give three curves to the spinal column. Along the column there are openings, at each joint, through which the nerves come to supply fibers to the different organs of the body.

The spine rests on the pelvis, which is a large double bone, basin-shaped, that sustains the abdominal organs. The pelvis, in turn, rests on the thigh bones.

The thigh bone, “femur,” is the largest single bone in the body; it is joined to the pelvis by a ball and socket joint, an arrangement which permits its motion in any direction. The femur is supported by the bones of the leg. These are two in number in each leg, the tibia and fibula. The former is much the larger, and the latter is often called the “splint bone.”

The bones of the arm corresponding to those of the leg are the humerus, the radius, and the ulna. The humerus is the bone of the upper arm, and is large and strong; it articulates with the body in so perfect a manner that the great variety of motions required to be performed is easily and gracefully accomplished. This bone is fastened by a ball and socket joint, but is not, like the thigh bone, firmly fixed to an immovable bone. It is attached to a broad bone called the scapula, or blade bone; this is fastened to the body by muscles which give it great freedom of motion. The bones of the fore arm are the radius and ulna: these are nearly of the same size; in this they differ from the leg bones.

The wrist, composed of eight bones, forms the support for the tendons, blood vessels, and nerves which pass to the hand. To these are attached the metacarpal bones which support the phalanges, or bones of the fingers. The arrangement of the bones of the foot is, in many respects, like that of the hand. However, the bones of the foot form an arch which acts like a spring when the weight of the body falls on it, thus helping to ward off shocks and jars.

The skull is a large bony case in which the brain is lodged, and by which it is protected. It is composed of several pieces firmly united by a very irregular line of union. The attachments of the skull are the jaw bones, nasal bones, and the cheek bones.

Joints have been provided wherever parts of the skeleton require motion; they are of several kinds, chief among which are the ball and socket, hinge, and flat joints. The hip has been mentioned as a perfect example of the ball and socket joint, while the knee presents an example of the hinge.

Muscles are necessary to all motion, and are found everywhere throughout the body. They are the active agents, which, under the control of the nerve centers, do all the work necessary to carrying on the functions of the organs. They are endowed with a remarkable property called contraction. How this is accomplished is not fully understood, but when stimulated by nerve force or electricity they shorten and widen. This brings their ends nearer together; also the parts to which they are attached. They are always attached to movable parts; their attachment is a tendon at one end, and a fan-like distribution of their fibers at the other. Tendons are inelastic fibrous tissue, very strong, and consequently much smaller than the muscle. They are to muscles what tugs are to horses. Muscles are either flexors or extensors; the former bend the joints, the latter straighten them.

The skin forms an exterior covering for the body, and is employed for other and very important functions. Its appendages are the hair and nails. Some of the uses of the skin are these: It is highly sensitive and capable of distinguishing various sensations; it has the property of excreting, of secreting, and of absorbing. It is an organized texture containing blood vessels, glands and nerves. It is composed of several layers; the outside, called the epidermis, is not supplied with blood vessels, and is produced by the under layers.

The glands of the skin are first, those which secrete the perspiration. This is poured out to keep the temperature of the body even, also to rid the circulation of many injurious substances. Then there are glands which secrete an oily matter which aids in preserving the softness and pliability of the skin. These glands are called sebaceous glands. Hair is said to be an appendage of the skin because its texture is essentially similar to that of the cuticle. Nails, also, are but modifications of the skin. The fact that the epidermis is being constantly cast off and renewed, and that matter is both secreted and excreted continually, renders frequent washing of the skin necessary to health.

Digestion—In animals the arrangements which exist for converting aliment into blood are more or less complicated according to the requirements of each class; and this conversion is called assimilation, a term which includes digestion and those changes which take place in the blood and tissues, by which new material is added to them.

The preparation of the food for assimilation by the tissues is accomplished in a long tube called the alimentary canal. This canal is made up of various parts having different functions and different construction. These parts we will briefly describe. They are the mouth, pharynx, Æsophagus, stomach, small and large intestine.

As solid food requires to be broken up or ground before it passes into the stomach, the mouth is provided with teeth firmly implanted in the jaws, while the lower jaw is moved by strong muscles in two directions, one vertical, the other lateral. Man is supplied with two sets of teeth; the first adapted to the jaws of childhood; the second larger, which replace the former, are designed to last through life. The rudiments of each set are found in the jaws before birth. During the grinding or mastication of the food it is moistened and softened by a fluid called saliva. This also acts chemically upon it, changing the starch into sugar. The food is carried from the mouth to the stomach in a long tube called the Æsophagus, by means of the muscular contraction of this tube.

The stomach is a flask-shaped organ consisting of a double wall of tissue, the outer one being muscular, the inner one vascular. This latter membrane has a large supply of blood vessels, which convey the blood out of which the gastric juice is manufactured and secreted by the little glands of this membrane. The gastric juice is a fluid which contains an active ingredient called pepsin. This, aided by the acid which this fluid contains, effects a remarkable change on the albumen of the food, making it a liquid. From the stomach the food passes into the small intestine, where it receives from a small tube the pancreatic juice which changes the fat into an emulsion.

The intestine is a tube, about twenty-five feet in length, which, like the stomach, has a double wall. Its inner coat contains multitudes of little projections called “villi.” These contain blood vessels which absorb and carry off the liquified food. It also secretes a fluid called intestinal juice, which acts upon the unchanged starch, making it into sugar. Its muscular wall by a continuous contraction produces a motion known as the “peristaltic motion,” which carries the food onward in its course. The intestine also receives the bile, a fluid produced by the liver.

The liver is the largest gland in the body. It receives the portal vein which conveys the blood from the intestines. This vein, after dividing and sub-dividing, thus bringing blood into communication with all parts of this organ, is again collected into a main trunk which passes on toward the heart. During its passage through the liver the bile is eliminated from the blood by the little cells of which this gland is composed. From the liver the bile is carried toward the intestine, into which it is poured to assist in digesting the food, and to be itself changed.

Besides the little blood vessels in the villi of the intestines there is another set of vessels called the lacteals, which aid in taking up the digested food and pouring it into the circulation; also, throughout the body a set of similar vessels collect the waste material and pour it into the great veins, returning to the heart, in order that it may be renovated or cast off; these are the lymphatics.

The kidneys are great excretory organs, and are similar in shape to those of a sheep, but are somewhat larger. They are glands, and excrete urea, as well as other salts and waste materials, all of which are highly poisonous if not removed from the blood.

Blood is the life-giving fluid of the body: it is the source from which all tissues are built, and it is the workman that carries the waste material away from the tissues. In order to accomplish its work it must circulate, and this requires a separate set of organs. First, the impure blood must be carried back from the different parts of the body; then it must be distributed again. It must also be made to flow onward in a continuous current. Blood is a viscid fluid, of a red color, containing over seventy per cent. water, with solid matter. Its color differs on each side of the heart. When it returns from the body it is blue, but when it leaves it is red. Under the microscope it is seen to contain minute globules, or disk-like cells; to these the blood owes its color. It possesses the remarkable property of spontaneous coagulation when drawn from the veins. It is forced on by the heart.

The heart is a muscle and consists of four cavities; two called auricles having weak walls, and two ventricles with strong muscular walls. The blood returning from the body is poured into the right auricle, thence into the right ventricle; from this cavity it is forced through the lungs and returns again to the heart, being poured into the left auricle, which empties into the left ventricle. This ventricle forces the blood throughout the body. The blood received by the heart from the body is impure, and is sent to the lungs, where it gives up part of its impurities and receives oxygen from the air.

Arteries are those vessels that distribute the blood, while the veins collect it and return it to the heart, thus all streams of blood leaving the heart are conveyed in arteries, and those pouring into it are carried in veins.

The lungs are the organs that purify the blood, and in order that this be thoroughly done, the blood is distributed throughout their substance in minute capillary vessels. The lungs themselves are vascular; being made up of a multitude of air cells, their surface is greatly increased; hence their power of absorption. The diaphragm is a muscular partition lying below the lungs. It is dome-shaped, and when its fibers are shortened it enlarges the cavity in which the lungs are situated. This creates a partial vacuum, causing the air to rush into the lungs. The blood absorbs the oxygen from the air and gives up carbonic acid gas. When the muscles of the diaphragm are relaxed the elastic force of the air cells in the lungs expels the remaining gases from the lungs. The diaphragm is assisted by the action of muscles situated between the ribs; these lift the ribs and enlarge the cavity of the thorax. The lungs also act as a reservoir for the air used in the production of vocal sounds. They communicate with the atmosphere by means of a tube called the trachea; this terminates in the pharynx, with which the nostrils also communicate, thus completing a passage to the outside air.

The voice is produced by the modifications which the teeth, tongue, lips and throat make upon the sound produced by the vocal cords. The vocal cords are stretched across the upper end of the trachea, which is called the larynx. The air from the lungs is forced past them, setting them in vibration, thus producing sound.

The nervous system consists first of all of the brain. This is composed of nervous matter and constitutes the mass contained in the skull. It is divided into three parts: the cerebrum, the cerebellum, and the medulla oblongata. These three parts seem to preside over different functions. Thus, the cerebrum is the seat of the faculties of the mind, while the cerebellum presides over the muscular actions. The brain is connected with distant parts of the body by means of fibers which are distributed to all parts. These fibers are of two kinds, one set carrying the impressions to the brain, the other carrying the stimulus from the brain to the organs. The former are called sensitive, the latter motor nerves. A deep fissure separates the brain into two lateral halves, and these parts are connected with opposite sides of the body. Besides these nerves, which originate in the brain and pass through the spinal cord, there are twelve pairs that pass directly through the skull to the organs which they supply. These are called cranial nerves, and are distributed to the eyes, the nose, the ears, the larynx, the lungs, the face, stomach, etc. Ten of these pairs of nerves originate in the medulla oblongata. Of the functions presided over by these nerves may be mentioned those performed by the tongue, (taste), eyes, ears, and nose.

Many of these nerves are not sensitive, in the ordinary use of the word. Thus, the retina of the eye is the expansion of the optic nerve, and, while it is sensitive to light, it is not to ordinary impressions, such as material contact. Also, the nerve of the ear is only sensitive to the vibrations of fluids. We see by light reflected from objects. This light passes through a set of lenses, and by means of these an image is formed on the retina, which impression is carried to the brain. Just how all this is accomplished is not known. The nerve of the ear floats in a fluid called lymph. This fluid receives the vibrations of bodies through the air, through the membranes and chain of bones, and thus the nerves receive and transmit them to the brain, which act constitutes hearing.

Volumes could not tell all that one single fiber of muscle contains that is instructive, much less the entire functions, constructions, and mysteries of a single organ of special sense. And to perform all the allotted functions every part must be in the best repair. This constitutes health. Health is maintained by cleanliness, by repose, by muscular activity, by moderate eating, by plenty of fresh air, by a contented disposition, and a clear and active mind. Watch over your body with a jealous care, for all your future depends upon its good condition.