no longer receive the necessary stimuli; and then the body falls in a heap on the ground. The erect position is not only maintained simply by virtue of the control of the nervous system; it must needs be learned in childhood by every individual, and long practice is necessary to give security in its assumption.

All forms of locomotion and all self-caused movements of the body are due to the muscular system. The various processes, such as walking. running, jumping, and propelling a bicycle, are all results of action involving many separate muscles in varying degree and at different moments. A careful analysis of the movements shows what muscles are involved and how their work is related; the well-known difficulty experienced in learning the processes is evidence of their complicated character, and that this is not in any way connected with possible immaturity of the organs in childhood may be seen, for instance, in the efforts of a full-grown man to learn to ride a wheel; his muscles are perfectly developed for other purposes, but are, nevertheless, unable to perform the work required in the new movement; they must be properly co-ordinated, and to this end the nervous system needs education, which it secures by repeated trials.

Muscle hygiene.

The infant needs not only to learn how to use its muscles, it must also strengthen them by continued use. This is noticeable long before the power of definite use is gained, for the child stretches and twists and wriggles in every possible way without apparent aim to its motions. Such exercise is necessary to the development of the organs, and conditions which put restrictions upon this liberty of motion limit the growth and health of the child. For this reason all tight clothing should be avoided, and, if possible, the child should be allowed to exercise untrammeled by apparel for a period every day. The child will not overdo the matter nor endanger weak bones and muscles by overwork unless unduly encouraged.

During youth any form of moderate exercise is to be encouraged; with most boys there is little danger that it will be neglected, but, unfortunately, a false idea of propriety has served, in some instances at least, to deter girls from engaging in active exercise. Fashionable dress and lack of exercise in youth are responsible for a very large percentage of the debility and ill-health prevalent among women.

But exercise may not cease with entrance into adult life. Activity is a condition of muscle-health, and thus directly of the general health of the body, since the circulation is increased, respiration is more active, the digestion is improved, and the tone of the whole. body noticeably raised. All forms of exercise, moderately engaged in, tend to produce these results, and interest will determine in most cases the form of exercise to be followed. As a nation we are, however, clearly lacking in forms of outdoor exercise which appeal to men and women in middle life. Walking, which is so generally practiced by our English cousins, has never been widely indulged in among us; cycling and golf have secured a better hold on this class, and it is to be hoped that regular exercise will be, in the coming years, a part of the every-day program. The nervous system.

The osseous, muscular, and nervous systems are very closely connected, and the performances of the one are largely dependent upon the physiological condition of the others. The nervous system is, however, the co-ordinating element which brings not only these, but all parts of the body into close relation and causes them to work together in harmony. It has been very aptly compared to an immense telegraphic system connecting the most distant parts of the body. Like such a system, it has certain centers, the chief of which are the brain and spinal cord, occupying the cavity of the cranium and vertebral column, and these give to this part the name of the cerebro-spinal system. Scattered throughout the body are large numbers of small nervous centers, known as ganglia, and a number of the most prominent of these in the body cavity are united to each other by nerve trunks. These constitute the sympathetic nervous system and give off nerves to the viscera and blood vessels chiefly. This system regulates in a general way what are known as the vegetative functions of the body.

The chief parts of the cerebro-spinal system. are seen in the illustration (Fig. 3). The delicacy and importance of the large central masses of the brain and spinal cord call not only for their inclusion in cavities of bone, as already mentioned, but also for their being enveloped in membranes, which, being well provided with lymph spaces, furnish protection against accident and also nourishment for the enclosed nervous tissue. From the centers are given off

nerve trunks or bundles of fibres, which are definite in number and leave the cavity of the cranium or column by specific openings. In certain regions, notably those opposite the attachment of the limbs, the trunks unite and separate, forming networks or plexuses, in which an interchange of fibres takes place. As the nerve trunks proceed outward from the center they split up into smaller and smaller

FIG. 8. The Cerebro-Spinal System.

trunks; some have been demonstrated to terminate in sense cells and others in muscle

fibres.

When examined microscopically, a nerve trunk is found to consist of a number of nerve fibres bound together by connective tissue; these fibres are very delicate and usually have a protective sheath. In the central masses there are, in addition to these fibres, groups of

large cells, with usually much-branched processes, and investigation has shown that the fibres are nothing more than these much-elongated processes of the cells, which also are dependent upon connection with the cell for life and functional activity, since, if the fibre be cut, that portion which remains in connection with the cell continues to act, while the severed portion undergoes complete degeneration.

Physiologically considered, two kinds of fibres may be distinguished, the one known as efferent fibres, the other as afferent fibres. The latter, also called sensory or centripetal fibres, carry impulses towards the centers; the efferent fibres carry impulses away from centers, and are designated centrifugal or motor fibres, since many of them, connecting with the muscles, produce through them movements. These fibres run side by side, except in the roots by which nerve trunks originate from the spinal cord, where all the sensory fibres are collected into the dorsal roots and all the motor fibres are grouped into the ventral roots. Elsewhere they not only run side by side, but are absolutely indistinguishable in structure. There are strong reasons for believing that the fibres are really alike, though the nervous impulses are transmitted only in one direction on each sort of fibre, and in opposite directions on the different fibres. Although the nervous system has been generally likened to a telegraph system, it should be carefully borne in mind that the impulses are not electric currents, as has been shown by abundant evidence. It is true that an electric stimulus may be applied to a nerve leading to a muscle and produce contraction of the muscle, as if a nervous impulse had been transmitted. Yet this is as little proof that the two are identical as the fact that a drop of glycerin placed on a muscle will cause it to contract is evidence of the similarity between the glycerin and normal muscle stimuli. Nerve fibres are, above all, bad conductors of electricity, and their function cannot be the transmission of electric currents. The nervous impulses travel, also, very much slower than electricity, their rate of progress along a frog's

nerve being less than 100 feet per second. The skin.

The external surface of the body presents. everywhere a comparatively uniform appearance, though it readily discloses, by feeling, the differences in thickness which characterize certain regions, such as the palm of the hand

or sole of the foot, with a much-thickened skin, or the lips, where, on the other hand, it is very thin. Two differentiations, the hair and the nails, are peculiar to certain regions; they are both excessive growths of the outer layer or epidermis, and in general are to be regarded as protective structures. The skin is composed of two distinct layers, which are closely connected to each other and are separated by loose tissue from the internal organs. The outer layer is known as the epidermis or cuticle, and consists of numerous layers of cells, nearly cubical at the inner surface of the layer and becoming gradually flattened as the surface is approached, where the hard dead cells flake off in wear as flat scale-like masses. The coloring matter of the skin is found in the epidermis, but there are no blood vessels at all in it. As it is worn off at the surface it is gradually renewed by growth from the deeper part of the layer.

The inner portion of the skin is known as the true skin, cutis vera or corium. Its upper surface is thrown into numerous conical elevations, the papillæ, which contain knots of blood vessels and nerve terminations. The papillæ are most numerous in those portions of the body which display a peculiarly well-developed sense of touch. The corium is made up of a mass of connective tissue fibres, and these are most closely felted together in the upper portion, next the epidermis, while below the meshes become looser and more open and enclose masses of fat globules.

Both of the varieties of the glands of the skin lie below in this loose tissue and their ducts, passing up through the two layers, open on the surface of the skin. Of the two types of gland, the sebaceous or oil glands are almost always found in connection with the hair follicles; they produce an oily secretion, which softens the hair and skin and seems to keep both from becoming stiff and brittle. The second type of gland, the sudiferous or sweat gland, is generally distributed over the body, but in greater numbers on the palm of the hand and on the forehead than elsewhere. The secretion of these glands is water containing a little saline material; it is given off moderately at most times, passing away by evaporation. This is known as insensible perspiration. When the When the air temperature is high or excessive exercise raises the temperature of the body, the glands secrete so rapidly that drops accumulate on

the surface, and then one speaks of sensible perspiration. The glands are under direct control of certain nerve fibres, and may secrete rapidly even with a limited blood supply in the skin, as in case of "cold sweats."

The dry scales of the epidermis, the sold matter left by evaporating perspiration, and the sebaceous material from the dermal glands all unite to cover the surface of the skin and make its constant cleansing necessary. The closed pores of the skin limit the functions of this organ and throw more work on lungs and kidneys, so that good health depends largely on frequent bathing. The friction of the towel after a bath serves also to quicken the circulation and raise the general tone of the body. For persons in good health a cold bath is best, and the water should always be left before the warm reaction which follows the first shock has passed off. About three hours after breakfast or dinner is the best time for a long bath, but a short dip or a shower bath just on rising is very beneficial. It is safe to bathe when warm if not in a profuse perspiration, but not when chilly or in poor condition. Owing to the stimulating effect of the salt water, a cold sea bath is more invigorating than one in fresh water, and can be enjoyed by many who are not strong enough to endure the latter.

The internal organs.

The cavity of the trunk is divided by a transverse dome-shaped partition (Fig. 4), the diaphragm, into two portions, an upper, thoracic, and a lower, abdominal, cavity. In the latter lies the major portion of the digestive system, while the former contains the main organs of respiration and the heart, with the great blood vessels immediately connected to it. The general arrangement of these organs, as shown in the figure, serves to emphasize certain hygienic facts of great importance.

The full inflation of the lungs is best attained by elevating the ribs and thus expanding the thorax laterally, while at the same time the muscles of the diaphragm are contracted and its dome lowered. It is clearly evident that tight clothing will interfere with the normal position of organs, and with the freedom of movement necessary for the exercise of their functions. This is particularly the case if the waist be confined; the movement of the diaphragm and of the lower part of the chest, just described, cannot be carried out. This reacts,

of course, unfavorably on respiration, but its effect does not end here; the subjacent organs, if not forced seriously out of place, are much limited in their movements, and this, with the lack of stimulus from abdominal breathing, is undoubtedly an important factor in the weak

FIG. 4.-Organs of Chest and Abdomen in situ. A, Heart; B, Lungs; C, Diaphragm; D, Liver; E, Gall Bladder; F, Stomach; G, Small Intestine; H, Large Intestine.

movements and consequent constipated condition of the digestive organs in many cases. Since the superficial muscles are unemployed, they will be reduced to a flabby, degenerate condition, and the effect upon the alimentary functions of their normal exercise is entirely

lacking. The effect of specific exercise on the abdominal muscles can always be seen in improved digestion and circulatory conditions, as well as in a better carriage of the body and in greater power and freedom of movement.

Furthermore, an improper position in sitting has also the effect of compressing and limiting the activity of these same organs, while a round-shouldered attitude serves to compress the upper angles of the lungs and makes them susceptible to the inroads of disease. On the other hand, exercise or habit that tends to straighten the shoulders develops the chest and strengthens and expands the lungs. Lung power is life force.

Many more illustrations might be adduced to show that the condition of the child physically determines his power of life-whether he may attain to ripe mental development by virtue of a strong body, or is destined to expend a large share of his energy in the struggle with disease. Men and women of great success in life have been, with rare exceptions, those who had an inheritance of strong physical nature developed by a period of growth under circumstances favoring the accumulation of physical force. Not only the power of the men and women of to-morrow, but also the vital energy of the next generation depends on the education of the children of to-day.

Lincoln, Neb. HENRY BALDWIN WARD, The University of Nebraska.

The figures illustrating this article are reproduced from Hutchi Merrill & Co., New York. son's Physiology through the kindness of the publishers, Maynard,