knee, the muscles that straighten the knee-joint, proceed from the front of the pelvis and from the femur itself, forming the fleshy mass in front of the thigh. The most important muscle of the foot is that which forms the calf of the leg (A, fig. 50). It is of immense strength, because in raising D a the heel it has to raise the weight of the whole body. It is attached above to the bones of the thigh and leg, and is inserted by the tendon of Achilles, as it is called (a), to the heelbone. Another muscle, B, is attached to the tibia, and inserted in one of the bones of the arch of the foot; and a third is attached to the fibula, and inserted in the outer metatarsal bone. These two last being inserted into the outer and the inner edges of the instep, turn round the outer and inner ankle respectively. The raising of the toes, the turning of the foot outward, and the straightening of the sole, are effected by other two muscles (F, fig. 50, and a similar one on the other side), the tendons of which pass down in front of the inner and outer sides of the ankle, and are inserted in the inner and outer edges of the instep respectively. Fig. 50. The Skin, Hair, &c. THE SKIN consists of two layers, the true skin and the scarf-skin ; the former being called the dermis1 or cutis,2 and the latter the epidermis3 or cuticle. The dermis forms the thick layer of the skin, and is a strong, supple, elastic membrane, formed of a compact kind of tissue, which, in its ordinary form, has a spongy appearance, being full of little cells. This tissue is composed of two elements—a yellow or elastic substance, and a white or inelastic, tough substance; and these substances are arranged in various proportions, according to the nature of the covering that is wanted at a particular place. The cutis also varies as to thickness, according to the amount of resistance or of protection required for the part covered; thus, on the sole of the foot, the cutis is very thick, and is composed principally of the tough white substance. On the outer surface of the cutis are a number of small flexible elevations, called papillæ or pimples, which will be mentioned again in treating of the 1 From Greek derma, skin. 3 From Greek epi, upon, derma, skin. 2 Latin, 'skin.' 4 From Latin cuticula, diminutive of cutis, skin. senses of touch and smell. Imbedded in the cutis are the sweat-glands,1 which separate from the blood the fluid which appears on the skin when one is heated. These glands send forth their fluid by ducts2 or canals of a spiral form. Besides these glands, there are, in the cutis, two kinds of follicles or 'little bags' connected with the hair. These are the hair follicles proper, which are little pits in the cutis, in which the roots of the hair are developed; and the sebaceous follicles, which are glands for the secretion of a fatty matter supplied to the hair when it comes above the skin, to prevent its being too much dried up. Each hair consists of a ‘bulb' and a ' shaft;' and the shaft, again, consists of a soft, pith-like part in the middle, with an outer and harder layer. The bulb consists of a mass of the cells of which the cutis is composed, developed at the bottom of the follicle. The inside of the shaft contains something of the same kind; while its outer layer is simply these cells dried and turned into a scaly nature. This is, in fact, exactly the composition of the epidermis; the cells which are on the surface are dried and flattened into scales (as seen in the scurf thrown off the skin of the head, especially under the hair), so as to form a transparent, insensible varnish for the sensitive cutis. So, too, the nails are formed; the horny substance of which they consist being composed of these dried and hardened cells, which are being continually pushed outward by a development of fresh cells at the root of, and under, the nail. Nutrition. Having described the framework of the body, we now proceed to consider the manner in which it is kept in repair. When anything is said to be kept in repair, it is understood that it is already made or formed; and so here it is understood to be a full-grown human body that is to be nourished. And this process is different from either 'growth' or 'development.' Development is the process by which each part of the body is first formed or so changed as to be adapted to perform a higher function. Growth, again, is the mere increase, principally in size and weight, without any change in form, of the different parts by the addition of matter similar to that composing them. During the whole period of life, there is continually going on in all parts of the body a process of decay, and the casting off of substances which have become useless and even positively injurious in the economy of the body. Nutrition, then, includes the different processes of Digestion, Absorption, Secretion and Excretion, Circulation of the Blood, and Respiration, by 1 A gland is an organ, having blood-vessels, absorbents, and nerves, for secreting a certain fluid from the blood. 2 From Latin duco, to lead. 4 From Latin sebum, fat. 3 Latin folliculus, diminutive of follis, a bag. which the decaying matter of the body is carried away and fresh material supplied in its place. The process of Digestion may be divided into the following parts1. The Mastication of the food; 2. The Insalivation; 3. The Deglutition or swallowing; 4. The Chymification or digestion in the stomach; 5. The Chylification or digestion in the intestines; and 6. The Absorption of the chyle. These are 1. Mastication or chewing is performed by the teeth. small, hard, white bodies, fixed in the jaws, so as to come against each other, and reduce the food by cutting and bruising it. They are formed of three substances-dentine, cement, and enamel. The inside and body of the tooth are formed of dentine, so called from Latin dens, dentis, a tooth; outside of this is a layer of cement, which is softer than dentine; but where the tooth appears above the jaw, the cement is replaced by a hard enamel, which forms a very strong protection for the exposed part of the tooth, called the corona [Latin, 'a crown']. There are in all 32 teeth in man, and these are of three kinds: the incisive1 or cutting teeth, in front; the canine2 or dog-teeth, next to the cuttingteeth on each side; and the molars 3 or grinders, behind, ordinarily called double teeth, because they have a double edge, instead of a single one like the front teeth. The incisors have a single fang or root, and the corona is bevelled behind, so as to present a sharp, chisel-like cutting edge. There are two on each side of both jaws, that is, eight altogether, and they are used for biting the food, aa, fig. 51. The canine teeth, so called because they are very prominent in dogs, have a single root, and their corona is more pointed than edged, bb, fig. 51. The molars are divided into true and false. First come (in a halfjaw, as A, fig. 51) two false molars or premolars, so called because they are placed before the true molars, c, which are of a form between that of the canine teeth and the true molars. Last of all come the true molars, d, which are larger than any of the other teeth, and have a corona well adapted for bruising the food. 1 From Latin incido, incisum, to cut. 3 From Latin molo, to grind. 2 From Latin canis, a dog. The different kinds of teeth with which man is furnished indicate that his food is intended to be of a mixed nature. Those animals that live entirely on flesh have back teeth with sharp, jagged edges, which fit into each other, and act very much like scissors, tearing and cutting their food; partly of this nature are the incisive, canine, and premolar teeth of man. Those animals, again, that live entirely on vegetable food, have broadheaded teeth, very uneven and rough on the top, the joint of the jaw having a freer motion, so that the jaws rub on each other sidewise, and thoroughly bruise the food, as if with millstones. This kind of teeth is found in the molars of man; and the under jaw has this side-motion, to a certain extent. 2. Insalivation. While the food is being chewed, it is mixed with saliva or spittle. This is a colourless, tasteless fluid, which is secreted by three pairs of glands, one pair in front of the ears, a second under the jaw, one on each side, and a third under the tongue, one on each side. The saliva, by moistening the food, renders it more easily swallowed; the moisture also helps in the separation of the particles of the food, and in the sense of taste. Another purpose served by the saliva is the conversion of the starchy matter of the food into sugar, which promotes its absorption. h d 3. Deglutition.-The pharynx1 or entrance to the gullet, is closed, while the food is being chewed, by the soft palate, which is a fleshy valve hanging down from the roof of the mouth. As soon, however, as the food is properly chewed and mixed with saliva, it is pushed backward by the tongue, and the soft palate is immediately drawn backward and upward, and the food enters the pharynx. As soon as this happens, it is beyond recall, except by the involuntary action of coughing, for it is carried into the stomach by the action of the muscles of the gullet, which are excited by the presence of the food. The principal muscles of the gullet are circular, or surround it like a number of elastic rings; so that, when a mouthful of food has been pushed into the pharynx by the Fig. 52. 1 Greek, a cleft.' tongue, it is moved onward through the gullet by the continual contraction of the elastic rings behind it. In this manner, the food finds its way into the stomach. The course taken by the food will be best understood from an outline sketch of the Alimentary Canal (fig. 52), which is the whole digestive apparatus through which the food passes. The gullet (a), in its passage to the stomach, passes through an opening in the diaphragm,1 a large, flat muscle, which forms the partition between the chest and the abdomen. Immediately below this, it enters the stomach, a large, muscular bag (b), widest at the end where the gullet enters (c); from the narrower end proceeds the smaller intestine, which is generally about twenty feet long, and, rolled up in numerous folds (e, e), occupies the middle of the abdomen. The small intestine is attached to the large intestine (h, i, j, k), which is about five or six feet long, and passes upward on the right side of the folds of the small intestine (the right side of the figure is the left side of the man), across to the left, and then down to the anus or lower orifice of the alimentary canal, by which its contents are discharged. 4. Chymification.-When the food enters the stomach, the process of digestion immediately begins. This consists in the reduction of the food to a thoroughly broken-up, thin, pulpy mass, so as to allow of the nutritive part being received into the system. For this object, it is mixed with a fluid, called the gastric 2 juice, which is secreted in a great many little bags in the walls of the stomach, and poured out whenever food enters the stomach. To facilitate this mixture, there is a provision made for stirring it about. The walls of the stomach are composed partly of strong muscles; and when the food enters, which it does at the upper left-hand corner (c, fig. 53), by the contraction of these muscles, it is pushed towards the right, along the top (a, a). When it comes to the right extremity, it is pushed back to the left, but now nearly in the middle (b); Fig. 53. and when it comes back to the left, the opening being firmly closed by muscles, it is divided into two streams -one going along the top, and the other along the bottom. By this means, the food is thoroughly mixed with the gastric juice, and reduced to a halffluid pulp, called chyme [Greek chymos, juice]. The gastric juice consists of two elements-an acid, and an organic matter called pepsine, from Greek peptō, to digest; and the uses of it are to dissolve and modify all animal 1 From Greek diaphrassō, to divide by a partition. 2 From Greek gastēr, the stomach. |