cess. It was necessary, therefore, to give the body the power of selfrenewal or repair; and hence the necessity for food. The conversion of food into the materials of the body is a complicated proBlood is the fluid employed, and is manufactured from the food by the processes of digestion and absorption, after which it is carried by the blood-vessels to all parts of the body, and made the vehicle by which waste matter is removed from the different parts of our frame, and new matter laid down in its place. DIGESTION. The first step in the alimentary process is the seizing, or Prehension, of the food, which man accomplishes with his hands, while the lower animals chiefly employ the lips, tongue, and teeth. Next follows the process of Mastication, or chewing, accomplished by the teeth, tongue, and cheeks. 1. The TEETH are very hard, and shaped so as to cut like knives or grind like millstones. The child has the same number of teeth as it has fingers and toes, and similarly arranged as four fives; the adult a dozen more. The first, or temporary set, generally begin to cut the gum between the seventh and eighth month, and are completed in two years and a half or three years. Being unable to grow, they are displaced by the larger permanent set, which appear between the sixth and seventh year, and are completed by the thirteenth year; except the backmost, which may be delayed till the twentieth or thirtieth year, and is hence called the wisdom tooth. The eight, on each side of each jaw, are (naming them from before backwards),-two incisor,1 one canine,2 two bicuspid, and three molar.4 3 The process of Mastication is assisted by the addition of the saliva, which is secreted by three pairs of salivary glands; the parotid, which pours its fluid in at the cheek; and the sub-maxillary, and sub-lingual, the ducts of which enter below the front of the tongue. 2. The next stage is Deglutition, or swallowing. First voluntarily carried backwards into the pharynx, the food is thence involuntarily carried over the opening to the windpipe, down into the 1 Incido, to cut into. 2 Canis, a dog; corresponding to the tusk of carnivorous animals. 3 Two-pointed, Cuspis, a point. 5 Parotid, para, near; ous, the ear. 4 Mola, a millstone. œsophagus or gullet, by which it is conveyed away down through the neck and chest to the stomach. Diagram of the course of the food, from the mouth to its entering the blood at the jugular vein. 88 The three salivary glands. Ph the Pharynx, the cavity behind the nose, mouth, and larynx. The two arrows show the course of the air from the nose to the larynx, and of the food from the mouth to the oesophagus. L Larynx. T Trachea or Windpipe proper. O Esophagus, or Gullet terminating in the stomach. P Pyloric orifice and valve, between Stomach and Duodenum. G Gall-bladder and its duct. B Bile duct, uniting with duct of Pancreas. 4 3. The STOMACH is a dilated part of the alimentary1 canal, for containing and reducing the food. It is composed, like most of the other hollow viscera,2 of three layers:-1. The external, serous, 3 which enables it to move smoothly about, gives it strength, and helps to keep it in its place. 2. The middle, muscular, which gives it contractile power, to act on the food by a kind of churning process, so as to mix up the digesting materials, and then force it onwards. 3. The internal, mucous, or lining coat, soft and velvety, which is thickly studded with the little tube-like glands which secrete the gastric juice. This gastric juice is the true digestive fluid, acting chemically, but formed in the stomach. It is an important fact that this fluid is formed and poured out, not in proportion to what we eat, but in accordance with the wants of the system. Hunger is a craving of the entire body for nourishment, to supply the waste attendant on living action, but is referred to, or felt in, the stomach. It is enough that we eat until this feeling is appeased; whatever is more, merely overloads and distresses. Eating, like other healthy actions, has been made pleasant to us, but it is not meant that we should follow the example of those who, instead of eating in order to live, seem to live in order to eat. No organ, perhaps, is more unfairly dealt with than the stomach. By some, it is first overloaded and then pampered, for the mere animal pleasure of eating; for which the simple cure is to put less in it. A famous physician well advised his over-fed patient, to live on sixpence a day and work for it. While, on the other hand, it is no less true that the want of sufficient food, among the poor, especially in early life, produces a degenerate frame and a weak constitution. The prevalent but decreasing custom of drinking alcoholic fluids-beer, wine, spirits, &c.,-is still more to be reprehend No ed. That the abuse of them is injurious to stomach, brain, and system generally, and that they destroy life, is well known. insurance office will accept the life of a drunkard. But it is now well known to physiologists that, even when used in moderation, they are only stimulants, and that the stimulation they produce is succeeded by a corresponding depression; that they are "physic, not food," and physiologically and medically on 1 Alimentum, food. 2 Viscus, any internal organ. 3 Serous membranes line the closed cavities, and facilitate motion, like the synovial membranes of the joints. 4 Mucous membranes line those cavities, and passages, which open externally, and there become continuous with the skin. 5 Gaster, the stomach. the same footing, for instance, as opium. Let it be known, then, that the deep-rooted drinking custom of society is not a necessity, whatever its source may be. Of the many excuses people assign, the most common is, that they feel the better of it, as the Chinese says of his opium. If all men could use liquors in moderation, the philanthropist would have little occasion to remonstrate; but as the use unavoidably leads to the abuse with many, it is highly important to proclaim, especially to the young, the great physiological truth, that all we require for health and enjoyment, whether of body or mind, is a sufficiency of good food to eat, water to drink, and plenty of fresh air to breathe. The digested food, now in the form of a pulp, called Chyme,1 is allowed to pass into the duodenum, having been previously kept back by the structure termed the Pylorus or gate-keeper of the stomach, which consists of a muscular ring, assisted by a fold of the lining membrane. 4. In the Duodenum,2 or first part of the small intestine,3 the chyme is mixed with the bile and the pancreatic juice, by which it is separated into two parts-the one, a finer part, called the Chyle, to be taken up by the absorbents, and the other consisting of matter which is useless, to be cast out of the body. The Bile to which we have referred is secreted by the Liver from the blood which passes through it. The bile ducts unite to form one large duct, from which, before it goes down to the duodenum, a side duct is given off, which soon ends in a dilated sac, the gall-bladder, of the shape and size of an average pear. The use of the gall-bladder is merely to hold for a time the bile which the liver is continually forming, although between the times of digestion it is not needed in the duodenum. The bile duct is joined by the duct from the Pancreas, and the common duct, resulting from the junction, enters the duodenum by a small orifice about three inches beyond the pylorus, having pierced through the soft coats so obliquely that regurgitation is checked as effectually as if it had been provided with a valve. (Fig. 12.) 5. Absorption.5-Next begins the process by which the chyle is 1 Chumos, juice; chulos, juice. 2 The duodenum (from duodem, twelve is twelve finger-breadths long. 3 Intestine, or bowel; intus, within. 5 Absorbeo, to suck up. 4 Pancreas, pan, all; kreas, flesh. sucked up by the absorbent vessels. To facilitate this, it is spread out over a large surface of intestine, which is many feet in length, and which, in order to increase the absorbing surface over which the chyle has to flow, is plaited inside. Some substances, such as water or alcohol, or solutions of salts, may be absorbed directly by the blood-vessels of the stomach, but the food proper is absorbed afterwards from the intestine, and is not, physiologically speaking, within the body until it has been absorbed. The absorbent vessels spread themselves thickly on the intestine, like the roots of a tree in the soil beginning, by closed or looped ends in the numerous little tongue-like villi1 which project into the intestine. Absorption takes place, not through apertures, but, like all the changes between the body and the exterior, by passing through the membranes, soaking through as it were, only in obedience to a vital force. The absorbent vessels from the intestine are termed the Lacteals,2 while those which take up lymph from other parts of the body are termed Lymphatics.3 Passing through a series of glandular bodies, the lymphatic glands, which further prepare the chyle, that thick white fluid then enters the thoracic duct, a vessel somewhat less than an ordinary quill, and, like the other lymphatic vessels, well guarded with valves. After travelling up through the back part of the chest, near the oesophagus, the thoracic duct, as seen in Fig. 12, at last ends in one of the veins at the root of the neck on the left side; and there the course of the food is completed, and the blood renewed or fed by the addition of the chyle. CIRCULATION. The organs concerned in the circulation of the blood are the heart, arteries, capillaries, and veins. The arteries take it out from the heart; the capillaries give out the nourishment; the veins return it to the heart; and the heart is the force-pump which keeps it moving on in a circle. Before being again sent round the body, it is sent to the lungs for purification, and hence we have a double, or figure of 8, circulation; the larger, or systemic, circle of distribution to the body generally, and the lesser, or pulmonic, circle to 1 Villus, a tuft. |
