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food, except fat, and all the blood-forming portion of vegetable food.

5. Chylification. When a certain quantity of chyme has been formed in the stomach, the valve which closes the pyloric1 orifice (d, fig. 52), or opening between the stomach and the small intestine, is opened, and it passes out; but no food is allowed to pass out that has not been properly mixed with and acted upon by the gastric juice. In the small intestine, the chyme is mixed with three new fluids—the bile, the pancreatic3 juice, and the intestinal juice. The bile is a greenish-yellow liquid, secreted by the liver, and kept stored in a little bag near it, called the gall-bladder, from which, as well as directly from the liver, it is discharged into the intestine, close to where the latter leaves the stomach, f, fig. 52. Its principal function is to digest the fatty part of the food, which the gastric juice does not act upon. The pancreatic juice is a fluid very like saliva, which is also poured into the intestine at the same place as the bile, g, fig. 52. One of its functions is to complete what was begun by the saliva-namely, to convert starchy matter into sugar. It also serves further to dilute the chyme, and performs a very important chemical process, too complex to be described here. The intestinal juice unites in itself the properties both of the saliva and of the gastric juice—namely, the power of converting starchy matter into sugar, and of dissolving all animal food but fat, and the blood-giving portion of vegetable food.

In the intestine, then, we have seen that there is a provision for reducing nutritious matter of every kind to a state fit for absorption, the fluid here acting on the food being compounded of the saliva, the gastric, the pancreatic, the intestinal juices, and the bile. And as the process of thus reducing the food is being carried on whilst the food moves through the intestine, so the nutritious part is being withdrawn by absorption. One part, which is perfectly reduced, and fit for passing into the system at once, is taken up by the blood-vessels (in the stomach as well as in the intestine); another part, which seems to be blood in an early stage of its formation, with a large excess of fatty matter,' is called chyle [Greek chylos, juice], and is taken up by the lacteals, a system of absorbent vessels, so called from the white, milk

B like appearance of their contents.

After the chyle has been taken up by the lacteals, there remain certain parts of the food which have been rejected as comparatively worthless for nutrition. This portion of the food, called the faces [Latin, 'grounds'], passes from the smaller into the larger intestine, in which most of the

1 From Greek pylouros, a porter, from pylē, a gate, and ouros, a guardian.

2 Latin bilis, connected with fel, fellis, the gall-bladder.

3 From Latin pancreas, a fleshy gland under and behind the stomach, by which the fluid is
secreted, from Greek pan, all, and kreas, flesh.
4 From Latin lac, lactis, milk.

remaining fluid is absorbed. This intestine, though not so long as the small intestine, is wider than it, because a greater capacity is required, on account of accumulation.

6. Absorption.-Both in the stomach and in the smaller intestine, part of the nutritive materials of the food is received directly into the system by the blood-vessels; but besides this, there is another system of absorption going on, on a more roundabout plan, by means of the lymphatics, which are absorbent vessels that originate in all parts of the body. From the body in general, they collect the fluid which has escaped from the blood-vessels into the different tissues, as well as the particles of worn-out tissues, and convey both into the venous circulation near the heart. At their extremities, the lymphatics are spread out in a minute network, from which they pass to the lymphatic glands, or to a larger trunk. In passing through these glands, which are small solid bodies found in the course of the lymphatic vessels-for example, in the neck, the armpit, the groin, &c.-the fluid receives a part of its peculiar properties.

The lacteals, already mentioned, are simply the lymphatics of the smaller intestine; but the fluid they carry is called chyle instead of lymph. On the inner coat of the small intestine are a great number of small conical projections, called villi, from Latin vellus, a fleece of wool. At the points of these villi, the lacteals take their origin. They then pass between the two folds of the membrane which connects the intestine with the cerebral column, and make their way to a pouch or bag in the lumbar region, from which the thoracic2 duct conveys the chyle upward along the spinal column to above the level of the heart; then crossing to the left side, it joins another vein by which its contents reach the heart.

Circulation of the Blood.

The process of digestion just described prepares a fluid called the BLOOD, which conveys nutriment to all parts of the body. It at the same time takes up all waste matter, which is carried to the lungs, where it is discharged, and a supply of new matter received from the air, as will be explained under RESPIRATION.

To effect all this, a continual circulation of the blood is necessary, which is produced by a very simple but effective pumping apparatus called the Heart.

The blood itself consists of a nearly colourless fluid called the liquor of the blood, and, floating in this fluid, an immense number of round, flat particles called globules,3 or corpuscles.*

1 From Latin lympha, water.

3 From Latin globulus, diminutive of globus, a ball.
4 From Latin corpusculum, diminutive of corpus, a body.

2 From Latin thorax, the chest.

The liquor of the blood consists of water in which are dissolved a quantity of a substance called fibrine,1 a good deal of albumen, to which all the tissue-forming elements of the food are reduced, a quantity of fatty matter, and other organic substances, with a quantity of mineral matter, principally salt. The fibrine is the substance which coagulates in the blood (it may be seen by beating fresh blood with a stick, to which it adheres in fine threads-hence its name); and it is this property that forms the beautiful provision for the repair of injuries. Were it not for this power of coagulating, a great quantity of blood might be lost by the smallest cut or scratch. The purpose of the albumen, along with the fatty matter also in solution in the blood, is to be expended in keeping up the solid tissues of the body, by which it is being continually appropriated. The liquid in which the fibrine and albumen are dissolved has the power of absorbing gases, so that the 'liquor of the blood,' besides continually building up the tissues of the body, also conveys to them oxygen from the lungs, and carries back the carbonic acid which is set free in the tissues. But in this duty, the heaviest part of the work is performed by the properly solid part of the blood, the red corpuscles; and these, again, besides this function, are principally concerned in the production and renewal of the muscular tissue.

The apparatus by which the blood is conveyed to every part of the body, to perform the functions just described, consists of two systems of tubes to convey the blood, called blood-vessels, and the heart, which is in reality a force-pump of great power. The two sets of blood-vessels are the arteries, which carry the blood from the heart throughout the body, and the veins, which bring it back to the heart. There is, besides this, a

second circulation. When the blood returns to the heart, it is unfit to be again sent out until it be purified by exposure to the air: this is accomplished in the lungs, so that all the blood thus returned to the heart is immediately sent off to the lungs, from which it comes back to the heart; and this is called the Pulmonary 2 or Lesser Circulation. There are thus two sets of arteries and two sets of veins, the vessels which carry the blood from the heart to the lungs, and vice versa, being called pulmonary arteries and veins respectively; although it is to be remembered that the kind of blood contained in the arteries and veins of the different sets is reversed, the pulmonary arteries containing true venous blood, and the pulmonary veins carrying back to the heart true arterial blood.

The HEART is a strong muscular bag, situated in the left side of the chest. It contains four chambers or divisions, the two in the upper end called auricles 3 (d the right auricle, and k the left, fig. 54), and the two in

1 From Latin fibra, a thread.

3 Latin auricula, diminutive of auris, the ear.

2 From Latin pulmones, the lungs.

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the lower end, ventricles1 (a the right, and m the left, fig. 54). And as the most important division of the heart is into two sides, there are a right and left auricle and a right and left ventricle. Between each auricle and the ventricle on the same side, there is a valve (c and 7, in fig. 54), like those in a force-pump [PHYSICS, page 17], through which the blood is forced, by the muscular contraction of the heart, from the auricle

into the ventricle. The blood brought back from the body is poured, by the two veins f and b, into the right auricle; the right auricle contracting on this blood, drives it through the valve, c, into the right ventricle; the right ventricle now contracting, drives the blood, which cannot pass back to the auricle, because the pressure on it closes the valve, along the pulmonary artery, branches of which, g and i, go to the right and left lung respectively. The blood is brought back from the lungs by the pulmonary veins, two from each lung, e, e', and poured into the left auricle; the contraction of the left auricle drives the blood through the valve, 7, into the left ventricle; the left ventricle contracts, and drives the blood into the large artery, h, called the aorta,2 because it seems to suspend the heart between the lungs.

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n Fig. 54.

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The whole of the arterial blood leaves the heart by the one large trunk of the aorta; this trunk soon divides into branches; these branches subdivide into smaller branches, and these into smaller still, until every part of the body, especially at the surface, is intersected with such a network of small arteries, that the point of a needle cannot be inserted into the flesh without wounding one or more of them, and thus drawing blood. To a certain extent, the veins might be said to run parallel with the arteries, branch for branch, with this difference, that they are carrying back to the heart the blood which has oozed into them through the capillaries from the arteries. It will easily be understood that the force of the blood in the arteries, as propelled direct from the heart, must be much greater than in the veins, in which the blood is found after it has passed through a succession of most minute tubes, where much of its velocity is lost. Accordingly, the arteries are of a much stronger structure than the veins. In consequence of the strong flow of blood through the larger arteries, they are, for the most part, placed at a distance from the surface, so as to be protected from injury; the veins, on the other hand,

1 Latin ventriculus, diminutive of venter, the belly. 2 Greek aorte, from aeiro, to suspend.

lie, for the most part, near the surface, which renders them more liable to injury; but the consequences are less serious than in the case of the arteries, on account of the less rapid flow of blood.

The minute vessels which form the connection between the smallest branches of the arteries and the veins are called the capillaries,1 from their extreme smallness. The figure (55) shews the arrangement by which the arteries and veins are connected in muscular tissue.

When the aorta leaves the left ventricle, it rises towards the neck, but soon turns downward, forming a curve called the arch; this trunk passes down in front of the spine, and divides into two main branches, which proceed to the legs. From the arch of the aorta are given off the arteries that supply the head and arms with blood: these are the two carotid arteries (c and e, fig. 56), which ascend on either side of the neck to the head, and the two sub-clavian arteries, which pass beneath the clavicles to the arms (the left of these is marked g on fig. 56). The descending aorta while passing down gives off branch arteries to the different parts and organs-the cœliac artery, which subdivides into three, to supply the stomach, liver, and spleen, one to each kidney, and two to the intestines. It at last divides into two branches, which supply the legs.

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Fig. 55.

The veins of the body unite to form two large trunks, the superior or descending, and inferior or ascending vena cava, which meet as they enter the right auricle of the heart, f and b, fig. 54. The descending vena cava is formed by the union of the veins bringing the blood back from the head, the jugular veins, b and f, fig. 56, and those bringing it from the arms, the sub-clavian veins, a and h. The ascending one brings it back from the legs, the organs in the abdomen, and the trunk. The Lesser Circulation will be described under Respiration.

1 From Latin capilla, a hair.

2 From Greek karoo, to stupefy, because a state of torpor is brought on by stopping these arteries.

3 From Latin cælia, the belly.

4 Latin, 'hollow vein.'

5 From Latin jugulum, the hollow at the neck above the collar-bone.

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