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DIAGRAM SHOWING THE COURSE OF THE MAIN TRUNKS OF THE ABSORBENT SYSTEM.

The lymphatics of lower extremities (D) meet the lacteals of intestines (LAC) at the

receptaculum chyli (RC), where the thoracic duct beg The superficial vessels are shown in the diagram on the right arm and leg (S), and the deeper ones on the arm to the left (D). The glands are here and there shown in groups. The small right duct opens into the veins on the right side. The thoracic duct opens into the union of the great veins of the left side of the neck (T).-From Yeo's Text-Book oj Physiology.

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These finally unite to form the portal vein, a short trunk about three inches in length. The portal vein enters the liver at the transverse fissure, after which it forms a fine capillary plexus ramifying throughout the substance of the liver; from this plexus the hepatic veins take their origin, which finally empty the blood into the vena cava inferior. (See Fig. 10.)

Absorption of Food.-Physiological experiments have demonstrated that the agents concerned in the absorption of new materials from the ali

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Diagram of the portal vein (pv) arising in the alimentary tract and spleen (s), and car

rying the blood from these organs to the liver.-From Veo's Text-Book of Physiology.

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mentary canal are : ist. The blood-vessels of the entire canal, but more particularly those uniting to form the portal vein. 2d. The lymphatics coming from the small intestine, which converge to empty into the thoracic duct. As a result of the action of the digestive fluids upon the different classes of food stuffs, albumins, sugars, starches, and fats, there are formed peptones, glucose, and fatty emulsion, which differ from the former in being

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highly diffusible, a condition essential to their absorption. In order that these substances may get into the blood, they must pass through the layer of cylindrical epithelial cells and the underlying basement membrane and into the lymph spaces of the villi and sub-mucous tissue. The mechanism by which the cells effect this passage of the food is but imperfectly understood. Osmosis and filtration are conditions, however, made use of by the cells in the absorptive process.

The products of digestion find their way into the general circulation by two routes :

1. The water, peptones, glucose, and soluble salts, after passing into the lymph spaces of the villi, pass through the wall of the capillary blood vessel ; entering the blood, they are carried to the liver by the vessels uniting to form the portal vein; emerging from the liver, they are emptied into the inferior vena cava by the hepatic vein.

2. The emulsified fat enters the lymph capillary in the interior of the villus; by the contraction of the layer of muscular fibers surrounding it its contents are forced onward into the lymphatic vessel or lacteal; thence into the thoracic duct, and finally into the circulation at the junction of the internal jugular and subclavian veins on the left side.

Absorption of Lymph.—Similar to the absorption of food from the alimentary canal, is the absorption of lymph from the lymph spaces of the organs and tissues. During the passage of the blood through the capillary blood-vessels, a portion of the liquor sanguinis or plasma op.lymph, passes through the capillary wall out into the lymph spaces. The tissue cells are thus bathed with this new material; from it those substances are selected which are necessary for their growth, repair, and all purposes of nutrition. An excess of nutritive material, far beyond the needs of the tissues, transudes from the blood vessels, and it is this excess which is absorbed by the lymphatics, and returned to the blood by the thoracic duct. It is quite probable also that a portion of this transudate is reabsorbed by the blood-vessels.

Properties and Composition of Lymph and Chyle.—Lymph as found in the lymphatic vessels of animals, is a clear, colorless, or opalescent fluid, having an alkaline reaction, a saline taste, and a specific gravity of about 1.040. It holds in suspension a number of corpuscles, resembling in their general appearance the white corpuscles of the blood. Their number has been estimated at 8200 per cubic millimeter, though the number varies in different portions of the lymphatic system. As the lymph flows through the lymphatic gland, it receives a large addition of corpuscles. Lymph corpuscles are granular in structure, and measure too of an inch in

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