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is given. Fibrin is also found in the chyle, which has evidently been formed out of the albumen of the chyle. The chyle corpuscles become more and more perfect, and more like true blood corpuscles as the chyle advances towards its destination, the jugular vein; until in the upper portion of the thoracic duct, they often assume the red colour of the latter. The fibrin also increases in quantity in the same proportion. Thus the more the chyle, the result of the digestive process, approaches the point at which it is thrown into the current of circulation, the more it assumes both the microscopic and chemical characteristics of the blood, which it is evidently destined

to renew.

By this same channel, the thoracic duct, is also thrown into the current of circulation the fluid collected by the lymphatic vessels from all parts of the body. These vessels, originating in nearly all parts of the economy by minute radicules, pass through ganglia like the lacteals, and converging, finally terminate in the thoracic duct. They contain, at first, an albuminous fluid like the lacteals, but clear instead of milky, owing to the absence of the fatty particles. Lymph corpuscles and fibrin appear on their emerging from the ganglia, and both the perfection of the first, and the quantity of the latter

increase as they approach their termination. The lymphatics, like the lacteals, evidently perform an important part in the renewal of the blood; but, unlike the lacteals, the renovating elements they contain and elaborate are drawn, not from food, but from the blood itself, and the tissues it creates.

To complete this sketch of the sources from which the blood, the grand element of nutrition, is derived, we must not forget that, as we have seen, part of the chymous fluid is absorbed by the intestinal veins, and is carried directly by the portal vein to the liver, and that there it is elaborated and eliminated in the shape of bile; the greater portion of this bile being again absorbed in the small intestine.

Thus we find the chyle which enters into the circulation by passing from the thoracic duct into the jugular vein is, in reality, new blood. This new blood is elaborated out of the chyme and bile absorbed in the intestines and out of the fluid brought by the lymphatic vessels from the capillary tissues. The quantity of chyle thus contributed in the twenty-four hours is very considerable, amounting, it is supposed, to as much as one-third or onefourth of the weight of the blood contained in the body. This fact proves how rapid and constant must be the

nutritive processes, for the weight of the blood of a healthy adult is supposed to be about twenty-eight pounds.

The new, or lacteo-lymphatic blood, reaches the right heart along with the venous blood, returning from the different parts of the body. It is propelled by the right ventricle into the lungs, and as it passes through the capillary vessels which form the connecting link between the pulmonary artery and vein, it comes in contact with the atmospheric air, introduced by respiration into the pulmonary cells, around which the capillaries in question are spread. It is at this stage of the pulmonary circulation that the chemical changes of respiration take place; changes which modify the air inspired on the one hand and the blood on the other.

The atmospheric air which enters the lung in inspiration is .composed of 79 parts of oxygen and 21 of nitrogen in every hundred. It also contains carbonic acid in the proportion of about four volumes in 10,000, and watery vapour, with occasional traces of ammonia, &c. The air which is emitted from the lungs in expiration, has lost part of its oxygen, and has gained a considerable amount of carbonic acid and watery vapour; the nitrogen remaining the

same.

The amount of carbonic acid thrown off is very considerable, amounting to about 160 grains per hour, or as much as eight ounces of carbon in the twenty-four hours. According to Liebig, the lungs and skin together would emit as much as 10.5 ounces during that time. The experiments which he and other observers have made prove that a considerable proportion of the carbonic acid eliminated by the blood, passes off through the skin. The skin thus becomes a species of respiratory organ, which greatly assists the lungs in purifying the venous blood of the surplus carbonic acid which it contains. Thence the evident necessity of keeping its pores open by frequent ablutions and by friction. In the lower tribes of animals, and especially in the Batrachia, such as frogs, toads, whose skin is thin and moist, life may be long supported by cutaneous respiration alone.

The carbonic acid thus evolved in respiration is to be attributed to the combination of the carbon of the blood with part of the oxygen, which disappears during respiration. This evolution of carbon from the lungs is necessary for the purification of the blood; ; and as it is retarded, indeed arrested to a great extent, by the presence of an undue proportion of carbonic acid in the air we breathe, the perni

cious influence of an atmosphere loaded with this gas can be easily understood. Thus is explained in part the injurious effects of the atmosphere of crowded, badly ventilated, brilliantly lighted rooms.

Part only of the oxygen which disappears during respiration is consumed in the formation of carbonic acid. The number of cubic inches of carbonic acid exhaled in an hour being about 1345 3, the quantity of oxygen absorbed should be 1583.6. The surplus oxygen is generally supposed to be consumed in the more intimate or capillary structures of the body, and to combine on the one hand with the albumen which forms the new tissues, and on the other with the nitrogen of the decomposing nitrogenous tissues, and with the sulphur and phosphorus of the body, to form the sulphates and phosphates that are excreted in the urine.

It has been questioned whether nitrogen is exhaled or absorbed during respiration, and it appears probable that in very small proportions both conditions may be observed, exhalation, however, rather being the rule. Neither the absorption nor the exhalation of nitrogen, however, is sufficiently active or important to deserve taking into consideration in recalling the changes that occur in the nutritive fluid, the blood, during respiration.

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