ABSORPTION OF CARBOHYDRATES AND PROTEIDS. 433 



There are thus two channels of absorption leading to the systemic 

 blood stream. One by the capillaries of the villus, passing through the 

 liver ; the other by the lacteals, vid the abdominal lymphatics, to the 

 thoracic duct leading directly to the subclavian vein. 



Absorption of water. It has been shown by Heidenhain l that by 

 far the greater share of the water absorbed from the small intestine 

 is taken up by the capillaries of the villus and not by the lacteals. 

 When large quantities of dilute saline solution (0-3 per cent.) are injected 

 into the small intestine, the rate of lymph flow in the thoracic duct 

 is not markedly increased, unless so much salt solution is injected at one 

 time that the intestine becomes forcibly distended. Zawilsky 2 also 

 found that even during active fat absorption there was no great increase 

 in the amount of lymph flowing from the thoracic duct; the lymph 

 became charged with an exceedingly fine emulsion of fat, but was not 

 largely increased in quantity. 



The considerable absorption of water which commences in the lower 

 end of the ileum, and goes on throughout the entire length of the large 

 intestine, causing the thin chyme of the upper part of the small intestine 

 to become semi-solid, and finally to assume the consistency of the faeces, 

 is also carried out by the agency of capillary blood vessels, so that 

 practically all the water absorbed from the intestine is taken up by the 

 blood stream. The blood is not diluted to a corresponding extent in the 

 process ; in fact, even with the absorption of an excessive amount of 

 water, as in Heidenhain's experiments above quoted, the composition of 

 the blood is little altered. The absorbed water in such a case of 

 excessive absorption passes at first into the lymph which bathes the 

 tissues, to be afterwards brought out and gradually eliminated by the 

 kidneys as the excess in the blood diminishes. 



Absorption of soluble constituents. 8 All those substances which leave 

 the epithelial cell in solution, are also carried away from the lymph 

 spaces of the villus by the capillaries. 4 This has been shown chiefly by 

 observations made during active absorption of these several constituents, 

 on the rate of flow in the thoracic duct, the constitution of the lymph so 

 flowing, and the effects of ligature of the duct or diversion of the stream 

 to the exterior. Direct analyses of the blood of the portal vein, as com- 

 pared with the systemic blood, do not yield trustworthy results ; partly 

 because of the difficulty of making very exact determinations in such a 

 complex fluid as blood serum ; still more because of the very small change 

 in composition which is sufficient to account for the carriage of a great 

 weight of absorbed substance, by reason of the copious flow which takes 

 place through the capillaries, especially when active digestion is in 

 progress. 



If a cannula be inserted into the upper end of the thoracic duct, and 

 the rate of flow of the lymph stream measured, as well as the amount of 

 proteid contained therein, neither of these is found materially to alter, 

 whether the animal (dog) be fasting, or active proteid digestion be going 



1 Arc-h.f. d. ges. Physiol., Bonn, 1888, Supp. Heft, Bd. xliii. S. 53; 1894, Bd. Ivi. 

 S. 579. 



2 Arb. a. d. physiol. Anst. zu Leipzig, 1876, Bd. xi. S. 161. 



3 For the Absorption of fats and fatty acids, see p. 443. 



4 It is often stated that all the dissolved intestinal contents are so absorbed, but if, as 

 is probable, fats are absorbed in soluble form, such a statement is obviously incorrect. Only 

 those constituents which remain soluble, after the action of the absorbing cells, pass into 

 the capillaries. 



VOL. I. 28 



