SECTION III. 



Absorption. 



TYe have already seen that the alimentary tube is developed as an 

 involution of the external integument. Hence, even after having under- 

 gone the most perfect digestion, food-stuff's are practically still outside 

 of the body, and can serve no nutritive purposes until they have entered 

 the lymph- or blood-currents. This entrance of the digestive products 

 into the circulation is termed absorption, and, as just indicated, sub- 

 stances reach the blood-current from the alimentary canal in one of two 

 ways : either directly through the walls of the minute blood-vessels in 

 the mucous membrane of the stomach and intestine, or through the 

 mediation of the lymph-channels. Both of these modes occur in the 

 absorption of digestive products. 



1. Venous Absorption. — The first of these modes of absorption is 

 frequently termed venous absorption, as the entrance of substances into 

 the blood in all probability occurs through the wajls of the minute 

 venous radicals, where there is not a sufficiently high pressure to inter- 

 fere with the passage of fluids from without to within the vessels under 

 the ordinary physical laws of filtration and osmosis. In fact, in the 

 very arrangement of the capillary blood-vessels in the intestine we find 

 the conditions at one time advantageous to the passage of fluid from 

 within to without the blood-vessels, and again directly the reverse hold- 

 ing. Thus, after undergoing subdivision into the minute arterioles, we 

 find the first capillary loops, where, consequently, the pressure is highest, 

 distributed around the deep ends of the intestinal tubules : the con- 

 ditions are there most favorable for the transudation of fluid from the 

 interior of the vessels and the consequent formation of the intestinal 

 secretion. The capillary loops, after leaving the deeper portions of the 

 intestinal mucous membrane, when the blood has been deprived of water, 

 then pass to the most superficial layers, and the conditions are then most 

 favorable to absorption. The blood is more concentrated, from the loss 

 of water in secretion ; it, therefore, from the affinity of the albumen of 

 the blood for water, favors the absorption of large quantities of water, 

 which may, of course, hold nutritive matters in suspension. More 

 than this, the blood-current is now becoming more accelerated, and the 

 internal pressure on "the walls of the vessels reduced, both of which 

 conditions are favorable to absorption; 



(453) 



