380 VETERINAKY PHYSIOLOGY 



blood capillaries or lacteals is excluded. In such a case osmosis 

 cannot play a part. Absorption is stopped or diminished when 

 the epithelium is removed, injured, or poisoned with fluoride 

 of sodium, in spite of the fact that this must increase the 

 facilities for osmosis and filtration. 



3. Channels of Absorption. There are two channels of 

 absorption from the alimentary canal (see fig. 107, p. 225) 

 the veins which run together to form the portal vein of the 

 liver, and the lymphatics which run in the mesentery and, after 

 passing through some lymph glands, enter the receptaculum 

 chyli in front of the vertebral column. From this, the great 

 lymph vessel, the thoracic duct, leads up to the junction of the 

 subclavian and innominate veins, and pours its contents into 

 the blood stream. The lymph formed in the liver also passes 

 into the thoracic duct. 



(1) Proteins. Peptones and the further products of their 

 digestion are formed from proteins in digestion, but they seem 

 to undergo a change in the intestinal wall before passing to the 

 tissues, since they are not found in the blood. That in some 

 altered condition they leave the intestine by the blood and 

 not by the lymph is shown by the fact that their absorption 

 is not interfered with by ligature of the thoracic duct. 



During the digestion of proteins the number of leucocytes 

 is enormously increased, sometimes to more than double their 

 previous number, and in all probability it is they which carry 

 the products of digestion from the intestine. According to 

 the observations of Pohl, the leucocytes are derived from the 

 lymph tissue in the intestinal wall, but more recent experiments 

 tend to show that they come from the bone marrow, being 

 probably attracted to the intestine by a positive chemiotaxis. 

 By breaking down in the blood stream they probably set free 

 the proteins for use in the tissues. 



When an excess of proteins is taken in the food, it is 

 broken down in the lining membrane of the gut, and the 

 nitrogen is rapidly excreted in the urine as urea, and thus the 

 entrance of an excess of nitrogen to the tissues is prevented. 

 Its non-nitrogenous part remains available as a source of energy. 



It has been pointed out that gastric juice does not dissolve 

 the nucleo-proteins, but that the pancreatic juice does so. 



