358 HUMAN PHYSIOLOGY 



sulphate, which is more diffusible than glucose, is absorbed 

 much less readily. Again, as shown by Reid, an animal can 

 absorb its own serum under conditions in which filtration 

 into 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. 105, p. 209) 

 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 recep- 

 taculum 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. Proteids. Peptones and the further products of their 

 digestion under the influence of erepsin are formed from 

 proteids in digestion, but the peptones 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 proteids 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 experi- 

 ments 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 proteids for use in the tissues. 



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

 broken down in the lining membrane of the gut, its nitro- 



