PHYSIOLOGY OF DIGESTION. 51 



Proteids. — These arc not affected Ijy the saHva, but are 

 changed to soluble peptone by the gastric and pancreatic juices, 

 then absorbed into the blood current and ultimately distributed 

 to tissue cells of the body to replace worn out parts or to build 

 up new parts in growing animals. 



It has been held that proteid matters were especially neces- 

 sary to repair tissue worn out by mechanical work ; but there 

 is good reason to think that this has been overestimated in the 

 past, and that the carbohydrates have much to do with the ulti- 

 mate source of physical power. 



It is probable that surplus proteids are extensively converted 

 into fat, and stored as such in the tissues of the fattening animal, 

 or excreted as butter fat by the milch cow or nursing mare. The 

 remaining proteids are finally oxidized, and excreted from the 

 body as urea and water. The urea is mainly excreted by the 

 kidneys and the water by all the excretory organs. 



Fats, or hydrocarbons. — These are digested mainly by the 

 pancreatic fluid. They are not acted on by either saliva or gas- 

 tric juice and but slightly by the bile. The pancreatic fluid emul- 

 sifies a portion which reaches the lacteals in this condition. The 

 remainder is digested by separating the glycerine from the fatty 

 acid (fat consists of a fatty acid plus glycerine). Glycerine is 

 readily soluble and easily absorbed. The fatty acid unites with 

 alkaline material from the bile and pancreas to form soluble soap 

 and thus the remainder of the fat is absorbed. Then comes ab- 

 sorption of this ennilsified and digested fat, distribution in the 

 blood current, and finally assimilation, i. e., building into cell 

 tissues or oxidation. It may be deposited either into and become 

 a part of the cell bodies or it may be deposited between the cells 

 and thus become a stored reserve. Under some conditions this 

 storing up of fatty matter may constitute a diseased condition, as 

 when the heart muscle undergoes fatty infiltration or degenera- 

 tion. The final changes which fats undergo are oxidation and 

 then excretion as CO2 and water. 



Suggestions. — Mature animals need a constant supply of 

 food to supply heat, and energy for physical work and in the 

 case of fattening animals for conversion into storage fat. Young 

 and growing animals must have food to supply heat and energy 

 for physical work, and in addition food from which new cells 

 mav be built. 



