20 



YETEEINARY PHYSIOLOGY 



by phosphoric acid linked to cholin — hydroxyethyl- 

 trimethyl-ammonium-hydroxide. 



Lecithin- 



( Fatty acid. 

 Glycerol < Fatty acid. 



( Phosphoric acid. 



Cholin. 



Cholin. 



Cholin has some action upon visceral muscle, and some 

 of the symptoms occurring in degenerative changes of the 

 nervous system may be due to its presence. It is closely 

 allied to muscarin, a very powerful jjoison of vegetable origin. 



In protoplasm, the lipoids are intimately associated 

 with the proteins, and form part of the colloidal complex. 

 By their tendency to adsorb to the surface, and thus to form 

 membranes, they help to differentiate masses of protoplasm 

 from their surroundings and to present a more or less per- 

 meable membrane, through which exchanges between the 

 living matter and its surroundings go on. 



3. Carbohydrates. 



Closely connected with the proteins and fats, and some- 

 times actually built into the molecule of the former, are 

 small quantities of carbohydrates, bodies belonging to the 

 class of starches and sugars (-p. 285). 



C. CRYSTALLOIDS. 



These ma}' either exist in true solution or be combined 

 with the proteins and lipoids in the colloidal state. Those 



