12 A Manual of Veterinary Physiology. 



melanin : it occurs in the skin, hair, eye, and is the chief 

 constituent of the melanotic tumours so common in the 

 horse. There are several other pigments, but nore so im- 

 portant as the above ; from the lipochromes, or fatty pig- 

 ments, is derived the visual purple of the retina. 



Fatty Nitrogenous Matters. — Though true fatty matters 

 contain no nitrogen, yet in the body there is found, 

 especially in the nervous system and blood, a fat -like 

 substance containing nitrogen known as lecithin, and other 

 fatty nitrogenous matters known as phospho-glyceric acid, 

 cholin or neurin, and cerebrin. Lecithin appears to furnish 

 the majority of these substances, which are found not only 

 in the animal but also in the vegetable body. 



In the Amide Group we have the important substance 

 urea, the chief end product of proteid decomposition in the 

 system. 



The Amido Acids are an important group, formed by the 

 breaking up of proteids in the body, and are intermediate 

 between these and their end products which are excreted 

 with the urine. Glycocin is connected with one of the bile 

 acids, and also with hippuric acid : leucin and tyrosin are 

 formed by the pancreatic ferment from proteids ; creatin 

 and creatinin are probably connected with the formation of 

 urea ; and taurin is found in the bile in conjunction with 

 oholic acid. 



Of the Nitrogenous Acids, some are found in the urine, 

 others in the bile. Sulpho-cyanic acid is said to occur in 

 the saliva, though I have never detected it in the horse. 

 Uric acid only occurs in herbivora when they are out of 

 health ; it is then one of the end products of proteid decom- 

 position. 



Hippuric Acid in the urine of herbivora takes the place of 

 uric acid ; it is not free but combined with potash or soda 

 as hippurates of these bases. It is readily formed after par- 

 taking of benzoic acid. In animals it is said to be derived 

 from cellulose ; if this were its sole origin it is clear that it 

 could not be looked upon as a proteid end product; it is 

 possible, however, that proteid does furnish glycocole and 



