THE FOOD OF YOUNG ANIMALS 185 



of carnivores, and in many other situations, and it is these which 

 may be most easily compared with the special glands of reptiles. 

 Mammals have also two kinds of skin glands not found in other 

 animals : the sweat glands which pour out a watery secretion that 

 is partly a waste product and partly helps to regulate the tempera- 

 ture by cooling the over-heated skin, and the sebaceous glands 

 which discharge an oily fluid at the roots of the hairs which auto- 

 matically keeps them soft and flexible. The milk glands of 

 all mammals * are simply masses of much enlarged sebaceous 

 glands, and milk is an oily fluid to be compared with the ordinary 

 sebaceous fluid. By what stages this became turned into milk 

 and used for the feeding of the young it is very difficult to under- 

 stand. 



However they may have come into existence, the mammary 

 glands of all mammals secrete milk in sufficient quantities for the 

 young and of much the same nature in every case. I should have 

 said the mammary glands of all female mammals, because although 

 the structures exist in the males, they are rudimentary, and although 

 in some abnormal instances they may secrete a small quantity of 

 a milky fluid, the suckling of the young is entirely the work of the 

 females. The milk of different mammals differs slightly in colour, 

 taste and odour, but these qualities are of little importance, and 

 just as the flesh of all mammals consists of the same kinds of sub- 

 stances in slightly different proportions, so the only food used to 

 build up the flesh of young mammals consists of the same kinds of 

 chemical materials. 



By far the greater part of the weight of the body of an animal is 

 made up of the water its tissues contain, and so from seventy 

 to ninety per cent, of milk is water. Next in importance come the 

 very complicated nitrogenous substances known as proteins, of 

 which the most familiar example is the white of an egg. In milk 

 the proteins form from one and a half to ten per cent., and are 

 present as casein, the chief component of the curd which is formed 

 when acid is added, and albumin, which becomes solid when milk 

 is boiled. Protein builds up the vital framework of the tissues ; 

 muscle, nerve, every living part of the body is simply living protein. 

 In a fully grown animal protein does little more than repair waste, 

 and under perfectly healthy conditions only so much protein is 

 required in the food as is necessary to repair the wear and tear of 



* The statement, common in text-books, that the mammary glands of mono- 

 tremes are derived from sweat glands and are therefore different from those of all 

 other mammals, is erroneous. 



