12 ZOOLOGY. 



Men or animals lose in weight if they take no food. 

 The reason for this is that certain substances leave 

 the body either as gases (through the lungs), or as 

 liquids (by the kidneys and sweat-glands), without 

 a corresponding compensation. An animal or human 

 being could not live without taking in fresh^ sub- 

 stances, which, according as they are solid or liquid, 

 are known as food or drink. The different kinds of 

 food and drink, which, with few exceptions (salts, 

 water), are taken from the animal and plant king- 

 doms, cannot, however, as such, replace the gradually 

 diminishing body-substance, for, to begin with, they 

 contaia useless matters, which pass out of the body in 

 the fcBces (dung). And even the nutritious parts of 

 the animal and vegetable substances taken into the 

 stomach, are not always in a form in which they can 

 be used at once. Digestion, which in all the higher 

 animals takes place in a food-tube (gut), serves to 

 reduce them to a suitable condition, at the same time 

 separating the useless matters. The action of several 

 fluids (saliva, gastric juice, bile, etc.) secreted by 

 glands, extracts the useful (nutritious) substances from 

 the food and drink, converting them also into a 

 suitable form. The smaller the pieces into which the 

 food is separated, the better can this purpose be 

 effected. In mammals the teeth serve to break down 

 the food ; in birds and many Invertebrates the same 

 part is played by special secretions of the stomach or 

 intestine provided with hard ridges. 



So long as the nutritious food-stuffs remain in the 

 food-canal, even though in a completely suitable form, 

 they cannot nourish the body. And since waste of 

 the substance of the body everywhere takes place, it 

 is absolutely necessary that the food-stuffs should 

 pass after digestion into a system of organs going to 

 all parts of the body. This system is the circulatory, 

 or vascular system. Food-stuffs enter it from the gut 

 directly or indirectly, reaching it in the latter case 

 through the lymphatic (lacteal) system. 



