16G THE CAT. [chap. vi. 



during life. Now every tissue, as vre liave seen, can ultimately bo 

 reduced to oxygen, hydrogen and carbon, with or without nitrogen, 

 and a few other elementary substances, in greater or less 

 quantity. But let the cat be supplied, however plentifully, with 

 those elements in v.'hatcvcr forms or combinations which are merely 

 chemical, and it would none the less infallibly starve ; for it has no 

 power of building up from inorganic matter, the very complex sub- 

 stances of Avhich its hody is formed. It absolutely requires to be 

 supplied with compovmds which have been ready formed for it by 

 other creatures — it must feed on li^'ing or recently dead animal or 

 vegetable substances. Such inorganic matters as water Avith the 

 salts which may be dissolved within it, do, however, form part of 

 its food. 



The organic substances on wliich it lives, may, like its own 

 tissues, be divided into the nitrogenous and the non-nitrogenous, 

 and there are two sets of each of these kinds. 



One set consists of albtiiii/iwid substances, such as the blood and 

 flesh of the animals on which it may prey. Their connective tissues, 

 cartilage, and bone, are examples of gchifinoid substances, and of 

 such the second set of nitrogenous foods consists. 



Olccifjiiions substances (or fats and oils) and (onylaccous substances 

 — sugar, starch, and gum — are the two sets of non-nitrogenous 

 foods. The last set are mainly of vegetable origin, but there is a 

 sort of starch (glycogen) in the livers of animals, while muscle has 

 a sugar of its own (inosite), and there is a sugar of milk. 



;Much oxygen (as we shall hereafter see) is also received into 

 the body by the lungs in respiration. 



The products of that waste of the tissues which is inseparable 

 from the wear and tear of life (and which necessitates the acquisition 

 of food) are eliminated in various ways by the lungs, kidneys, and 

 skin, and the undigested residue of what has been eaten is cast forth 

 from the alimentary canal itself. 



The process of nutrition effected by food i*?, in the early life of 

 the animal, greatly in excess of waste, but at maturity a prac- 

 tical equilibrium is established, wliich is maintained till, with the 

 advance of age, the balance at first existing becomes reversed. 



§ 3. As has been said, secretion is closely connected with alimen- 

 tation. That it must bo so will clearly appear if we reflect that 

 "secretion" is an action by wliicli certain portions of the body 

 extract from the Wood now sul)stances (the various secretions) 

 which do not exist as such within it, and that " nutrition " (the 

 culmination of the alimentary process) is an action by which 

 certain portions of the body extract from the blood new substances 

 (the various tissue-substances) which do not exist as such within it. 

 Every part of the cat's body which can be nourished nmst necessarily 

 have this power or the cat could not repair the effects of its own 

 • waste when adult, or "grow " during immaturity. In "nutrition," 

 however, the formed product enters into the composition of the 

 body itself, while in secretion this is not (directly at least) the case 



