STOMACH AND INTESTINE. 



391 



large proportion. And it is impossible to 

 avoid connecting this maximum of fat in the 

 food, with the large amount of heat that has 

 to be evolved from the body in these cold 

 regions, as well as with the energy of the 

 combustion on which this evolution of tem- 

 perature depends. As a rule, however, but 

 a small quantity of fatty matter can be really 

 digested at a time. Any excess over this 

 amount is merely expelled from the intes- 

 tinal canal with the faeces. 



The digestibility of fat depends chiefly on 

 two circumstances : its mechanical arrange- 

 ment, and its chemical composition. In the 

 adipose tissue, the fatty substances are en- 

 closed in large nucleated cells ; the membranous 

 walls of which consist of a proteinous sub- 

 stance that is rather difficult of solution, and 

 yet requires to be dissolved before its contents 

 can enter the lacteals as chyle. And the 

 three substances (stearine, elain, and marga- 

 rine), which form the greater part of the fat 

 of the Mammalia ordinarily slaughtered for 

 food, possess very different degrees of solu- 

 bility. Hence they are by no means equally 

 easy of digestion; the first resisting its in- 

 fluence much more obstinately than either of 

 the other two. 



An animal which is fed exclusively on fat 

 increases in size during a short period. Its 

 nutrition, however, soon suffers ; and it finally 

 dies, with those appearances of inanition 

 which have already been mentioned as attend- 

 ing all attempts to maintain life by the in- 

 gestion of only one ingredient of the normal 

 food. In the later stages of this process of 

 starvation, its body gives off a repulsive odour, 

 which appears to be due to the evolution of 

 volatile fatty acids from the skin and lungs. 

 The production of these acids may be re- 

 garded as probably due to an imperfect oxida- 

 tion of the hydro-carbons accumulated in the 

 organism. 



The alimentary properties of various other 

 tissues and organs of the animal body may be 

 passed over with a very brief notice. 



The blood itself appears to be a far less 

 valuable article of food than its composition 

 would lead us to suppose: abounding, as it 

 does, in the important protein-compounds 

 of fibrin and albumen. Some authors have 

 supposed, that its digestion is rendered diffi- 

 cult by the dense state of aggregation which 

 its fibrin is so apt to assume m the act of 

 coagulation. But however this may be, still 

 its large albuminous constituent appears to be 

 in a condition such as would eminently fit it 

 for fulfilling the requirements of the organism. 

 We are thus left to remark upon its almost 

 total want of hydrocarbons *: as well as upon 

 the contrast offered by its salts f to those of 

 the muscular substance. 



The brain and nervous centres are so rich 

 in albumen and fat, as to form highly nutri- 

 tious articles of food; especially when they are 

 mixed with other substances, which are more 



* Seep. 386. of this article, 

 t Compare pp. 332. and 389. 



capable of affording the requisite mechanical 

 stimulus to the digestive organs. 



The various glands possess a dietetic 

 value which is derived, partly from their 

 physical structure and arrangement, partly 

 from their chemical composition. Thus, the 

 dense mechanical texture of the liver and 

 kidney must decidedly oppose their usefulness 

 as food : while the bile and urine which they 

 respectively contain, necessarily superadd the 

 properties of these secretions to those of 

 the proteinous parenchyma that forms the 

 bulk of their mass. And conversely, from 

 both mechanical and chemical reasons, the 

 pant'reas is highly digestible and nutritious. 



The hard solid texture of bone, and its large 

 gelatinous and calcareous constituents, to- 

 gether render it of comparatively little use 

 as an article of food. 



The eggs of oviparous animals contain, in 

 addition to the embryo itself, a quantity of 

 nutritive matter, which is destined for its nou- 

 rishment during the process of incubation. 

 Hence, the large eggs of many Birds form an 

 excellent article of food, the dietetic virtues 

 of which resemble, to some extent, those pre- 

 viously attributed to milk. The white of egg 

 contains about 15 per cent, of albumen. The 

 yolk is composed of about 20 per cent, of the 

 same protein compound ; together with about 

 30 per cent, of fatty matter chiefly margarin 

 and elain in a state of subdivision and ad- 

 mixture which eminently adapt it to digestive 

 purposes. 



The general composition of the milk which 

 forms the food of young Mammalia has already 

 been mentioned. It only remains for us to 

 notice its chief varieties, and the products 

 which its artificial preparation adds to the bill 

 of fare of the adult. 



The peculiarities exhibited by the various 

 kinds of milk, are chiefly referrible to the species 

 of the parent animal, the date of its lactation, 

 the nature of its food, and its habits. Thus 

 the milk of the Human female* contains about 

 half the quantity of casein, and two-thirds 

 the butter, of that of the Cow : while that of 

 the Ass, which is still poorer in each of 

 these constituents, greatly surpasses them 

 both in the amount of its saccharine ingre- 

 dients (being as 3 to 2). The rich colo- 

 strum which is yielded in the puerperal 

 state -f- soon gives place to a milk which is 



* From some analyses of this secretion in two 

 persons, L'Heritier concludes that the milk of Bru- 

 nettes contains nearly twice as much casein, as that 

 of Blondes; together with about one-half more butter, 

 and one sixth more sugar. This statement con- 

 firms a belief generally entertained as to the superior 

 qualifications of women of dark complexion as nurses. 

 But, without much more extensive observations, it 

 cannot be accepted as an established fact. If true, it 

 would remarkably complete what we may venture to 

 call the structural and functional homologies of the 

 mammary gland, all of which concur in regarding 

 it as a highly-developed offshoot of the general 

 integuments. 



f The composition of the colostrum seems to in- 

 dicate, that it is partly derived from milk which 

 has been concentrated in the breast subsequently 

 to its secretion, by the re-absorption of a portion of 



c <: 4 



