470 OP GENERATION AND DEVELOPMENT. 



long enough, a spontaneous change takes place in its composition ; an 

 incipient change in the caseine being the cause of the conversion of the 

 sugar into lactic acid ; and this coagulating the caseine, by precipitating 

 it in small flakes. The same precipitation may be accomplished at any 

 time by the agency of various acids, especially the acetic, which does 

 not act upon Albumen ; but Caseine cannot be coagulated, like albu- 

 men, by the influence of heat alone. The most complete coagulation 

 of Caseine is effected by the agency of the dried stomach of the calf, 

 known as rennet; which exerts so powerful an influence as to coagulate 

 the caseine of 1,800 times its weight of milk. It is thus that, as in 

 the making of cheese, the curd is separated from the whey ; the former 

 consisting chiefly of the caseine ; whilst the latter contains a large 

 proportion of the saline and saccharine matter, which entered into the 

 original composition of the milk. These may be readily separated by 

 evaporation. 



833. The principal characters of Caseine have been already stated 

 ( 172). The Oleaginous matter consists, like the fats in general, of 

 the two substances, elaine and stearine ; but it also contains another 

 substance peculiar to it, which is termed butyrine. This last (to which 

 the characteristic smell and taste of butter are due) is conveyed by 

 saponification into three volatile acids, of strong animal odour, to which 

 the names of butyric, capric, and caproic acids have been given. This 

 change may be effected, at any period, by treating the butyrine with 

 alkalies ; but it may also take place by spontaneous decomposition, which 

 is favoured by time and moderate warmth. The Sugar of Milk is pecu- 

 liar as containing nearly 12 per cent, of water: so that it may be con- 

 sidered as really a hydrate of sugar. It is nearly identical in its com- 

 position with starch ; and may, like it, be converted into true sugar by 

 the agency of sulphuric acid. But it is chiefly remarkable for its 

 proneness to conversion into lactic acid, under the influence of a fer- 

 ment or decomposing azotized substance. The Saline matter contained 

 in Milk appears to be nearly identical with that of the blood ; with a 

 larger proportion, however, of the phosphates of lime and magnesia, 

 which amount to 2 or 2J parts in 1000. These are held in solution 

 chiefly by the Caseine, which has a remarkable power of combining 

 with them. 



834. Thus ordinary Milk contains the three classes of organic prin- 

 ciples, which form the chief part of the food of animals, namely, the 

 albuminous, the saccharine, and the oleaginous ; together with the mi- 

 neral elements, which >are required for the development and consolida- 

 tion of the fabric of the infant. It would appear, however, that the 

 combination of all these is not necessary ; but rather has reference to 

 the composition of the food on which the animal is destined to be after- 

 wards supported. Thus it has been lately shown that, in the Carnivora, 

 the milk contains no sugar ; which principle is altogether wanting in 

 the food of the adult. Amongst the different species of Herbivorous 

 animals, the proportion of the several ingredients varies considerably ; 

 and it is also liable to considerable variation in accordance with the 

 nature of the food, the amount of exercise taken by the animal that 

 affords it, and other circumstances. Thus in the milk of the Cow, Goat, 



