COMPOSITION OF MILK. 471 



and Sheep, the average proportions of Caseine, Butter, and Sugar are 

 nearly the same one with another, each amounting to from 3 to 5 per 

 cent. In the milk of the Ass and Mare, on the other hand, the pro- 

 portion of Caseine is under 2 per cent., the oleaginous constituents are 

 scarcely traceable, whilst the sugar and allied substances rise to nearly 

 9 per cent. In the human female, the saccharine and oleaginous ele- 

 ments are both present in large amount ; whilst the Caseine forms a 

 moderate proportion. The proportion of the saccharine and oleaginous 

 elements appears to be considerably affected by the amount in which 

 these are present in the food ; and by the degree in which the quantity 

 ingested is consumed by the respiratory process. Thus, a low external 

 temperature, and out-door exercise, by increasing the production of 

 carbonic acid from the lungs, occasion the consumption of the oleagi- 

 nous and saccharine matters, which might otherwise pass into the 

 milk, and thus diminish the amount of cream. On the other hand, exer- 

 cise favours the secretion of caseine ; which would seem to show, that 

 this ingredient is derived from the disintegration of the azotized tissues. 

 Thus in Switzerland, the cattle which pasture in exposed situations, and 

 which are obliged to use a great deal of muscular exertion, yield a very 

 small quantity of butter, but an unusually large proportion of cheese ; 

 yet the same cattle, when stall-fed, give a large quantity of butter, and 

 very little cheese. 



835. The Milk first secreted after parturition, known as the Colos- 

 trum, is very different from ordinary milk, and possesses a strongly- 

 purgative action, which is useful in clearing the bowels of the infant, 

 from the various secretions which have accumulated in them at birth, 

 constituting the meconium. The Colostrum, when examined with the 

 Microscope, is found to contain a multitude of large yellow granulated 

 corpuscles ; each of which seems composed of a number of small grains 

 aggregated together. The Colostric character is sometimes retained 

 for some time after birth, and severely affects the health of the infant. 

 This may happen without any peculiarity in the ordinary characters 

 of the secretion, which has all the appearance of healthy milk ; but the 

 Microscope at once detects the difference, by the presence of the colos- 

 tric corpuscles. 



836. The formation of this Secretion is influenced by the Nervous 

 system, to a greater degree, perhaps, than that of any other. The 

 process may go on continuously, to a slight degree, during the whole 

 period of lactation ; but it is only in animals that have special reser- 

 voirs for the purpose, that any accumulation of the fluid can take 

 place. In the Human female, as we have seen, these are so minute as 

 to hold but a trifling quantity of milk ; and the greater part of the 

 secretion is actually formed whilst the child is at the breast. The 

 irritation of the nipple produced by the act of suction, and the mental 

 emotion connected with it, concur to produce an increased flow of 

 blood into the gland, which is known to Nurses as the draught ; and 

 thus the secretion is for the time greatly augmented. The draught 

 may be produced simply by the emotional state of mind, as by the 

 thought of the child when absent ; and the irritation of the nipple may 

 alone occasion it ; but the two influences usually act simultaneously. 



