MAMMARY GLAND- SECRETION OF MILK. 545 



from Albumen, by the peculiar readiness with which it is precipitated by^ 

 feeble organic acids, such as the lactic and acetic ; and by its non-coagulability 

 by heat alone. The Casein of Human milk, however, is much less precipi- 

 table by acids than is that of the Cow ; very commonly resisting the action of 

 the mineral acids, and even that of the acetic ; but being always coagulated 

 by rennet, though the curd is long in collecting. It is remarked by Dr. G. O. 

 Rees,* that the casein of human milk thus bears somewhat the same relation 

 to that of the cow, that the albumen of chyle bears to that of the blood.' The 

 Sugar of milk, which may be obtained by evaporating whey to the consistence 

 of a syrup and then setting it aside to crystallize, contains a large proportion 

 (12 per cent.) of water, so that it may be considered as really a hydrate of 

 sugar ; it is nearly identical in its composition with starch, and may, like it, be 

 converted into true sugar by the action of sulphuric acid ; and when in contact 

 with a. ferment, or decomposing azotized compound, it is extremely prone to be 

 converted into lactic acid, by appropriating to itself the elements of water. It is, 

 in fact, through this process, that the coagulation of the casein is effected, by 

 means of rennet ; for as soon as a very minute quantity of lactic acid is gene- 

 rated, it withdraws from the casein the free alkali which kept it in solution, 

 and the casein is consequently precipitated. The same effect will be produced 

 by incipient decomposition of the casein itself; \vhich will soon occasion lactic 

 acid to be generated from the sugar, in sufficient quantity to give to the milk 

 a distinctly acid reaction. The Saline matter contained in milk, appears to be 

 nearly identical with that of the blood ; with a larger proportion of the phos- 

 phates of lime and magnesia, which amount to 2 or %%, parts in 1000. These 

 phosphates are held in solution chiefly by the Casein ; which seems to have 

 a power of combining with them, even greater than that of Albumen : the 

 presence of a minute proportion of free alkali, also, assists their solution. A 

 small portion of iron in the state of phosphate, together with the chlorides of 

 potassium and sodium, may also be detected in milk.t 



687. It is very interesting to observe that Milk thus contains the three 

 classes of principles which are required for human food, the albuminous, 

 oleaginous, and saccharine : and is the only secreted fluid, in w r hich these 

 all exist in any considerable amount. It is, therefore, the food most perfectly 

 adapted for the young animal; and is the only single article supplied by 

 nature, in which such a combination exists. Our artificial combinations will 

 be suitable to replace it, just in proportion as they imitate its character ; but 

 in none of them can we advantageously dispense with milk, under some form 

 or other. It should be remembered that the saline ingredients of Milk, espe- 

 cially the phosphates of lime, magnesia, and iron, have a very important func- 

 tion in the nutrition of the infant, affording the material for the consolidation 

 of its bones, and for the production of its red blood-corpuscles ; and any fluid 

 substituted for milk, which does not contain these, is deficient in essential con- 

 stituents. It is very justly remarked by Dr. Rees, that, of all the secreted 

 fluids, Milk is most nearly allied in its composition to Blood. 



688. The proportion of the different ingredients in the Milk of different ani- 

 mals, is subject to considerable variation : and this fact is of much practical 

 importance in guiding our selection, when good Human milk cannot be con- 

 veniently obtained for the nourishment of an infant 4 The first point to be 



* Art. MILK, in the Cyclopaedia of Anatomy and Physiology, 

 f Haidlen in Annalen der Chemie und Pharmacie, xlv. p. 263. 



* It appears, from the recent inquiries of Dr. Playfair, that the proportion of two, at 

 least, of the principal ingredients of the Milk, is liable to great variation with the circum- 

 stances of the animal. That of the butter depends in part upon the quantity of oily matter 

 in the food; and in part upon the amount of exercise which the animal takes, and the 

 warmth of the atmosphere in which it is kept. Exercise and cold, by increasing the 



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