MILK OF DIFFERENT MAMMALS. 979 



pregnancy should supervene ; 1 so that the older child is only displaced by 

 the arrival of another infant. And it seems to be chiefly among those who 

 have thus forced the mammary gland into a state of unnaturally persistent 

 activity, that the spontaneous and irrepressible flow continues, after the de- 

 mand for it has ceased. 2 



816. It is very interesting to observe that Milk contains the three classes 

 of principles which are required for human food, the Albuminous, the Oleag- 

 inous, and the Saccharine; and it is the only secreted fluid in which these 

 all exist to 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, es- 

 pecially the phosphates of lime, magnesia, and iron, have a very important 

 function in the nutrition of the infant, affording the material for the consol- 

 idation 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 constituents. It is very justly remarked by Dr. Rees, 3 that, of all 

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



817. The proportion of the different ingredients in the Milk of different 

 animals, is subject to considerable variation ; and this fact is of much prac- 

 tical importance in guiding our selection, when good Human milk cannot 

 be conveniently obtained for the nourishment of an infant. The first point 

 to be inquired into, is the quantity of solid matter contained in each kind ; 

 this may be determined either by evaporation, or by the specific gravity of 

 the fluid. The specific gravity of Human Milk is stated by Dr. Rees (loc. 

 cit.) to vary between 1030 and 1025; others, however, have estimated it 

 much lower. That of the cow appears to be usually about the same ; that 

 of the cream, however, being 1024, and that of the skimmed milk about 

 1035. The variation will in part depend (as in the case of the urine) upon 

 the quantity of fluid ingested, and in part, it is probable, upon the manner 

 in which the milk is drawn ; for it is well known to milkers, that the last 

 milk they obtain is much richer than that with which the udder is distended 

 at the commencement. The quantity of solid matter obtainable from Cow's 

 Milk by evaporation, seems to be usually considerably greater than that 

 yielded by Human Milk ; and there is also a considerable difference in the 

 relative proportions of their ingredients, there being far more casein and less 

 sugar in the milk of the Cow, than in that of the Human female. The fol- 

 lowing table exhibits the average proportions of the different ingredients, in 

 the Milk of various animals from which that fluid is commonly obtained ; 

 these proportions, however, are liable to wide variations : 



1 See Erman's Travels in Siberia (translated by Cooley), vol ii, p. 527; and the 

 Narrative of the United States Expedition, vol. ii, p. 138 



2 Thus Dr. Green has published (New York Journ. of Med. and Surg., Sept. 1844) 

 the case of a lady, set. 47, the mother of four children, who had an abundant supply 

 of milk for twenty-seven years previously. A period of exactly four years and a half 

 occurred between each birth ; and the'children were permitted to take the breast 

 until they were running about at play. At the time when Dr. Green wrote, she had 

 been nine years a widow, and was obliged to have her breasts drawn daily, the secre- 

 tion of milk being so copious. 



3 Cyclopaedia of Anatomy and Physiology, Art. Milk. 



