546 OF SECRETION. 



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 to vary between 

 1030 and 1035 ; others, however, have estimated it much lower. That of the 

 Cow appears to be usually about 1027; that of the cream 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 Human and from Cow's Milk by evaporation, seems, like the 

 specific gravities of the fluids, to be nearly the same, varying from 11 to 12| 

 per cent. In the relative proportion of the ingredients, however, there is a 

 considerable difference ; there being much more butter, and less casein, in 

 Human milk than in that of the Cow ; so that the former would be most nearly 

 represented by the cream of the latter, mingled with about half its proportion 

 of skim milk, and the other half water. The cream of Human Milk has been 

 found by Sir A. Cooper to vary in proportion to the milk, from one-fifth to one- 

 third of the whole volume ; the largest amount being given by well-fed women, 

 free from mental anxiety, and at an early period after parturition. It is curi- 

 ous, however, that although it diminishes to as little as one-seventh, between 

 the fourth and ninth months of lactation, it increases again between the twelfth 

 and the eighteenth ; being sometimes, at the latter period, almost a third of 

 the whole. In conformity with this change in the character of the secretion, 

 it is found that the specific gravity increases during the early weeks, as the 

 oleaginous part gives place to the albuminous and saccharine. The following 

 table exhibits the relative proportion of the different ingredients, in the Milk 

 of various animals, from which it is commonly obtained. It appears from 



Woman. Cow. Goat. Sheep. Ass. Mare. 



Casein ..... 2-95 4-48 4-02 4-50 1-82 1-62 

 Butter ..... 5-20 3-13 3-32 4-20 -Oil traces 



6 ' 34 *" *.00 6-08 8.75 

 Saline Matters . . 0-45 0-60 0-58 0-68 0-34) ftQ fi{ > 

 Water ..... 85-06 87-02 86-80 85-62 91-655 



this, that, whilst the milk of the Cow, Goat, and Sheep do not differ from each 

 other in any very prominent degree, that of the Ass and Mare is a fluid of very 

 dissimilar character, containing a comparatively small proportion of casein and 

 butter, and abounding in sugar. Hence it is, that it is much more disposed 

 to ferment than other milk ; indeed, the sugar of Mare's milk is so abundant, 

 that the Tartars prepare from it a spirituous liquor, to which they give the 

 name of koumiss. It appears from these details that no milk more nearly 

 approaches that of the Human female than that of the Sheep and Goat ; these 



respiration, eliminate part of the oily matter in the form of carbonic acid and water; 

 whilst rest and warmth, by diminishing this drain, favour its passage into the milk. 

 The proportion of Casein, on the other hand, is increased by exercise; which would 

 seem to show that this ingredient is derived from the disintegration of Muscular tissue, 

 and thus adds strength to the Author's view ( 468), that of the matter thus set free, a 

 part only is destined to immediate excretion, and that a part may again be subservient to 

 the operations of Nutrition. Dr. Playfair's experience on this head seems to correspond 

 with the results of common observation in Switzerland, where the cattle pasture in very 

 exposed situations, and are obliged to use a great deal of muscular exertion. The quan- 

 tity of Butter yielded by them is very small; whilst the Cheese is in unusually large 

 proportion. But these very cattle, when stall-fed, give a large quantity of Butter and 

 very little Cheese. 



