MAMMARY GLAND SECRETION OF MILK. 543 



whilst in others it is an inch, or even two inches, in diameter. In its structure it corre- 

 sponds exactly with that of the female, but is altogether on a smaller scale. It is com- 

 posed of lobules containing follicles, from which ducts arise; and these follicles and 

 ducts are not too minute to be injected, although with difficulty. The evolution of the 

 gland goes on part passu with that of the body, not undergoing an increase at any par- 

 ticular period; it is sometimes of considerable size in old age. A fluid, which is proba- 

 bly mucus, may be pressed from the nipple in many persons; and this in the dead body, 

 with even more facility than in the living. That the essential character of the gland is 

 the same in the male as in the female, is shown by the instances, of which there are now 

 several on record, in which infants have been suckled by men.* Corresponding facts 

 are also recorded of the male of several of the lower animals. 



684. The secretion of Milk consists of Water, holding in solution Sugar, 

 various Saline ingredients, and a peculiar albuminous substance termed Casein; 

 and having Oleaginous globules suspended in it. These globules appear to 

 be surrounded by a thin pellicle (probably originating simply in the contact of 

 oil with albuminous matter, which is known to give rise to a membranous 

 film), that keeps them asunder, so long as the milk remains at rest. The con- 

 stitution of this fluid is made evident by the ordinary processes, to which it is 

 subjected in domestic economy. If it be allowed to stand for some time, 

 exposed to the air, a large part of the oleaginous globules come to the surface, 

 being of less specific gravity than the fluid through which they are diffused. 

 At the same time, there is reason to believe that they undergo a change, 

 which will be presently described. The cream thus formed does not, how- 

 ever, consist of oily particles alone ; but includes a considerable amount of 

 casein, with the sugar and salts df the milk. These are further separated by 

 the continued agitation of the cream ; which, by rupturing the envelops of 

 the oil-globules, separates it into butter, formed by their aggregation, and but- 

 termilk, containing the casein, sugar, &c. A considerable quantity of casein, 

 however, is entangled with the oleaginous matter ; and this has a tendency to 

 decompose, so as to render the butter rancid. It may be separated by melting 

 the butter at the temperature of 180 ; when the casein will fall to the bottom, 

 leaving the butter pure, and much less liable to change. -The milk, after the 

 cream has been removed, still contains the greatest part of its casein and sugar. 

 If it be kept long enough, spontaneous change takes place in its composition ; 

 the sugar is converted into |actic acid, and this coagulates the casein, precipi- 

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

 time, by the addition of an acid ; all the acids, however, which act upon albu- 

 men, do not precipitate casein, as will presently be pointed out in detail ; the 

 most effectual is that contained in the dried stomach of a calf, known as rennet. 

 This exerts so powerful an influence over it, that, according to the experiments 

 of Berzelius, a piece of the membrane coagulated the casein of 1800 times its 

 weight of milk, with the loss of only l-17th part of its own weight : so that 

 the active principle, dissolved from the rennet, must have coagulated the 

 casein of about 30,000 times its weight of milk. The whey left after the curd 



* See the case described by the Bishop of Cork, in the Philosophical Transactions, 

 vol. xli. p. 813: one mentioned by Captain Franklin (Narrative of a Journey to the 

 Polar Sea, p. 157) ; and one which fell under the notice of the celebrated traveler Hum- 

 boldt (Personal Narrative, vol. iii. p. 58). The following is given by Dr. Dunglison 

 (Physiology, vol. ii. p. 437). " Professor Hall, of the University of Maryland, exhibited 

 to his Obstetrical class, in the year 1837, a coloured man, fifty-five years of age, who had 

 large, soft, well-formed mammse, rather more conical than those of the female, and pro- 

 jecting fully seven inches from the chest; with perfect and large nipples. The glandu- 

 lar structure seemed to the touch to be exactly like that of the female. This man had 

 officiated as wet-nurse, for several years, in the family of his mistress ; and he repre- 

 sented that the secretion of milk was induced by applying the children entrusted to his 

 care, to the breasts, during the night. When the milk was no longer required, great 

 difficulty was experienced in arresting the secretion. His genital organs were fully 

 developed." 



