766 SPECIAL PHYSIOLOGY. 



That of the mare and of the ass, are characterized by the small quantity of 

 butter and casein, and the large quantity of sugar which they contain. The 

 milk of the mare is most remarkable for its enormous proportion of sugar ; this 

 may explain its disposition to undergo the alcoholic fermentation, a fact turned 

 to account by many Tartar tribes, in order to make an intoxicating drink. The 

 milk of the ass, notvyithstanding its difference from human milk, is, perhaps 

 from the difficulty with which its casein is precipitated, and from the delicacy 

 of its curd, the most easily digested by the human infant. Cow's milk, which 

 is the great source of milk for human food and the great substitute for human 

 milk in the case of infants, contains more casein and more butter than human 

 milk, but only about 3-5ths of the quantity of sugar. The specific gravity of 

 good milk is about 1030, that of the cream being 1024, and that of the skimmed 

 milk about 1035 ; by means of a proper lactometer, the quality of milk may be 

 determined approximately by every householder. Considered as infant food, 

 the milk of the cow is too rich in casein and butter, and too poor in sugar ; 

 hence it should be diluted and sweetened, either with common white sugar, 

 or, what is better, with sugar of milk. Half a pint (imperial) of good fresh 

 cow's milk, with half an ounce of milk-sugar and half a pint of water, will 

 form a tolerably near approximation to ordinary human milk, but it is de- 

 ficient in the due proportion of saline, earthy, and ferruginous salts. As an 

 infant advances in age, the sugar and water may be diminished, and farina- 

 ceous food may be added. It is well known that the milk of certain breeds of 

 cattle, is richer than that of others. Authorities differ as to the relative rich- 

 ness of the milk of cows fed in town dairies or country pastures, but country 

 milk must be more natural, and better as food, than the artificially forced 

 production of the town-fed animal, other circumstances being equal. 



The mammary glands, as is well known, differ in arrangement and position 

 in different orders of Mammalia ; sometimes, as in the Carnivora and in the 

 pig, they are divided into numerous portions, disposed along nearly the whole 

 length of the under side of the trunk, each symmetrical mass having its own 

 nipple ; sometimes, as in the Ruminants, and in the genus Equus, they are post- 

 abdominal ; in the Cetacea, they are situated even still further back ; in the 

 Quadrumana, as in Man, they are found in the pectoral or thoracic region only. 

 The microscopic structure resembles that of the human gland, excepting in the 

 lowly organized Monotrematous ornithorynchus, in which the milk glands 

 consist merely of clusters of simple blind follicles, opening in a group on the 

 skin. This simple structure suggests an homologous relation between the 

 mammary and the cutaneous gland. 



Mucous Secretion and Mucus. 



Mucus is the clear, or slightly turbid, colorless, viscid fluid found 

 on mucous membranes. It is partly secreted by the epithelial cells of 

 the compound racemose glands, but in part, also, by those of the sur- 

 face of such membranes. It is a special secretion from the plasma of 

 the blood, and differs from it chemically. Mucus is commonly alka- 

 line, but often speedily becomes acid ; normally, perhaps, it is neutral. 

 It is composed chiefly of water, holding in it from 4 to 6 per cent, of 

 solids. It contains desquamated epithelial cells, mucous corpuscles 

 which closely resemble the white blood corpuscles, and pus corpuscles, 

 and also certain nucleated cells intermediate between the true mucous 

 cells and epithelial cells. Its chief constituent is a special albuminoid 

 substance, called mucin, which, precipitable by alcohol, acetic and 

 other acids, but not by boiling, swells up, rather than dissolves, in 

 water, and is the cause of its natural viscidity ; besides this, it con- 

 tains a small amount of extractives, and salts like those of the blood. 

 It is sometimes very thin, as when secreted from the nose during a 



