764 SPECIAL PHYSIOLOGY. 



of lactation. It supplies materials to the infant for the re-formation 

 of albuminoid compounds. The oily matters derived from the animal 

 fats, or from sugar, and the sugar itself, the source of which is yet 

 unknown, are not only directly adapted for respiratory purposes, and 

 the production of animal motion and heat in the infant, but also, as 

 well as the casein, are doubtless employed, in part, in various impor- 

 tant nutritive and secretive processes. The salts of the milk are also 

 those which are essential to the formation of blood, salts of potassium 

 and iron for the corpuscles, and salts of soda, calcium, and magnesium 

 for the liquor sanguinis. The phosphates of lime and magnesia are 

 absolutely necessary for the growth of the young skeleton. Of all the 

 secretions milk is especially nutritive, and most closely resembles blood 

 in composition, its chief distinction from that fluid being the large 

 quantity of sugar in it. Milk alone contains albuminoid, fatty, and 

 saccharine elements combined. Its secretion is not essential to the 

 system, being, ordinarily, limited to one sex, and, in that, being tem- 

 porary or periodic. From its general resemblance to blood, the arrest 

 of its secretion is not so pernicious as the non-secretion of the bile ; 

 nevertheless, its retention within the gland, besides causing obstruction 

 of the ducts, inflammation of the organ, and its consequences, may 

 likewise, perhaps, prove injurious through reabsorption, especially of 

 the crystalloids, lactin, and lactic acid. It has been supposed that its 

 constituents may be retained in the blood, and so account for the con- 

 stitutional disturbance which follows the sudden arrest of the secre- 

 tion ; but the proper constituents of the milk are probably not, nor- 

 mally, pre-existent in the blood, though they may, like those of other 

 secretions, be reabsorbed. Cases of vicarious secretion of milk, which 

 are very numerous, may depend on reabsorption, and distant exuda- 

 tion, of the absorbed constituents. A case has been recorded of the 

 expectoration of milk following sudden arrest of the secretion. In- 

 stances of so-called vicarious secretion of milk from the inguinal region, 

 have been supposed to be due to the presence of supernumerary mam- 

 mary glands in that position. This would correspond with the normal 

 situation of these glands in some of the lower animals, and rudiments 

 of more than one pair of mammary glands are sometimes met with in 

 the human body. 



The quantity and quality of the human milk vary according to 

 many circumstances. Thus, it is not only most abundant, but most 

 nutritious, in nursing women, from the age of 15 to 20, whilst it is 

 least so, in those from 35 to 40. The constitution also greatly in- 

 fluences the character and nutritive qualities of the milk ; hence the 

 necessity for the selection of healthy wet-nurses. In the early periods 

 of lactation, the casein is at first relatively small in quantity, but 

 afterwards becomes increased and attains a determinate ratio; whilst 

 the sugar is at first abundant, but afterwards reduced in proportion. 

 From experiments on the cow, the fatty matters seem to vary most, 

 chiefly according to the nature and quantity of the food, the tempera- 

 ture in which the animal lives, and the amount of exercise it is per- 

 mitted to take. Thus, warmth and rest, increase the quantity of oily 

 matter, whilst cold and exercise diminish it. (Playfair.) Exercise, 



