PHYSIOLOGICAL ANATOMY OF THE MAMMARY GLANDS. 369 



As regards the mechanism of the formation of the peculiar and characteristic con- 

 stituents of the milk, the mammary glands are to be classed among the organs of secre- 

 tion and not with those of elimination or excretion; for none of these elements preexist 

 in the blood, and they all appear first in the substance of the glands. 



During the period of secretion, the glands receive a much larger supply of blood than 

 at other times. Pregnancy favors the development of the secreting portions of the 

 glands but does not induce secretion. On the other hand, when pregnancy occurs dur- 

 ing lactation, it diminishes, modifies, and it may arrest the secretion of milk. The secre- 

 tion is destined, however, for the nourishment of the child and not for use in the economy 

 of the mother an important point of distinction from all other secretions and its pro- 

 duction presents one or two interesting peculiarities. 



In the first place, the secreting action of the mammary glands is nearly continuous. 

 When the secretion of milk has become fully established, while there may be certain 

 periods when it is formed in greater quantity than at others, there is no absolute inter- 

 mitteucy in its production. 



Again, in all the other glandular organs, the epithelial cells found in their secreting 

 portion seem to be the active agents in the production of the secretions; but, in the mam- 

 mary glands, as we have already noted, the epithelium entirely disappears from the secret- 

 ing culs-de-sac during the period of greatest functional activity of the gland, and nothing 

 is left to perform the work of secretion but the amorphous membrane of the vesicles. 



Conditions which modify the Lacteal Secretion. Very little is known concerning the 

 physiological conditions which modify the secretion of milk. When lactation is fully 

 established, the quantity and quality of the milk secreted become adapted to the require- 

 ments of the child at different periods of its existence. In studying the composition of 

 the milk, therefore, it will be found to vary considerably in the different stages of lacta- 

 tion. It is evident that, as the development of the child advances, a constant increase of 

 nourishment is demanded; and, as a rule, the mother is capable of supplying all the 

 nutritive requirements of the infant for from eight to twenty months. 



During the time when such an amount of nutritive matter is furnished to the child, 

 the quantity of food taken by the mother is sensibly increased ; but observations have 

 shown that the secretion of milk is not much influenced by the nature of the food. It is 

 necessary that the mother should be supplied with good, nutritious articles ; but, as far as 

 solid food is concerned, there seems to be no great difference between a coarse and a deli- 

 cate alimentation, and the milk of females in the lower walks of life, when the general 

 condition is normal, is fully as good as in women who are able to live luxuriously. It 

 is, indeed, a fact generally recognized by physiologists, that the secretion of milk is little 

 influenced by any special diet, provided the alimentation be sufficient and of the quality 

 ordinarily required by the system and that it contain none of the few articles of food 

 which are known to have a special influence upon lactation. So long as the mother is 

 healthy and well-nourished, the milk will take care of itself; and the appetite is the 

 surest guide to the proper variety, quality, and quantity of food. It is very common, 

 however, for females to become quite fat during lactation ; which shows that the fatty 

 elements of the food do not pass exclusively into the milk, but that there is a tendency, 

 at the same time, to a deposition of adipose tissue in the ordinary situations in which it 

 is found. It is a matter of common experience, that certain articles, such as acids and 

 fermentable substances, often disturb the digestive organs of the child without producing 

 any change in the milk, that can be recognized by chemical analysis. The individual 

 differences in women, in this regard, are very great. 



The statements with regard to solid food do not apply to liquids. During lactation, 



there is always an increased demand for water and for liquids generally ; and, if these be 



not supplied in sufficient quantity, the secretion of milk is diminished, and its quality is 



almost always impaired. It is a curious fact, which has been fuUy established by obser- 



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