482 TEXT-BOOK OF PHYSIOLOGY 



which are discharged separately into the lumen, while others remain for a 

 time associated with the detached portion of the cell (Fig. 217). From 

 these histologic changes it is inferred that the caseinogen and fat are products 

 of the metabolism of the cell protoplasm and not derived directly from 

 the lymph from the blood. The lactose apparently has a similar origin, as 

 appears from the fact that it is not found either in the blood or any other 

 tissue, and that it is formed independently of carbohydrate food. The water, 

 and especially the inorganic salts, are the result of secretor activity rather 



than of diffusion and filtration. This is ren- 

 dered probable from the fact that the propor- 

 tions of the inorganic salts of milk are more 

 closely allied to those of the tissues of the new- 

 born child than to blood. With the passage of 

 the water and salts into the lumen of the acinus 

 the proteids undergo disintegration and solution 

 and the liquid assumes the characteristics of 

 milk. 



The discharge of milk is occasioned by the 

 FIG. 217. SECTION OF THE suction efforts on the part of the child, aided by 

 MAMMARY GLAND OF A CAT atmospheric pressure and the contractions of 

 TAT'ON E Tcav"y E oraive A oU the non-striated muscle-fibers of the lactiferous 



filled with granules and glob- ducts. 



of fat. i, 2, > Epithe- xhe Developmental Changes and the Se- 



cretion of Milk.-Previous to menstruation the 

 glands are quite rudimentary, but with the estab- 

 lishing of this function they increase in size as a result of the growth of con- 

 nective tissue and the deposition of fat. The glandular tissue is, however, 

 more or less inactive, but with impregnation it too undergoes a rapid devel- 

 opment. The alveoli increase in number and size and toward the end of 

 the period of pregnancy begin to secrete the forerunner of milk, the fluid 

 colostrum. From the foregoing it is apparent that there is a close relation- 

 ship between the activities of the ovaries and the uterus and the activities 

 of the mammary glands though the mechanism by which the two groups of 

 activities are mediated is not very clear. Starling and Lane-Claypon have 

 thrown some light on the mechanism by the demonstration that the injection 

 of water extracts of developing feti into the body of a non-pregnant mam- 

 mal a rabbit for a period of from 15 to 17 days was followed by the devel- 

 opment of the mammary glands, similar if not identical with the develop- 

 ment that occurs during an actual pregnancy. From this fact the inference 

 is drawn that during intra-uterine life, the fetus as it develops secretes an 

 agent which, entering the maternal blood, is carried to the mammary glands 

 and by it stimulated to growth. The agent thus produced by the fetus 

 would belong to the class of agents known as hormones. Since those kata- 

 bolic changes in the cells of the glands which eventuate in the formation of 

 milk do not arise during the intra-uterine life of the fetus, but which soon 

 arise after birth, the further assumption is made that this hormone has an 

 inhibitor influence on the gland-cells which prevents the formation of milk; 

 but with its withdrawal after birth secretion at once begins. 



Influence of the Nerve System. Judging from analogy, it is 

 probable that the secretion of milk is regulated by impulses emanating 



