202 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



The chief carbohydrate in milk is the milk-sugar or lactose. Hammarsten * 

 has succeeded in isolating from the mammary gland a nucleo-proteid contain- 

 ing a reducing group. He designates this substance as nucleo-glyco-proteid. 

 It seems possible that a compound of this character might serve as the parent 

 substance for both the casein and the lactose of the secretion. The mineral 

 constituents are varied and, considered quantitatively, show an interesting rela- 

 tionship to the mineral composition of the body of the suckling (see p. 296). 

 The fact that the inorganic salts of the milk vary so widely in quantitative 

 composition from those of the blood has been used to show that they are not 

 derived from the blood by the simple mechanical processes of filtration or 

 dialysis, but are secreted by the epithelial cells of the gland. Traces of 

 nitrogeneous excreta, such as urea, creatin, and creatinin, are also found in 

 the milk-plasma, together with some lecithin and cholesterin and a small 

 amount of citric acid occurring as citrate of calcium. 



Histological Changes during- Secretion. The simple fact that sub- 

 stances are found in the milk which do not occur in the blood or lymph is 

 sufficient proof that the epithelial cells are actively concerned in the process 

 of secretion. Histological examination of the gland during lactation confirms 



fully this a priori deduction, and enables 

 us to understand the probable origin of 

 some of the important constituents. 2 In 

 the resting gland during the period of 

 gestation, or in certain alveoli during 

 lactation, the alveoli are lined by a single 

 layer of flattened or cuboidal cells, which 

 have only a single nucleus, present a 

 granular appearance, and have few or 

 no fat-globules in them (Fig. 83). 

 When such alveoli enter into the active 

 formation of milk the epithelial cells 

 increase in height, projecting in toward the lumen, the nuclei divide, and as a 



FIG. 83. Section through the middle of two 

 alveoli of the mammary gland of the dog ; con- 

 dition of rest (after Heidenhain). 



A B 



FIG. 84. Mammary gland of dog, showing the formation of the secretion : A, medium condition of 

 growth of the epithelial cells ; B, a later condition (after Heidenhain). 



rule (Steinhaus 3 ) each cell contains two nuclei (Fig. 84). Fat-droplets de- 

 velop in the cytoplasm, especially in the free end of the cell, and according to 



1 Zeitschrift fur physiologische Chemie, 1894, Bd. xix. S. 19. 



2 See Heidenhain : Hermann's Handbuch der Physiologic, 1883, Bd. v. S. 381. 



3 Du Bois-Reymond's Archiv fur Physiologic, 1892, Suppl. Bd., p. 54. 



