8 



MiJli and Its Froduds. 



part of the albumin is in some way changed dur- 

 ing its passage from the capillaries, and appears in 

 the cavit}^ of the follicle as the casein of the milk. 



Fig. 1. Section tlirougli alveoli of the mammary glaud of the dog in first and 

 second stages of secretion. From Meade Smith, after Heidenhain. 



When milk is being secreted, the lining cells of 

 the follicle are in a state of constant activity. 

 New cells are constantly being formed by budding 

 or fission (the cell elongates, a partition forms 

 across it, and the two halves so divided enlarge to 

 the size of the mother cell, and there are two cells 

 where but one existed before), and older cells are 

 as constantly breaking down. ^Yhile this is going 

 on, the cell contents, consisting mainh' of protoplasm, 

 become changed into a globule of fat, and the 

 globules so formed are either extruded through the 

 cell wall into the cavity of the follicle, or set free 

 by the breaking down and reabsorption of the cell 

 wall. In all probability both processes take place. 

 Small portions of the fat may also be carried over 

 directly from the blood and appear in the milk 

 without change. The milk sugar is probably formed 



