4492 ANIMAL PHYSIOLOGY. 
which run together more and more till they may wholly re- 
place the original protoplasm. 
The history of the mammary gland is, perhaps, still more 
instructive. In this case, the appearance of the cells during 
lactation and at other periods is entirely different. Fat may 
Fic. 335.—Section of mammary gland (udder and nipple) of cow (after Thanhoffer). Ma, sub- 
stance of gland; N, nipple; 4, acini of gland: m.d, milk-ducts; C, milk-cisterns; f, 
folds in wide milk-ducts ; S, section of sphincter muscle ; s, external skin; n.m.d, narrow 
milk-duct in nipple. 
be seen to arise within these cells and be extruded, perhaps in 
the same way as an Ameba gets rid of the waste of its food. 
So far as the animal is concerned, milk is an excretion in a 
limited sense 
