URINARY ORGANS AND MAMMARY GLANDS. 59 



Nerve supply. — The mammary glands have their nerve sup- 

 ply through the first lumbar pair of spinal nerves. The nerve 

 trunk which reaches the gland on each side, divides into three 

 branches, the second and third of which regulate in an inter- 

 esting way the various phenomena of cell activity, blood supply 

 and calibre of the milk ducts. These processes are all directly 

 under control of the nervous system. 



Why last milk drawn is richer in fat than the first : There 

 is a comparatively small amount of milk stored in the cisterns 

 and ducts when milking begins, and then as milking goes on the 

 secreting cells manufacture the fat, casein, sugar, etc., more rap- 

 idly in proportion than the process by which water and matters 

 in solution are taken from the blood and thus the later milk has 

 less water or more solids than the first drawn. 



Mare's milk differs from cow's milk in possessing more fat 

 and sugar, and less protein. Her udder differs from the cow's 

 udder in having but one gland in each half. There are from 

 two to four cisterns at the base of the teat instead of one and 

 each cistern has an excretorv canal to the point of the teat. 



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