EXCEETION OF MATTEE FROM THE BODY 431 



glands and in the sinus below the nipple, and is expelled from 

 these by the contraction of the muscular fibres in their walls, 

 and by the suction of the young animal. The excretion of milk 

 from the ducts is directly under the control of the nervous 

 system, but the evidence as to the way in which the central 

 nervous system influences the secretion of milk is by no means 

 satisfactory. Clinical experience shows that it is profoundly 

 modified by nervous changes, but so far stimulation of the 

 nerves to the glands has not yielded definite results. 



The chemistry of milk is considered upon p. 332, and the 

 way in which fats and proteins are formed is dealt with on 

 p. 385. The production of the disaccharid, lactose, and of the 

 phospho-protein, caseinogen, implies a synthetic process in the 

 protoplasm of the cells of the mammary glands. 



The comparative composition of the milk of the cow, mare, 

 and sheep is given below : 



