EXCRETION OF MATTER FROM THE BODY 411 



occurs the mammary glands are largely composed of fibrous 

 tissue, with a large amount of fat, in which run the branch- 

 ing tubules of the glands as small solid blocks of cells 

 completely filling the lumen. 



As pregnancy advances these tubules grow outwards and 

 increase, and the cells begin to divide, some remaining 

 attached to the basement membrane, some coming to lie 

 in the middle of the tubules. These latter undergo a fatty 

 change and break down, and they are shed in the first milk 

 which is secreted, which is known as the colostrum. The 

 cells left upon the basement membrane elaborate the con- 

 stituent of milk, and the presence of fat globules in their 

 protoplasm is very manifest. 



The milk, after being secreted, collects in the ducts of the 

 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 excre- 

 tion 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. 



2. Characters of Milk. The white colour of milk is due 

 to its being a fine emulsion of fat globules floating in a clear 

 plasma. Hence, when the cream is removed, milk becomes 

 less white and less opaque. Its specific gravity is about 

 1030, and in man and herbivorous animals its reaction is 

 alkaline. 



Its composition in the human subject and in the cow is 

 given on p. 318. 



Proteids. The chief proteid is caseinogen, which exists as 

 a soluble calcic compound. It is of the nature of a phospho- 

 proteid, but contains very little phosphorus, and does not 

 yield purin bases. It is not coagulated on boiling, but its 

 combination with lime is split under the action of acids and 

 the casein is precipitated. Under the influence of rennet it 

 splits into whey albumin and paracasein lime, which is in- 

 soluble, and separates out as a curd which generally holds 



