478 THE METABOLIC PROCESSES OF THE BODY. 



stance occupied with a few or with many oil globules and other granules, 

 and the free border more or less jagged. 



425. The dormant resting mammary gland, that for instance of an 

 animal which has never been pregnant, is much smaller than a suckling 

 gland, owing to the alveoli being both smaller and less numerous. Each 

 alveolus, moreover, is not a cavity lined with a single layer of epithelium, but 

 a solid cylinder or mass of comparatively small, rounded or polyhedral 

 cells. So long as pregnancy does not occur the growth of these is exceed- 

 ingly slow, and the products of such metabolism as goes on in them are 

 carried away by the blood, so that under normal circumstances no secretion 

 takes place. 



When pregnancy occurs rapid growth of the mamma takes place, numer- 

 ous new alveoli being formed by budding, but all for a time remaining solid 

 cylinders of cells. At the approach of the birth of the offspring, the 

 central cells undergo metabolic changes, especially a fatty transformation, 

 and either before or after birth are cast off, leaving a single layer to line the 

 alveoli and to carry on the work of secretion as described above. It is 

 generally supposed that these shed cells supply the so-called " colostrum cor- 

 puscles " characteristic of the first rnilk, of which we shall speak presently. 

 At the end of lactation an absorption of some of the alveoli takes place, 

 and in old age still further absorption goes on with great diminution of the 

 lamina. 



426. The connective tissue, joining together the lobules of various 

 sizes, surrounding the lobules and running in between the projecting blind 

 ends of the alveoli within the lobules, is rich in bloodvessels, which form 

 capillary networks round the alveoli ; it also carries a considerable number 

 of lymphatic vessels which arise in lymph-spaces around the alveoli and else- 

 where. Leucocytes are numerous in the spaces of this connective tissue, 

 and some of them may make their way through the basement membrane 

 and between the secreting cells into the cavities of the alveoli and so appear 

 in the milk. 



427. The nature of milk. Human milk has a specific gravity of from 

 1028 to 1034, and when quite fresh possesses a slightly alkaline reaction. 

 It speedily becomes acid ; and cow's milk, even when quite fresh, is some- 

 times slightly acid, the change of reaction taking place during the stagna- 

 tion of the milk in the mammary ducts. 



The constituents of milk are : 



1. Proteids, viz., casein, and an albumin, agreeing in its general features 

 with ordinary serum-albumin, but which, since it is said to differ somewhat 

 in its solubilities and rotary power from serum-albumin, has been called 

 lactalbumin. The casein, as we have seen ( 193), undergoes through the 

 action of rennin a change whereby insoluble casein (tyrein) makes its ap- 

 pearance and the milk is curdled. Casein may, however, be precipitated in 

 an unchanged form by saturating milk with neutral salts, or by the careful 

 addition of acetic acid to diluted milk, or by first adding to the diluted milk 

 a slight quantity of acetic acid and then passing through it a stream of 

 carbonic acid. In the filtrate the presence of the lactalbumin, which occurs 

 in small and variable quantities, may be shown by coagulation with heat, or 

 by precipitation with potassium ferrocyanide, etc. In the process of curdling 

 the casein, as stated in 193, appears to be not simply changed into tyrein, 

 but to be split up into tyrein and into another proteid, which unlike the 

 lactalbumin is not coagulated by heat and which appears to be allied to 

 peptone or alburnose. This or a similar peptone-like body has also been 

 found in small quantities even in milk which has not curdled ; it has been 

 called lactoprotein. The lactalbumin, though coagulated by heat when 



