474 THE MILK. [SECT. 210. 



bulbous ends ; by the excavation of these, which keeps pace with 

 the above process, there arises a highly ramified duct, whose 

 branches are beset with whole groups of hollow gland-vesicles ; 

 these stages of development, however, follow each other much 

 more slowly in the mamma than in any other organ of secretion. 

 According to Langer, to whom we are indebted for careful observ- 

 ations on the subject, true terminal vesicles never exist before the 

 commencement of puberty, but only imperfectly developed ducts 

 with swollen ends. True gland-vesicles are formed at the com- 

 mencement of puberty, at first, however, only at the border of the 

 gland, until, at last, the whole gland becomes completely developed 

 with the first pregnancy. After the first lactation, the gland, 

 indeed, diminishes again, but remains persistent in all its parts, 

 and, during subsequent conceptions, simply enlarges, without the 

 addition of new parts. At the period of involution — perhaps, also, 

 when too long a period elapses after a pregnancy without the 

 gland being called into action — it enters into a retrogressive me- 

 tamorphosis, until at last, in old age, all the gland-vesicles have 

 disappeared, and only the lacteal ducts of inconstant width, and 

 with their epithelium in a state of fatty degeneration, are to be 

 found imbedded in the cushion of fat which now occupies the place 

 of the true gland-tissue. 



The milk — the secretion of the lacteal glands — consists of a 



fluid portion or plasma, holding in suspension innumerable, round, 



dark, shining corpuscles, varying in size,, from a maximum of 



O'ooi'" to 0002'" and upwards, to others which are too small to 



be measured. These are the milk-globules, which, in all- proba- 



Fig. 192. bility, possess a delicate envelope of 



I , caseine around the particles of fat, of 



r. -, '} .>'' °., which thev consist ; it is to these cor- 



ou °- puscles that the milk owes its white 



- ly colour. With regard to the formation 



of the milk, it is to be remarked, that 

 nuiKaSel 35" a °M&! ^ other periods than those of lactation 

 f^'wJii «fflStaXS& and Pregnancy, the glands contain no- 

 coiostrum, the one d.with a nucleus, thing but a small quantity of yellowish 



viscid mucus, with a certain number of epithelial cells, and are lined 

 to their extremities with a pavement-epithelium, which becomes cy- 

 lindrical externally. On conception, this structure becomes altered. 

 The cells of the gland-vesicles begin to enlarge and to develop fat 

 in their interior (at first but little, afterwards more and more), so 

 that they completely fill the terminal vesicles. To this is super- 



