664 SPECIAL HISTOLOGY. 



however, go on more slowly in the lacteal gland than in any other secre- 

 tory organs. According to Langer, to whom we are indebted for care- 

 ful researches upon this subject, true terminal vesicles are never met 

 with in childhood, before menstruation is established, but, universally, 

 only undeveloped ducts with clavate ends. On the occurrence of puberty, 

 true gland-vesicles are formed, but at first only at the borders of the 

 gland, until, ultimately, in the first pregnancy, the entire gland is fully 

 developed. After the ^rst lactation, it is true, the gland again dimi- 

 nishes in size, but all its constituent parts remain, and again enlarge in 

 the succeeding occasions of conception, without the addition of any new 

 parts. At the period of involution probably also, if after a pregnancy, 

 too long a time has elapsed without the functions of the gland being 

 called into play it undergoes a retrograde metamorphosis, until finally, 

 in old age, all the gland-vesicles have disappeared, and nothing but the 

 more or less persistent lactiferous ducts, with their epithelium in a state 

 of fatty degeneration, are to be found in the adipose cushion which sup- 

 plies the place of the glandular tissue. 



The milk, the secretion of the mammary glands, consists of a fluid, 

 the milk-plasma, and innumerable spherical, opaque corpuscles, with the 

 brilliant aspect of fat-drops, suspended in it. These corpuscles the 

 milk-globules vary in size, from immeasurable minuteness up to O'OOl- 

 0*002 of a line and more, and most probably do not consist of the fatty 

 part of the milk alone, but have also a delicate investment of casein, 



and it is to them that the whiteness of the 

 rig<274 - g milk is owing. With respect to the for- 



mation of the milk, it is to be remarked 

 that, except at the periods of lactation and 

 pregnancy, the glands contain nothing but 

 a small quantity of a yellowish viscid 

 mucus, with a certain number of epithelial 



cells, and are lined up to their extremities by an epithelium, which in 

 that situation is tessellated, but externally is more cylindrical. With 

 conception, this state of things is altered. The cells of the gland-vesi- 

 cles begin to develop, at first a little, and subsequently more and more 

 fatty matter within them, and to enlarge, so as entirely to fill the ter- 

 minal vesicles. To this is added, before the end of pregnancy, a new 

 formation of fat-containing cells in them, by which the older cells are 

 forced into the lactiferous ducts, which they gradually fill. Thus it 

 happens, that although a true secretion is not at that time set up, still 

 in the latter half of pregnancy a few drops of fluid may be expressed 

 from the gland, which, as is shown by its yellow color, is not milk, but 

 nevertheless contains a certain tfumber of fat-globules from the more or 



FIG. 274. Elementary forms in milk, magnified 350 diameters : a, milk-globules ; 6, colos- 

 trum corpuscles; c, rf, cells with fat-globules from the colostrum, one (cf) with a nucleus. 



