THE SKIN AND ITS APPENDAGES 



405 



nothing dcrmite is known. Active secretion does not as a rule take 

 place in all the alveoli of a lobule at the same time. Each lobule 

 thus contains both active and inactive alveoli. The smallest ducts 

 are lined with a low columnar or cuboidal epithelium. This in- 

 creases in height with increase in the diameter of the duct until 

 in the largest ducts the epithelium is of the high columnar type. 



The secretion of the gland is milk. This consists microscopic- 

 ally of a clear fluid or plasma in which are suspended the milk 



Fig. 2S6. — From Section of Mammary Gland of Guinea-pig during Lactation, 

 X500. (Osmic acid.) (Szymonowicz.) a, Basement membrane; 6, lumen oi alveolus; 

 c, tangential section of alveolus; d, fat globules. 



globules. The latter are droplets of fat from 3 to 5," in diameter, 

 each enclosed in a thin albuminous membrane which prevents the 

 droplets from coalescing. Cells, probably leucocytes, containing fat 

 droplets may also be present. In the secretion of the gland during 

 the later months of pregnancy, and also for a few days following the 

 birth of the child, a relatively large number of large fat-containing 

 leucocytes — colostrum corpuscles — are found. 



Blood-vessels. — These enter the gland, branch and ramify in the 

 interlobar and interlobular connective tissue, and finally terminate in 

 capillary networks among the alveoli and ducts. From the capillaries 

 arise veins which accompany the arteries. 



