THE FEMALE REPRODUCTIVE ORGANS 547 



The blood-vessels of the mammary gland are specially abundant. 

 They form rich capillary plexuses about the walls of the active "alveoli. 

 Many of the venules coming from these plexuses converge toward the 

 areola, where they form an incomplete venous circle ( circulus venosus of 

 Hatter) from which the efferent veins take their origin. 



The lymphatics of the mammary gland are also numerous. They 

 take origin from broad channels among the alveoli and enter a rich plexus 

 about the interlobular ducts. From here several vessels pass to the lymph 

 nodes of the axilla. 



The nerves of the mammary gland include both spinal (sensory) and 

 sympathetic fibers. The latter are distributed to the vascular walls, to 

 the smooth muscle of the areola and nipple and to the alveolar epithe- 

 lium. The sensory fibers supply the connective tissue of the nipple and 

 areola where they occasionally terminate in tactile and Pacinian cor- 

 puscles. 



Among the secreting alveoli the nerve fibers form an epilemmal 

 plexus beneath the membrana propria, from which fibrils penetrate be- 

 tween the epithelial cells, upon which they end in minute granular 

 varicosities (Arnstein, Anat. Anz., 1895). 



Milk. Milk, secreted by the active mammary gland, consists of an 

 emulsion, in which fat droplets, varying in size from two to twenty 

 microns or more, are suspended in a watery albuminous fluid. Water 

 constitutes about eighty-six per cent, of the secretion; the protein con- 

 stituent (three per cent.), which is largely nuclein, is derived in part 

 from degenerating nuclei. Milk contains also a small amount of sugar 

 (five per cent.) and a trace of salt. Each fat droplet is presumably 

 invested with a thin coat of casein, derived from the cytoplasm of the 

 secreting epithelium. Occasionally leukocytes occur in the milk, but 

 never in large numbers, and like the similar colostrum corpuscles, they 

 are mostly confined to the earlier periods of lactation. 



