898 THE EXTERNAL SECRETIONS 



lobular ducts are enlarged into sinus-like reservoirs, which may be- 

 come highly distended during the periods of active secretion of this 

 gland. The orifice of the lactiferous duct is invested by areolar tissue 

 and smooth muscle fibers, the latter effecting the erection of the nipple 

 on reflex stimulation. Around its base winds a narrow zone of dark- 

 tinted skin which is beset with very sensitive papillae and contains 

 numerous minute secretory glands of the sebaceous type. Although 

 doubtlessly belonging to the group of the cutaneous glands, it is 

 difficult to classify the mammae either as modified sweat-glands or 

 as sebaceous glands. Their alveolar character as well as the fatty 

 character of their secretion, might prompt us to homologize them with 

 the latter, but since their secretory cells are short columnar in outline 

 and are arranged in a single row, they really present a much closer 

 '•esemblance to the former. Their size, number, and position vary 

 greatly in different mammals. In man, they are placed one upon 

 each side of the anterior aspect of the thorax and are copiously supplied 

 with blood-vessels, lymphatics and nerves. 



The histological character of these glands varies considerably 

 and especially during pregnancy and lactation. When milk is first 

 formed, the epithelium of the alveoli becomes sharply differentiated 

 from that of the ducts. While the lining of the latter remains cub- 

 oidal in shape, that of the former becomes elongated toward the lumen, 

 and shows a prohferation of the nuclei as well as numerous new granules 

 and fat-globules. This state is soon followed by one of active secre- 

 tion. The cells then enlarge still further and project markedly into 

 the lumen of the acini. Fat-droplets now appear in much greater 

 numbers, while the granules which during the early secretory period 

 presented a spherical outline, are now elongated and spiral in shape. -^ 

 A part of the inner segment of each cell then disintegrates, its fragments 

 being forced into the duct. This fully explains the fact that the early 

 secretion invariably embraces many epithelial cells which are only 

 partly transformed, and are, therefore, known as colostrum corpuscles. ^ 

 The places previously occupied by these fragmented cells, are 

 again taken up by new ones formed by karyokinetic division from 

 neighboring cells. In many cases, however, the ruptured inner part 

 of the cell is again closed, whereupon the cytoplasm is slowly reformed. 



During pregnancy, the mammae enlarge and become firm and ten- 

 der to the touch. Their blood-supply then increases enormously, as 

 is evinced particularly by the formation of prominent plexuses of 

 veins. The areola investing the base of the nipple, becomes broader 

 and darker in color and shows very prominent papillae. The nipple 

 itself increases materially in size. Evidently, these macroscopic 

 changes find their origin in a gradual proliferation of the secretory 

 cells and the formation of many new acini. This process of evolution 

 begins soon after conception and does not cease until shortly after 



1 Steinhaus, Archiv fiir Physiol., 1892, 54. 



2 Heidenhain, Hermann's Handb. der Physiol., 1883. 



