GLANDS. 79 



The intimate structure of the lactiferous ducts is inter- 

 esting and important. They are possessed of three distinct 

 coats. The external coat is composed of anastomosing fibres 

 of elastic tissue, with some fibres of inelastic tissue. The 

 middle coat is composed of non-striated muscular fibres, ar- 

 ranged longitudinally and existing throughout the duct, 

 from its opening at the nipple to the secreting culs-de- 

 sac. The internal coat is an amorphous membrane, lined 

 with roundish or elongated cells during the intervals of 

 lactation and even during pregnancy, but deprived of epi- 

 thelium during the period when the lacteal secretion is most 

 active. 1 



The acini of the gland, which are very numerous, are 

 \isible to the naked eye, in the form of small, rounded gran- 

 ules, of a reddish-yellow color. Between these acini there 

 exist a certain quantity of the ordinary white fibrous tissue 

 and quite a number of adipose vesicles. The presence of adi- 

 pose tissiie in considerable quantity in the substance of the 

 glandular structure is peculiar to the mammary glands. 

 Each acinus is made up of from twenty to forty secreting 

 vesicles, or culs-de-sac. These vesicles are irregular in form, 

 often varicose, and sometimes enlarged and imperfectly bifur- 

 cated at their terminal extremities. During lactation their 

 diameter is from -%%-$ to yj-g- of an inch. During pregnancy, 

 and when the gland has just arrived at its full development, 

 the secreting vesicles are formed of a structureless membrane, 

 lined 'with small, nucleated cells of pavement-epithelium. 

 The nuclei are relatively large, ovoid, and embedded in a 

 small amount of amorphous matter, so that they almost touch 

 each other. Sometimes the epithelium is segmented, and 

 sometimes it exists in the form of a continuous nucleated 

 sheet. When the secretion of milk becomes active, the epi- 

 thelium entirely disappears, and reappears as the secretion 

 diminishes. This observation is due to Robin, 8 and has an 



1 SAPPEY, Traite cT anatomic descriptive, Paris, 1857, tome Hi., p. 697. 



8 LITTRE ET ROBIX, Dictionnaire de medecine, Paris, 1865, Article, MameUe. 



