778 STRUCTURE OF THE MAMMARY GLAND. [BOOK n. 



of the section appear to be closed spaces and at others to com- 

 municate with each other, or with a passage in the centre of the 

 lobule leading to the lumen of the duct. The appearances thus 

 presented, at least by a suckling gland, contrast markedly with 

 those of an ordinary gland, such as the submaxillary, by reason of 

 the large alveoli with their conspicuously wide lumina, often 

 occupied by remains of the milk. 



The ducts consist of an epithelium resting on a connective 

 tissue basis which in the case of the main ducts is strengthened 

 with longitudinally disposed plain muscular fibres continuous with 

 the muscular fibres present in the dermis of the nipple. Over the 

 greater part of their course the ducts are lined with a single layer 

 of columnar epithelial cells, but at the mouths of the main ducts 

 on the nipple these pass into an epidermis of more than one layer 

 of flattened cells. Just before opening on to the nipple each 

 main duct is widened into a flask-shaped enlargement. At the 

 termination of the small ducts in the lobules, the columnar 

 epithelium is said to give place to flattened cells, so that this part 

 of the duct might be called a ductule corresponding to the ductule 

 of a salivary gland. 



510. The appearances presented by the alveoli differ widely 

 according as the gland is one which is being used for suckling or 

 is one in a resting or dormant condition, that is to say before 

 any pregnancy at all has taken place or in the interval between 

 two suckling periods. In the suckling gland each alveolus consists 

 of a basement membrane, presenting the usual characters, lined 

 with a single layer of cells leaving a wide lumen; but the 

 appearances presented by the cells differ from time to time 

 according to circumstances and are not the same in all the alveoli 

 at the same time. We may however distinguish two conditions 

 which, since they seem to correspond to the loaded and discharged 

 conditions of an ordinary gland, we may call the loaded and the 

 discharged phase respectively, conditions intermediate between 

 the two being met with. 



In the discharged phase the alveolus is lined by a layer of low 

 cubical or even flattened cells, so that the relatively large area of 

 the alveolus is almost wholly occupied by the lumen in which 

 some of the constituents of the milk may still be retained. Each 

 cell consists of granular cell-substance in which is placed a 

 rounded or oval nucleus. Sometimes the free edge of the cell is 

 jagged and uneven as if a portion of the free border had been torn 

 away. 



In a fully loaded phase the appearances are very different. 

 The alveolus is now lined with a layer of tall columnar cells 

 projecting unevenly into the lumen, the outline of which is 

 correspondingly irregular and the area of which is much reduced. 

 While the broader base of each cell rests on the basement 

 membrane, the other end, conical or irregular, stretches towards the 



