CHAP, iv.] METABOLIC PKOCESSES OF THE BODY. 611 



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 re- 

 duced. While the broader base of each cell rests on the base- 

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

 towards the centre of the lumen. Instead of one nucleus, two 

 or even more are now present, one well formed and normal be- 

 ing placed nearer the base, and the others, often shewing signs 

 of breaking up or degeneration, nearer the free end. Some- 

 times constrictions are seen whereby the free peripheral portion 

 of the cell, including one or more of the nuclei, is apparently 

 being separated from the basal portion in which the remaining 

 nucleus is lodged ; and occasionally portions or fragments of 

 cells, nucleated or nucleusless, may be seen lying in the cavity 

 of the alveolus. In the cell-substance, especially towards the 

 free border of the cell, are numerous oil globules of various sizes 

 as well as granules or particles of other nature ; some of the 

 larger oil globules may be seen projecting from the surface as 

 if about to be extruded from the cell ; and in the cavity of the 

 alveolus oil globules with a thinner or thicker coating of cell- 

 substance are frequently present. 



Between such a fully loaded phase, and a completely dis- 

 charged phase, various intermediate conditions may be observed, 

 the cells being of greater or less height, containing one nucleus 

 only or more than one, the cell-substance occupied with few or 

 with many oil globules and other granules, and the free border 

 more or less jagged. 



405. The dormant resting mammary gland, that for in- 

 stance of an animal which has never been pregnant, is much 

 smaller than a suckling gland, owing to the alveoli being both 

 smaller and less numerous. Each alveolus moreover is not a 

 cavity lined with a single layer of epithelium, but a solid cylin- 

 der or mass of comparatively small, rounded or polyhedral cells. 

 So long as pregnancy does not occur the growth of these is 

 exceedingly slow, and the products of such metabolism as goes 

 on in them are carried away by the blood, so that under normal 

 circumstances no secretion takes place. 



When pregnancy occurs rapid growth of the mamma takes 

 place, numerous new alveoli being formed by budding, but all 

 for a time remaining solid cylinders of cells. At the approach 

 of the birth of the offspring, the central cells undergo metabolic 

 changes, especially a fatty transformation, and either before or 

 after birth are cast off, leaving a single layer to line the alveoli 

 and to carry on the work of secretion as described above. It 

 is generally supposed that these shed cells supply the so-called 



