THE MAMMARY GLAND. 477 



THE MAMMARY GLAND. 



423. Since milk is a secretion, and indeed an excretion, the mammary 

 gland ought not to be classed as a metabolic tissue, in the limited meaning 

 we are now attaching to those words. Yet the metabolic phenomena giving 

 rise to the secretion of milk are so marked and distinct, have so many 

 analogies with the purely metabolic events which take place in adipose 

 tissue, and so strikingly illustrate metabolic events in general, that it will 

 be more convenient to consider the matter here, rather than in any other 

 connection. 



The mammary gland, formed like a sweat gland, of which it may be con- 

 sidered an extreme development, by an ingrowth of the Malpighian layer of 

 the epidermis, is a compound racemose gland, constructed after the general 

 plan of such a gland and thus composed of branching ducts, ending in 

 secreting alveoli. 



424. 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 charac- 

 ters, lined with a single layer of cells leaving a wide lumen ; but the appear- 

 ances presented by the cells differ from time to time according to circum- 

 stances 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 toward the 

 centre of the lumen. Instead of one nucleus, two or even more are now 

 present, one well formed and normal being placed nearer the base, and the 

 others, often showing signs of breaking or degeneration, nearer the free end. 

 Sometimes 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 toward 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 coat- 

 ing of cell substance are frequently present. 



Between such a fully loaded phase and a completely discharged 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 sub- 



