SEC. 7. THE MAMMARY GLAND. 



403. Since milk is a secretion, and indeed an excretion, 

 the mammary gland ought not to be classed as a metabolic tis- 

 sue, in the limited meaning we are now attaching to those 

 words. Yet the metabolic phenomena giving rise to the secre- 

 tion 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 considered an extreme development, by an ingrowth 

 of the Malpighian layer of the epidermis, is a compound race- 

 mose gland, constructed after the general plan of such a gland 

 and thus composed of branching ducts ending in secreting al- 

 veoli. 



404. 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 inter- 

 val 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 distin- 

 guish 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, con- 

 ditions 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 



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