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MANUAL OF HISTOLOGY. 



a deeper line of staining round the margin ; a nucleolus is not 

 always prominently seen." 



Structure of involuted mamma. Having thus briefly indi- 

 cated the main histological features of a fully evolved gland, 

 we are now prepared to examine the mamma in a condition of 

 advanced involution. By involution, in this sense, is meant 

 the periodical return to inactivity, and not to final retrograde 

 metamorphosis, which culminates in complete senile atrophy. 

 The glandular lobules, then, in the involuted organ are again 

 found to be composed of closely crowded alveoli. But all the 

 lobules appear to have become smaller, and 

 their acinous components are likewise shrunk- 

 en. The basement-membrane of the latter 

 does not appear to be materially altered, but 

 its cellular contents are considerably changed. 

 In place of the beautiful mosaic characteristic 

 of the active gland, there now appears only 

 an aggregation of nucleated corpuscles to the 

 number of five or ten. Creighton describes 

 them as " nothing else than a somewhat ir- 

 regular heap of naked nuclei, with no fringe 

 of protoplasm round them, and in size little, 

 if at all, larger than the nucleus alone of the 

 P^fect epithelium." ' This description, how- 

 ever > applies only to hardened specimens, for 

 in fresh preparations the nuclei, as a rule, 

 show a broader or narrower surrounding zone of protoplasm. 

 As regards the diameter of the involuted acini, it is about one- 

 fourth that of the actively secreting alveoli. 



Owing to the shrinkage in the glandular parenchyma, the 

 blood-vessels and excretory ducts, as already stated, are more 

 prominent in an involuted than in an active gland. 



It is not our purpose here to trace, step by step, the various 

 processes by which a gland passes from the resting state to 

 that condition of complete evolution which is alone compatible 

 with active secretion. For the details of this interesting subject, 

 the reader is referred to the work of Creighton. We may, how- 

 ever, very briefly summarize this author's account of the trans- 

 formations in question. The one essential circumstance char- 

 acterizing the whole change is a process of vacuolation, which 

 Creighton assumes to take place in the secreting cells. "The 



