1216 



VESTIGIAL AXD ABNORMAL STRUCTURES 



To understand the position in -which the majority of additional mammary 

 organs are found, it must l)e remembered that in many mammals the mammary 

 glands are developed as thickened ridges of epithelium, which extend from the 

 axilla3 to the groin. .Subsequently the ridges are broken up into separate patches, 

 each of which is capable of development into a distinct gland. 



Embryonic mammary ridges have not yet been found in human eml^ryos, but 

 the majority of additional mammae appear along lines extending from the axillae 

 to the groins, — that is, along the lines where we should expect the mannnary ridges 

 to be formed during embryonic life, — and additional mammary glands or nipples 

 appearing along these lines must be looked upon as of atavistic nature, inasmuch 

 as they are indications of a reversion to a more primitive organisation. By the 

 careful observation of a large number of cases it has been shown that along a line 

 running from the axilla to the groin, on each side, ten mammary glands may be 

 found in the human suliject, and of these the normal gland is the sixth. The 

 complete series does not occur in any one case, and more than half the numl^er of 

 accessory mammary organs which have been observed appear below the normal 

 nipple. 



In addition, however, to atavistic mammary organs appearing along the lines 

 indicated, mammary glands are occasionally found on the thigh, shoulder, neck, 

 and cheek. Such glands are most probably developed in situ by the abnormal modi- 

 fication of cutaneous glands, and in connection with the formation of such abnormal 

 structures it must not be forgotten that all mammary glands are merely modified 

 sebaceous glands. 



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