i6 2 EVOLUTION AND DISEASE. 



instances dichotomy is responsible for the abnormality, 

 and it is only after careful dissection by an experienced 

 anatomist that it is possible to determine to which of 

 these causes the accessory digit or digits can be ascribed. 



Supernumerary Mammce and Nipples. It is so diffi- 

 cult to distinguish atavistic phenomena from counter- 

 feit manifestations that the great need for care in 

 this direction will be further illustrated by discussing 

 supernumerary mammae and nipples. Up to the present 

 time accessory mammae, nipples, or teats, have been 

 reported on reliable authority in the following situa- 

 tions : the anterior wall of the chest, abdomen, axilla, 

 arm, shoulder, the cheek, buttock, and even in ovarian 

 dermoid cysts. If we accept unreservedly, that super- 

 numerary mammae are always atavistic, we force ourselves 

 into the absurd position of imagining ancestors better 

 supplied with milk-glands than even Diana of Ephesus ; 

 or if we protect our opinion by stating it in this way : 

 When a mamma occurs in an abnormal situation in man, 

 but corresponds to the normal situation of the mamma 

 in some lower animal of our class, it is then atavistic, we 

 save ourselves much trouble. 



In a few cases the guess would be correct, but in most 

 it would be erroneous, and such loose methods of deter- 

 mining the nature of abnormal organs will in the long 

 run lead to much confusion. At the outset we are faced 

 by the question, What is a mamma ? A mamma is a 

 collection of modified sebaceous glands secreting milk, 

 and having close relation to the function of reproduction.. 

 As a rule such glands are furnished with a cutaneous 

 papilla, known as the nipple, or teat, which is either 



