io8' THE PHYSIOLOGY OF REPRODUCTION 



mammalian types in which they have been studied, yet there is a 

 considerable amount of variation in the severity and duration of 

 the prooestrous phenomena. The extent of the congestion, and the 

 destruction which usually succeeds it, are greatest, as a rule, in the 

 highest Mammals, and comparatively slight in the Eodentia and 

 Ungulata. 



The purpose or meaning of the procestrum, and the- factors which 

 contribute to its occurrence, will be considered as fully as the present 

 knowledge of the subject permits, after the changes which take place 

 in the ovaries have been dealt with, in a future chapter of this work. 

 It may be at once stated, however, that most authorities are now 

 agreed that the menstrual process is in some sense a preparation for 

 the attachment of an ovum to the wall of the uterus, but opinions 

 differ as to the precise nature of the preparation. On the other 

 hand, it is evident that the changes involved in menstruation are not 

 absolutely essential, since there are records of pregnancy occurring 

 in individuals who had never experienced menstruation. Moreover, 

 there is evidence that the prooestrous discharge may be not only of 

 no utility to the organism, but may even become injurious, as in the 

 more severe cases of menstruation among women. 



In view of these facts it may be called in question whether the 

 prooestrous changes in the uterus should not be regarded merely as 

 the result of a wave of disturbance which ushers in the period of 

 desire, and is of the nature of a consequence rather than a purpose. 

 This is in accord with Metchnikoff"ssuggestion,i that the catamenia 

 in women are essentially a " disharmony " of organisation, which has 

 been brought about as the result of modifications acquired recently in 

 the history of the species. If this is so, a similar explanation must 

 be adopted in the case of those animals which experience an especially 

 severe prooestrum. According to such a view as tliis the phenomena 

 of menstruation must be looked upon as belonging to the borderland 

 of pathology. In this connection the large number of leucocytes 

 which attend the menstrual process, some of them clearly phagocytic 

 in function, is not altogether unsuggestive. 



1 MetchnikofF, The Nature of Man, Mitchell's Translation, London, 1903. 



