600 THE PHYSIOLOGY OF REPRODUCTION 



EFFECT OF PROLONGED LACTATION 



It has been recorded that the continuance of lactation 

 commonly exerts an inhibitory influence on menstruation in 

 women and on heat in animals, though this is very far from being 

 invariable (see p. 74). There can be no doubt that in the case 

 of sows, for example, early weaning is conducive to a more 

 frequent recurrence of oestrus and an increased number of 

 litters (see p. 50). In a similar way long-continued lactation 

 is believed to reduce the fecundity of women, who sometimes 

 refrain from weaning their babies in the belief that by so doing 

 they are less liable to become pregnant again. Moreover, 

 Haddon's observations l upon the Eastern Islanders of the 

 Torres Straits show that with these people also prolonged nursing 

 tends to reduce the size of the families, and that a single lactation 

 may be continued for three years. 



EFFECT OF DRUGS 



There is little evidence as to the effects of drugs upon egg- 

 or sperm-production, but innumerable substances have been 

 recommended as cures for impotence. 2 Cantharides and various 

 other drugs are said to produce sexual excitement, but this result 

 is probably due simply to the increased flow of blood to the 

 generative organs which these substances induce. 3 Wallace says 

 that the practice adopted by some grooms of giving cantha rides 

 to stallions is strongly to be deprecated. Bloch is disposed to 

 recommend the use of phosphorus and strychnine in the treat- 

 ment of impotence in men, but the most favourable results have 

 been obtained by yohimbine, an alkaloid prepared from the bark 

 of a West African tree. Bloch mentions several cases where, in 

 his own experience, treatment by yohimbine has been entirely 

 successful, and there are numerous others on record. Many 



1 Haddon, Reports of the Cambridge Anthropological Expedition to Torres 

 Straits, vol. vi., Cambridge, 1908. 



2 For the distinction between sterility and impotence see below (p. 606). 



3 Bloch, The Sexual Life of our Time, English Translation, London, 

 1908. 



