64 THE PHYSIOLOGY OF REPRODUCTION 



of menstruation during lactation when calcium salts in quantity are 

 required for the milk). (2) Uterine contractions are, like other 

 involuntary muscle contractions, largely dependent upon the calcium 

 salts circulating in the blood. (.">) Calcium salts have a powerful 

 effect on the vasomotor system, which is greatly affected during 

 menstruation and the menopause. (4) Menstruation does not begin 

 until puberty, when the bony framework has been laid down. 1 



According to Hare 2 menstruation is the result of a progressive 

 accumulation of carbonaceous material in the blood. In animals 

 excessive muscular activity is a substitute 1 . On this view, for which 

 Hare presents general evidence, menstruation is a means of getting 

 rid of an anabolic surplus. 



According to Martin, 3 and certain other writers, the human 

 female often experiences a distinct post-menstrual oestrus, at which 

 sexual desire is greater than at other times ; so that, although 

 conception can occur throughout the inter-menstrual periods, it 

 would seem probable that originally coition was restricted to definite 

 periods of oestrus following menstrual or prooestrous periods in 

 women as in the females of other Mammalia. On this point Heape 

 writes as follows : " This special time for oestrus in the human 

 female has very frequently been denied, and, no doubt, modern 

 civilisation and modern social life do much to check the natural 

 sexual instinct where there is undue strain on the constitution, or 

 to stimulate it at other times where extreme vigour is the result. 

 For these reasons a definite period of oestrus may readily be 

 interfered with, but the instinct is, I am convinced, still marked." 4 



Mall, 5 as a result of the study of thirty-six cases, has come to the 

 conclusion that fertile coition is most likely to occur between the 

 fourth and thirteenth days after the first day of the appearance of 

 the menstrual discharge. This is further evidence of a post-menstrual 

 oestrus. Mall supposes that there is on an average a one-day interval 

 between coition and the fertilisation of the ovum. 



Heape has also given a brief resumed of the evidence that 

 primitive mart resembled the lower Primates in having a definite 



1 Blair Bell, The Principles of (it/niwi/of/i/, London, 1910. 



2 Hare, " The Meaning and Mechanism of Menstruation," Clin!<-<il .l<n-l, 

 1916. 



3 Martin, "The Physiology and Histology of (halation, Menstruation, and 

 Fertilisation " Hirst's ,s>7<'//< <>f <>l>xt<>tr!<-*, vol. i., London, 1888. 



4 Heape, Inf. <-it. According to Slopes (.lA'/v/W An/v, London, 1918) there 

 may lie two periods of increased sexual desire in the human subject during 

 one menstrual cycle, but it is not suggested that the second period is in any 

 way correlated with the " Mittelschrnerz." If Stopes is correct it is difficult to 

 compare the menstrual cycle of man with the o-strous cycle of the lower 

 Mammals. 



6 Mall, "On the Age of Human Knibrvos," .1 ///. .///>. <>f .\//f., vol. xxiii., 

 1918. 



