THE CESTROUS CYCLE IN THE MAMMALIA 69 



Birnbaum and Osten ^ state that in the blood of menstruating 

 women coagulation is retarded. This statement is based on 

 experiments made by adding fibrinogen to menstrual blood 

 serum. 



Blair Bell ^ states that in connection with menstruation 

 there is a marked drop in the calcium content of the systemic 

 blood, and that this is most marked just before bleeding begins. 

 This is correlated with an excretion of calcium salts in the 

 menstrual discharge, an examination of which revealed the 

 presence of a considerable quantity of calcium, both free and 

 within the leucocytes (see p. 87). This excretion of calcium 

 during menstruation is regarded by Blair Bell as connected 

 phylogenetically with the process of egg-formation by birds and 

 other lower Vertebrates. 



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

 female often experiences a distinct post-menstrual CBstrus, 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 

 Mammaha. On this point Heape writes as follows : — " This 

 special time for oestrus in the human female has very fre- 

 quently been denied, and, no doubt, modern civihsation and 

 modern social hfe 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." * 



Heape has also given a brief resume of the evidence that 

 primitive Man resembled the lower Primates in having a definite 

 sexual season. The evidence is based largely upon the works of 



' Birnbaum and Osten, " Untersuchungen tiber die Gerinnung des 

 Blutes wahrend der Menstruation," Arch. f. Oyndk.. vol. Ixxx., 1906. 



^ Blair Bell, " Menstruation and its Relation to the Calcium Metabolism." 

 Proc. Hoy. Soc. Med., July 1908. 



' Martin, "The Physiology and Histology of Ovulation, Menstruation, 

 and Fertilisation," Hirst's System of Obstetrics, vol. i., London, 1888. 



* Heape, loc. eit. • 



