THE CESTROUS CYCLE IN THE MAMMALIA 59 



Little is definitely known concerning the length of the gestation 

 period in the various apes and monkeys. Pocock's observations 

 show that in Macamis nemestrinus this period is between six and 

 seven months. Blandford 1 states his belief that about seven months 

 is the usual period for the genus Macacus. Sdnyal, according to 

 information recorded by Solater,^ found that a female of Cercopithecm 

 cynesunis in the Zoological Gardens, Calcutta, carried her young for 

 seven months. 



'Gestation in the Quadrumana is dealt with at some length by 

 Breschet,^ who cites many of the older observations. He shows that 

 th6 question as to the duration of the period is complicated by the 

 fact that monkeys, unlike the majority of Mammals, may copulate at 

 other times than the breeding season, and that they are said 

 occasionally to experience menstruation during pregnancy. 



Man. — As is well known, menstruation recurs normally in the 

 non-pregnant human female at intervals of from twenty-eight to 

 thirty days. The exceptions to this general rule are, however, very 

 numerous, and have often been noticed. Thus the interval may be 

 extended to five weeks, or be abbreviated to two weeks without any 

 derangement to the general health. " In one hundred women, sixty- 

 one [were found] to menstruate every month, twenty-eight every 

 three weeks, ten at uncertain intervals, and one, a healthy woman 

 aged twenty-three years, every fortnight."* The duration and 

 amount of the discharge may also vary considerably both in diffierent 

 women and in the same woman at different times. 



It has been supposed by many from classical times onwards, that 

 menstruation is directly _ associated with lunar periodicity. Thus 

 Aristotle^ says that it occurs during the waning of the moon. In 

 recent times Arrhenius,^ as a result of a statistical examination of 

 12,000 cases, found a periodicity corresponding to the tropical lunar 

 month of 27-32 days (and not to the synodic period of 29'53 

 days, and hence with the moon's phases); that is to say, that 

 although women menstruate at all times, yet more do so at a certain 



' Blandford, The Fauna of British India, vol. i., London, 1888. 



■ ^ Sclater, Mam/mals of South Africa, London. 



' Bresohet, " Recherches anatomiques et physiologiques sur la Gestation 

 dea Quadrumanes," Memoires de I' Acad, des Sciences, vol. xix., 1845. 



* Laycock, loc. cit., and Havelock Ellis, loc. cit. There is in .some cases a 

 tendency for the cycle to become divided up into two cycles, separated by 

 the so-called " Mittelschmerz," or inter-menstrual pain, which is occasionally 

 accompanied by a sanguineous discharge. (Halliday Groom, " Mittelschmerz," 

 Trans. Edin. Obstet. Soc, vol. xxi., 1896.) 



^ Aristotle, Bistoria Animalium (The Works of Aristotle, vol. iv., Thompson's 

 Translation, Oxford, 1910) and De Generatione Animalium (vol. v., Piatt's 

 Translation, Oxford, 1912). , 



•5 Arrhenius, " Die Einwirkung kosmischer Einilusse an physiologischen 

 Verhaltnisse," Skandin. Arch. f. Physiol., vol. viii., 1898. 



