BEPR OD UCTION. 4 (i 1 



the ovary subject the ovarian nerve-fibres, and through them the spinal cord, 

 to a constant slight stimulation. Through the summation of the stimuli within 

 the cord a reflex dilatation of the vessels in the genital organs is produced. 

 The excessive blood-supply leads in turn to the tumefaction of the uterus, and 

 frequently to the ripening of a Graafian follicle. The bleeding follows, and 

 at the same time or slightly later the rupture of the follicle occurs, provided 

 the latter be sufficiently advanced in growth. The menstrual flow and ovulation 

 are, therefore, two phenomena conditioned usually by the same cause, namely, 

 the menstrual congestion, yet either may occur without the other. Pfliiger's 

 hypothesis accounts clearly for the absence of menstruation after removal of 

 the ovaries. Numerous other theories have been proposed, no one of which 

 can be said to be widely and generally accepted. The present tendency in 

 belief is as follows : Ovulation and menstruation are in great part independent 

 phenomena ; they may or they may not coexist ; the uterine growth is a prep- 

 aration for the future embryo ; the tissue of the decidua menstrualis is the fore- 

 runner of the decidua graviditatis (p. 471) ; if an ovum, whenever it is discharged, 

 be fertilized, it attaches itself to the thickened uterine wall, the tissues become 

 the decidua graviditatis, pregnancy follows, and the decidua is not discharged 

 until the time of parturition ; if, however, fertilization does not take place, 

 there is no attachment, the tissues degenerate and become the decidua men- 

 strualis, and the flow occurs. The suggestion of Jacobi * is not an extreme 

 one: "The menstrual crisis is the physiological homologue of parturition." 

 Its periodicity, which is approximately that of a tropical month (27. 32 days), 

 has been the subject of much hypothesis. In a suggestive paper based upon 

 much careful statistical study Arrhenius 2 ascribes it to the influence of 

 atmospheric electricity, which he finds to undergo a periodic variation of 

 similar length. Regarding the mechanism of menstruation the above hy- 

 pothesis of Pfluger seems not unreasonable, and, moreover, seems to be sup- 

 ported by the experiments of Strassmann, 3 who by pressure artificially pro- 

 duced in the ovary by means of injections into it of salt solution, produced 

 hypersemia and swelling of the uterine mucous membrane, congestion of the 

 external genitals, and mucous and bloody discharges. 



The mystery of menstruation largely ceases when we recognize what is un- 

 doubtedly a fact, that the phenomenon is a highly developed inheritance from 

 our mammalian ancestors, and that, although in the human race under the 

 influence of civilization and social life it has largely lost its technical sexual 

 significance, it is, nevertheless, primarily a reproductive phenomenon derived 

 directly from the lower females. Nature has endowed the latter, in a manner 

 yet unknown, with reproductive periods that are pronounced in the wild state 

 and are coincident with certain of the seasons. A primitive seasonal period 

 may perhaps still be shown in woman by the greater proportion of births that 

 take place during the winter months than at other times of the year : (his sig- 



1 Mary Putnam Jacobi; American Journal of Obstetrics, L885, xviii. 

 '■'Arrhenius: Skandinavisches Archivfiir Physiologie, L898, viii. S. M07. 

 s Strassmann : Archivfiir Oynakologie, 1896, lii. S. 134. 



