460 AN AMERICAN TEXT-HOOK OF PHYSIOLOGY. 



cated animals and some monkeys, 1 sheds some valuable light upon the phe- 

 nomenon in woman. In animals lower than man, in a wild state, the desire 

 arid power of reproduction are usually limited to seasonal periods. At such 

 times conception is possible, and probably usually takes place. Such periods 

 are known as "rut," "heat," and "oestrus." During the rest of the year 

 sexual activities are in abeyance. Domestication, with its artificial condi- 

 tions of regular food-supply, warmth, and care, has increased productiveness 

 (Darwin) and rendered the reproductive periods more frequent. If impregna- 

 tion be prevented, as is often the case in domesticated animals, the periods of 

 "heat" appear for awhile with great frequency and regularity (monkey, 

 mare, buffalo, zebra, hippopotamus, al intervals of four weeks; cow, three 

 week-; sow, fifteen to eighteen days; sheep, two weeks; bitch, twelve to 

 sixteen weeks). They arc characterized by general nervous excitement, desire 

 and power of conception, congestion and swelling of the external genital 

 organs, and a uterine discharge. The latter is scanty, mucous, and bloody, 

 the amount of blood increasing in ascending the evolutionary scale. The 

 histological processes occurring in the uterus have been studied carefully by 

 Retterer in the dog and by Heape in the monkey. In the latter the proc- 

 esses seem to be nearly identical with those of man. In the dog, growth 

 and congestion of the mucosa occur, and are followed by rupture of the capil- 

 laries, extravasation of blood, and degeneration of the tissues; but it is doubt- 

 fid whether the epithelium is actually shed. It is generally believed that 

 " heat " in the lower mammals is accompanied by ovulation. It is not neces- 

 sarilv so in monkeys. The phenomena of " heat" are thus closely similar to 

 those of human menstruation, the similarity being most marked in the 

 monkeys. In addition to these more hidden phenomena there is present 

 sexual desire, which in the human female is largely absent at such periods, 

 although it may be pronounced just before and just after the actual flow. 



Theory of Menstruation. — The significance of menstruation is in great dis- 

 pute. All modern theories agree in regarding it as associated in some way 

 with the function of ohildbearing. The How was early believed to be a means 

 employed by the body to get rid of a plethora of nutriment. This was 

 followed by the well-known hypothesis, put forward especially by Pfliiger 

 (1865). According to this hypothesis, 2 the menstrual bleeding and the 

 uterine denudation occur for the purpose of providing a fresh uterine surface 

 to which tli'- egg, if impregnated, can readily attach itself, just as, in graft- 

 ing, the gardener provides a wounded surface upon which the young scion is 

 set, or, in uniting two membrane-covered tissues, the surgeon first wounds or 

 freshens their surfaces. This conception of menstruation i> not now commonly 

 accepted. Pfliiger regards the mechanism of the uterine process to be as fol- 

 lows -. The constant growth of the ovarian cells and the consequent swelling of 



1 Cf. A. Wiltshire: British Medical Journal, March, 1883; I-'.. Retterer: Comptea rendm <I<'s 



memoires '/* /<< soeiete • /• biologie, 1892; W. Heape: Philosophical Transactions of the 

 Royal Society, 1894, (B), vol. 185. pt. i. ; and Proceedings of the Royal Society, 1897, lx. p. 202. 



2 E. F. W. Pfliiger: Untersuchungen aus dem physiologischen Laboratorium zu Bonn, 1865. 



