898 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



which is a gradual process extending over several months, usually marks the 

 climacteric (menopause) or end of the sexual life, and occurs usually at the 

 age of forty-four to forty-seven. Exceptionally the flow may cease early in 

 life or extend to extreme old age. 



Comparative Physiology of Menstruation. The comparative physiology of 

 menstruation, although it has been studied only incompletely in a few domesti- 

 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 

 and 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 with great frequency and regularity (monkey, mare, buffalo, 

 zebra, hippopotamus, four weeks ; cow, three weeks ; sow, fifteen to eighteen 

 days ; sheep, two weeks ; bitch, nine to ten days.) They are 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 processes seem to be nearly identical with those of man. In 

 the dog, growth and congestion of the mucosa occur, and are followed by rup- 

 ture of the capillaries, extravasation of blood, and degeneration of the tissues, 

 but it is doubtful whether the epithelium is actually shed. It is generally 

 believed that " heat " in the lower mammals is accompanied by ovulation. It 

 is not necessarily 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. 



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 childbearing. The flow was early believed to be a means 

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

 lowed by the well-known hypothesis, put forward especially by Pfliiger (1865), 

 and even now widely accepted. According to this hypothesis, 2 the menstrual 

 bleeding and the uterine denudation occur for the purpose of providing a fresh 

 uterine surface to which the egg, if impregnated, can readily attach itself, just 

 as, in grafting, the gardener provides a wounded surface upon which the young 



1 Cf. A. Wiltshire: British Medical Journal, March, 1883; E. Retterer : Comptes rendus des 

 seances et memoires de la Societe de biologic, 1892; W. Heape : Philosophical Transactions of the 

 Royal Society (B), vol. 185, pt. i., 1894. 



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



