78 THE PHYSIOLOGY OF REPRODUCTION 



by a stratified scaly epithelium, surrounded by erectile tissue. 

 The entrance to the vagina from the exterior is guarded by a 

 thin fold of mucous membrane, which usually becomes per- 

 forated at the first coition. This structure, which is called the 

 hymen, is peculiar to the human race. 1 



The vulva comprises the female generative organs which 

 are visible externally. These include the mons veneris, the 



FIG. 4. Cross-section through cervical canal of human uterus. (From 

 Williams' Obstetrics. Appleton & Co.) 



labia majora and minora, and the clitoris. The last-mentioned 

 structure is a small erectile organ, which is homologous with the 



penis. 2 



THE CYCLE IN MAN 



In giving an account of the changes which take place 

 in the uterus during the menstrual cycle of the human female, 

 it will be convenient to adopt the scheme of classification 

 employed by Milnes Marshall 3 in his work on Vertebrate 



1 The significance or function of the hymen is not certainly known. 

 Metchnikoff (The Nature of Man, English Edition, London, 1903) suggests 

 that it may have been useful in the earlier history of the race, when sexual 

 intercourse probably occurred at an early age, before the reproductive organs 

 were mature. Under such circumstances the hymen, instead of being a 

 barrier, may have facilitated successful coitus. Metchnikoff supposes the 

 aperture to have become gradually dilated by repeated intercourse without 

 being torn, until it admitted of the entrance of the adult male organ. 



2 The outer part of the vagina into which the female urethra opens is 

 often called the vestibule or urogenital sinus. 



3 Milnes Marshall, Vertebrate Embryology, London, 1893. 



