1224 



PHYSIOLOGY 



a fatty pigment known as lutein. The corpus luteum. as the body so 

 formed is called, attains its greatest size about a week after ovulation, and 

 then gradually undergoes regressive changes. If, however, the ovum, 

 which has been discharged, undergoes fertilisation, and pregnancy results, 

 the corpus luteum continues to grow for a considerable time and attains its 

 largest size at about the third month of pregnancy. It does not entirely 

 disappear until after the end of pregnancy. The big corpus luteum found 

 in pregnancy is often spoken of as the * true ' corpus luteum, and is dis- 

 tinguished from the corpus luteum spurium of menstruation or of ovulation 

 without fertilisation. There is, however, no essential difference other than 



FIG. 562. Fully developed corpus luteum of the mouse. (SOBOTTA. 



that of size between these two kinds of corpus luteum. It must not be 

 imagined that all the 70,000 primordial follicles found in the ovary of a 

 new-born child undergo this series of changes ; it is probable that in the 

 human female ovulation occurs, as a rule, once every four weeks during the 

 thirty-five years of sexual life. A vast number of the Graafian follicles, 

 after developing to a certain extent, undergo regressive changes, both during 

 childhood and during adult life. The cellular elements degenerate, leuco- 

 cytes wander into the follicle and attack the degenerating ovum, so that 

 finally the follicle is replaced by connective tissue, without the formation of 

 any corpus luteum. 



MENSTRUATION. Puberty in the girl is marked by the onset of 

 menstruation. Under this term is understood a flow of blood and mucus 

 from the uterus, which recurs every four weeks and lasts each time from four 

 to five days. Before the first menstrual period, other signs of puberty, i.e. 

 of approaching sexual maturity, are usually observed. These include 



