REPRODUCTION 251 



in the strength and rate of the heart beat. These reach their 

 maximum a few days before the menstrual flow, and then undergo 

 a rapid fall, reaching a minimum with the cessation of the 

 flow. The first menstruation is an index of puberty, and occurs 

 in temperate climates at the age of from fourteen to seventeen. 

 The time varies with the climate, food, growth, environment, 

 etc. Occasionally menstruation may be entirely absent in 

 otherwise normal women. The removal of the ovaries puts an 

 end to further menstruation. Its cessation at the age of forty- 

 five to forty-eight marks the menopause or climacteric. 



The meaning of menstruation has been much discussed. In 

 the lower mammalia reproduction is limited to seasonal periods, 

 which are characterized by sexual excitement, congestion and 

 swelling of the external genital organs, and a uterine discharge. 

 During the remainder of the year sexual excitement is absent. 

 These periods of excitement are known as rut or heat. Domes- 

 tication with its regular food supply and care has increased 

 productiveness by rendering the reproductive periods more 

 frequent. This has taken place in like manner in the human, 

 but has progressed farther in that woman during the menstrual 

 flow has largely lost sexual desire. According to Pfliiger, men- 

 struation is a preparation of the uterine surface for the recep- 

 tion of the impregnated egg. An alternative view is that men- 

 struation takes place because the ovum has not been fertilized. 

 The nutrition of the uterus seems to be intimately dependent 

 upon the ovaries through an influence exerted by a hormone 

 formed by the corpus luteum. In monkeys, menstruation 

 takes place after the ovaries have been transplanted, but the 

 flow stops when the transplanted ovaries are removed from the 

 body. It has also been stated that in a young woman suffering 

 from amenorrhea a regular flow appeared after the transplan- 

 tation of an ovary from another woman into her uterus. The 

 influence of the ovary on the formation of the decidua has been 

 shown in the artificial production of deciduomata. If a number 

 of incisions are made into the uterus of a rabbit within a certain 

 interval after the estral period, a structure with the histological 

 characters of the decidua develops at each wound. Ovulation 

 seems indispensable to the production of this phenomenon. 



While in the uterus the growing fetus derives by far the greater 

 part of its nourishment from the mother by means of the placenta. 



