228 THE FEMALE REPRODUCTIVE SYSTEM 



MENSTRUATION. 



In humans, ovulation occurs about every twenty-eight days. 

 The changes undergone by the uterus may be indicated by separat- 

 ing this hmar period into four parts, understanding, of course, that 

 the tissue changes are gradual and grade into each other. Begin- 

 ning to count the days with the onset of menstruation, there are 

 four stages: 



Menstrual First to fourth day. 



Postmenstrual Fifth to tenth day. 



Intermenstrual Eleventh to twentieth day. 



Premenstrual Twenty-first to twenty-eighth day. 



The description already given for a resting uterus applies to the 

 intermenstrual period. At the beginning of the premenstrual period 

 the mucosa gradually increases in thickness until it is almost twice 

 as thick as during the resting condition. (Figs. 142, 143, 144.) The 

 glands become longer, assume a twisted course, and may branch 

 externally. Droplets of glycogen and lipoid material develop in 

 the cytoplasm of the gland cells, and the lumen of each gland 

 becomes distended with secretion. The cells in the connective 

 tissue immediately below the epithelium increase greatly in size. 

 Capillaries in the tunica propria enlarge and are filled with blood 

 to the point that their walls begin to give way and blood issues forth 

 to collect in pools beneath the epitheliinn. The glands also begin 

 to rupture laterally, and their secretion mingles with the escaped 

 blood. The surface epithelium eventually can no longer withstand 

 the distention, due to the continued collection of su})jacent fluids, 

 and begins to give way at numerous points. With the ruptures in 

 the epithelial covering, the underlying fluids pour into the lumen 

 of the uterus, carrying along small masses of epithelial tissue, mouths 

 of glands, and tunica propria. P^Nentually the entire spongy inter- 

 nal lining of the uterine mucosa is sloughed off. Muscular contrac- 

 tions in the uterine wall begin during this period and continue imtil 

 material accumulated in the lumen is expelled. 



The postmenstrual period begins with the closure of the small 

 blood vessels and cessation of loss of blood. E])ithelial cells from 

 the broken ends of the uterine glands migrate out to the surface, 

 where an epithelial lining is reformed. The glands are likewise re- 

 formed and the connective tissue is restored to its former condition, 

 so that by about the tenth day following the beginning of menstrua- 

 tion the uterine mucosa is entire again. The uterus then enters 



