458 THE HUMAN EMBRYO. 



The epithelium lining the uterus is pitted to form the uterine 

 glands. These (Fig. 176) are tubular glands, embedded in large 

 numbers in the connective-tissue layer of the mucous membrane, 

 vertically to the inner surface of the uterus ; they are straight, or 

 slightly convoluted ; theil' blind or outer ends are usually slightly 

 dilated; and they secrete a transparent, glutinous, alkaline fluid. 



Changes in the mucous membrane accompanying menstruation. 



These changes commence with congestion and tumefaction of the 

 mucous membrane lining the entire uterus. This swells up 

 considerably, becoming softer and more vascular than before, 

 and forming ridge-like folds which project into the cavity of the 

 uterus. The connective-tissue cells increase considerably in 

 number, and the uterine glands become longer, wider, and more 

 convoluted. The whole layer of mucous membrane increases 

 in thickness from 1'5 mm. to from 3 to 5 mm. ; while the glands 

 increase in diameter from 0'08 to 0'12 mm. This swollen and 

 hypertrophied mucous membrane forms what is called the 

 menstrual decidua. 



At the menstrual period, the superficial layer of the mucous 

 membrane, about a fourth of the entire thickness, breaks down 

 and is thrown off, usually in detached fragments, but sometimes, 

 in cases of dysmenorrhoea membranacea, as a single piece, 

 forming a complete cast of the interior of the uterus. Fatty 

 degeneration has been noticed in these cast-off cells, but only in 

 the later stages, after the menstrual discharge has actually 

 commenced. 



This disintegration, and casting ofi^, involves the loss of 

 the epithelial lining of the uterine cavity, of the mouths of the 

 uterine glands, and also of about one-fourth of the entire thick- 

 ness of the swollen mucous membrane. It of necessity causes 

 rupture of the blood-vessels of the detached portions, and so occa- 

 sions more or less free haemorrhage ; and the blood so discharged, 

 together with the broken-down mucous membrane of the uterus, 

 and with a certain amount of mucus from the uterine glands, 

 forms the menstrual or catamenial flow. 



The menstrual flow lasts, as a rule, from three to five days, 

 but may be protracted for a week or more. It is accompanied 

 by nervous and other disturbances, which are fully described iii 

 works on obstetrics. 



