REPRODUCTION. 895 



some mammals the passage occupies three to five clays; the time in woman is 

 not known. 



The Uterus. The uterus (Fig. 309, u\ or womb, receives the ovum from 

 the Fallopian tube and passes it on, if unimpregnated, to the vagina ; on the 

 other hand, it receives from the vagina spermatozoa and transmits them to the 

 Fallopian tubes ; it is the seat of the function of menstruation ; when impreg- 

 nation has taken place, it retains and nourishes the growing embryo, and ulti- 

 mately expels the child from the body. Its structure accords with these func- 

 tions. Its thick walls consist largely of plain muscular tissue arranged 

 roughly in the form of three indistinctly marked layers. Of these, the exter- 

 nal and the middle coats are thin ; the fibres of the former are arranged in 

 general longitudinally, those of the latter more circularly and obliquely. 

 The third, most internal layer, which is regarded by some as a greatly hyper- 

 trophied muscular is mucosa?, forms the greater part of the uterine wall. Its 

 fibres are arranged chiefly circularly ; toward the upper part they become trans- 

 verse to the Fallopian tubes, and at the cervix longitudinal fibres lie within 

 the circular ones. The individuality of the muscular layers and uniformity 

 in the course of the fibres is largely interfered with by the numerous blood- 

 vessels of the uterine walls. The uterus is lined by an epithelium composed 

 of columnar ciliated cells, except in the lower half of the cervix, where a stratified 

 non-ciliated epithelium exists. The direction of the ciliary movement in woman 

 is not definitely known ; in other mammals the cilia appear to sweep toward the 

 os uteri. The mucous membrane is thick, and contains very numerous branch- 

 ing tubular glands that are lined by ciliated epithelium and have a tortu- 

 ous course, terminating in the edge of the muscular layer. They secrete a 

 viscid mucous fluid. Between the glands are branched connective-tissue 

 cells that are not unlike the connective-tissue cells of embryonic structures, 

 and wandering cells. Lymph-spaces and blood-capillaries exist. The 

 development of the tissue goes on slowly up to the time of puberty, and, 

 as we shall see, after puberty the mucous membrane is subject to constant 

 change. 



Menstruation. Except during pregnancy the most striking activities of 

 the uterus are associated with that peculiar female function which, from its 

 monthly periodicity, is called menstruation. The most obvious external fact 

 of this phenomenon is the discharge every month of a bloody, mucous fluid 

 through the vagina ; the most obvious internal facts are the bleeding and the 

 degeneration and disappearance of a portion of the mucous membrane of the 

 body of the uterus. This curious process, though having analogies in lower 

 animals, occurs most markedly in the human female, and from before the time 

 of Aristotle to the present, among both primitive and civilized races, its signifi- 

 cance has been the cause of much speculation. The detailed phenomena of 

 menstruation are not as well known as they should be. Experimentation is 

 practically out of the question, and the opportunities of careful post-mortem 

 study of normal healthy uteri at different stages are rare. The main facts are 

 as follows : 



