234 NORMAL HISTOLOGY. 



tions become more pronounced and result in the formation of the 

 decidua. When incidental merely to the phenomena of menstrua- 

 tion, the flow of blood following the rupture of the over-distended 

 capillaries is accompanied by a degeneration of the inner portions 

 of the uterine mucous membrane, including the glands, which are 

 cast off as far as the deepest layers next the muscular tissue ; from 

 this external zone of remaining unimpaired tissue the regeneration 

 of the mucous membrane proceeds. 



The muscular coat of the uterus consists of bundles of involun- 

 tary muscle separated by bands of connective tissue and surrounding 

 numerous vascular, especially venous, channels. While more or less 

 irregularly arranged, the muscular tissue is disposed in three general 

 strata, an inner, a middle, and an outer layer. The inner layer, 

 upon which directly rests the mucosa, is often regarded as belonging 

 to the mucous membrane, being in fact the hypertrophied muscu- 

 laris mucosae ; it is composed principally of irregular longitudinal 

 or oblique bundles, and contributes about 1.5 mm. of the entire 

 muscular tunic. The middle layer is the most robust, forming the 

 greater part of the muscular wall, consisting chiefly of bundles having 

 a general circular disposition. This layer is also distinguished by the 

 numerous large venous channels enclosed between its bundles; 

 hence the name, stratum vasculare. The outer layer of muscle, 

 about .1 mm. in thickness, is made up partly of circular and partly 

 of longitudinal bundles, the latter predominating and being closely 

 related to the overlying serous coat. Many bundles of this outer 

 layer pass obliquely across the fundus and into the broad ligament ; 

 some of these enter the round ligaments and accompany the areolar 

 tissue and the blood-vessels composing these structures towards the 

 groin, while others extend along the oviducts ; strong muscular bands 

 also run from the uterus into the ovarian ligaments. The muscu- 

 lature of the cervix is characterized by greater regularity in its 

 arrangement, a distinct inner longitudinal, a middle circular, and an 

 outer longitudinal layer being present. During pregnancy the 

 muscular tissue of the uterus becomes enormously increased, the 

 augmentation depending not only upon the excessive size of the 

 already existing individual muscle-cells, but also upon the appear- 

 ance of additional new muscle-cells. 



The serous coat of the uterus is composed of the usual constitu- 

 ents of the peritoneum, the fibro-elastic stroma being covered by the 

 outer sheet of endothelium. 



The blood-vessels supplying the uterus are very numerous. The 

 arteries, branches from the ovarian and the uterine, pass beneath the 

 serous coat into the muscular tunic, where many twigs are given off 

 for distribution to the tissue of this layer ; the capillary vessels pass 



