CHAP. XX.] FEMALE GENERATIVE ORGANS. 239 



is comparatively small. The cavity is triangular in shape 

 and has three openings, one at each upper angle, communicating 

 with the Fallopian tubes, and one, the os internum, or internal 

 mouth, opening into the cavity of the cervix below. The 

 cavity of the cervix, which is, of course, continuous with the 

 cavity in the body, is constricted above, where it opens into 

 the body by means of the os internum, and below, where it 

 opens exteriorly by means of the os extern um, 1 or external 

 mouth. Between these two openings, the cavity of the cervix 

 is somewhat enlarged. 



The walls of the uterus consist mainly of bundles of plain 

 muscular tissue, arranged in layers which run circularly, longi- 

 tudinally, spirally, and cross and interlace in every direction. 

 A part of the external surface is covered by a portion of the 

 peritoneum in the form of a broad ligament, and the inner sur- 

 face is lined by a mucous membrane. This mucous membrane 

 is continuous with that lining the vagina and Fallopian tubes. 

 It is highly vascular, provided with numerous mucous glands, 

 and is covered with ciliated epithelium. 



The uterus is abundantly supplied with blood-vessels, lym- 

 phatics, and nerves. The blood reaches the uterus by means of 

 the uterine arteries from the internal iliacs, and the ovarian 

 arteries from the aorta. Where the neck joins the body of the 

 uterus, the arteries from both sides are united by a branch 

 vessel, called the circumflex artery. If this branch is cut dur- 

 ing a surgical operation, or a tear of the neck during parturi- 

 tion extends so far as to sever it, the hemorrhage is very 

 profuse. The arteries are remarkable for their tortuous course 

 and frequent anastomoses. The veins are of large size and cor- 

 respond in their behaviour to the arteries. 



During pregnancy all the tissues of the uterus become much 

 enlarged, undergoing what is called a physiological hypertrophy. 

 The uterus increases in weight from two or three ounces to two 

 or three pounds. After parturition, it goes back to nearly its 

 former size. The tissues all go through a gradual shrinkage, or 

 what is called a physiological atrophy. The enlarged muscles 

 especially undergo fatty degeneration and absorption, called 

 "involution," in contradistinction to "evolution" or develop- 



1 The os externum is bounded by two folds or lips of the mucous membrane, 

 the anterior of which is thick, and the posterior narrow and long. 



