838 MENSTRUATION. 



which is added to it. The ejaculatory ducts, which are formed by the 

 union of the ducts of the vasa deferentia and vesiculse seminales, open into 

 the prostatic portion of the urethra. Their coats are thinner, but have 

 essentially the same structure as the vasa deferentia, with which they are 

 continuous. 



778. The seminal fluid is a complex secretion, being composed of the 

 anatomical elements of spermatozoa, which are formed in the testes, and of 

 the secretions of the vasa deferentia, vesiculse seminales, the prostrate and 

 Cowper's glands, and the mucous glands of the urethra. The seminal fluid 

 is of a thick, whitish striated appearance, and, if examined microscopically, 

 is seen to contain innumerable bodies which are in active motion. These 

 are the spermatozoids, and are the essential male elements concerned in the 

 fecundation of the ovule. Each of these bodies (Fig. 214) consists of a 

 flattened, ovoidal head, having at its base a tapering caudate appendage in 

 active vibratile motion. These anatomical elements were at first considered 

 animalcula, but they are now looked upon as free mases of protoplasm with 

 ciliary appendages, which endow them with the power of migration. 



The spermatozoa are developed from the nuclei of vesicles which are 

 formed in the tubules of the testes. The nuclei are metamorphosed into 

 the heads of the spermatozoa, the ciliary appendages being afterward devel- 

 oped as a sort of outgrowth. Different stages of the development and other 

 interesting features are shown in the above figure.] 



CHAPTER II. 



MENSTRUATION. 



779. FROM puberty, which occurs at from 13 to 17 years of age, to 

 the climacteric, which arrives at from 45 to 50 years of age, the human 

 female is subject to a monthly discharge of ova from the ovaries, accompanied 

 by special changes, not only in those organs but also in the Fallopian tubes 

 and uterus, as well as by general changes in the body at large, the whole 



[FiG. 216. 



5 v ' fi 5 4 3 



FIG. 215. Section of Graafian Follicle of a Mammal. (After Von Barr.) 1, stroma of the ovary 

 with bloodvessels ; 2, peritoneum ; 3 and 4, layers of the external coat of the Graafian follicle ; 5, 

 membrana granulosa : 6, fluid of the Graafian follicle ; 7, granular zone, or discus proligerus, con- 

 taining the ovule (8).] 



FIG. 216.-0vula of the Sow. (After Barry.) 1, germinal spot ; 2, germinal vesicle ; 3, yolk ; 4, 

 zona pellucida; 5, discus proligerus; 6, adherent granules or cells.] 



constituting "menstruation." The essential event in menstruation is the 

 escape of an ovum from its Graafian follicle (Fig. 215). The whole ovary 

 at this time becomes congested, and the ripe follicle bulges from its surface. 

 The most projecting portion of the wall of the follicle, which has previously 



