148 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



finds the anatomical relation of the vesicle to the vas deferens in the human 

 being to be such that liquids injected into the testicular end of the vas 

 deferens pass Brs1 into the vesicle before going out the urethra; and lie 

 believes strongly thai the vesicles exert the double function of serving as 

 storehouses for spermatozoa and finding liquid for the semen. The ejacuki- 

 tory duct (de) on each side is a -hurt, thin-walled muscular tube, passing 

 partly through the substance of the prostate -land and serving to convey the 

 semen to the urethra. 



The Urethra. — The urethra (Fig. 219, u), the common excretory duct 

 for the urine and the semen, is commonly described as consisting of three parts, 

 named, respectively, the prostatic, the membranous, and the spongy portions. 

 The first is characterized by the presence of the prostate gland, the second by 

 the absence of special features, and the third by the presence of Cowper's glands 

 and the penis. Throughout its length the wall of the urethra contains plain 

 muscular tissue arranged longitudinally within and circularly without ; and, 

 except at the external opening, the small racemose mucous glands of Littre. 

 It- wall is hence contractile and its lumen is kept moist. Beyond these its 

 special physiological feature- are given it by the organs above mentioned. 



The Prostate Gland. — The prostate gland (Fig. 21!», pr) is a compound 

 tubular gland whose alveoli are mingled with a large quantity of plain mus- 

 cular tissue. Jt completely surrounds the urethra at the base of the bladder, 

 and open- into it by numerous small ducts situated about the openings of the 

 vasa defereniia. Its function is to contribute prostatic fluid to the semen. 

 The composition of this fluid and its specific use so far as it is known have 

 been already mentioned (p. 446). 



Cowper's Glands. — Courper's glands | Fig. 219, Og), two in number, are 

 tubulo-racemose glauds, the duets of which open into the spongy portion of 

 the urethra by two orifices situated some two inches below the openings of the 

 vasa defereniia. Their viscid secretion is thought to be one of the components 

 of the seminal fluid, but its specific function is unknown. It has been sug- 

 • I thai Cowper's fluid cleanses the urethra of urine and of semen, instead 

 of contributing actually to the seminal fluid. 



The Penis. — The penis (Fig. 219, pn) has as its constant function merely 

 the conveying of the urine to the outside world, and for this purpose it has no 

 Bpecial features beyond those belonging to the urethra, which runs throughout 

 it- whole length. Specifically, however, it is the intromittent organ, and 

 serves to convey the -emeu into the genital passages of the female. This 

 (unction is based upon it- power of erection, and this power is dependent 

 upon the presence of the erectile tissue which constitutes the bulk of the 

 organ. The erectile tissue is arranged in the form of three long cylindrical 

 masses imperfectly separated from, but parallel to. one another and extending 

 lengthwise. < >f these, the two corpora cavernosa lie at the sides, and meet each 

 other in the middle line along the upper side of the penis ; the corpus spongi- 

 osum lies in the middle line below, and is pierced throughout its length by the 

 urethra. At its proximal end each corpus is enlarged into a bulbous part, 



