746 TEXT-BOOK OF PHYSIOLOGY 



The vas defer ens, the excretory duct of the testicle, is about 60 cm. in 

 length and from 2 to 3 mm. in diameter, and extends upward from the 

 epididymis to the inguinal canal, through which it passes into the abdominal 

 cavity and then to the under surface of the base of the bladder, where it 

 unites with the duct of the vesicula seminalis to form the ejaculatory duct. 



The vesiculae seminales are two lobulated pyriform bodies, about 40 

 mm. in length, situated on the under surface of the bladder. Each vesicula 

 seminalis consists of an external fibrous coat, a middle, muscular coat, and an 

 internal mucous coat. The mucous coat contains a number of small 

 tubular albumin-producing glands which secrete a characteristic fluid. 



The ejaculatory duct, formed by the union of the vas deferens and the 

 duct of the vesicula seminalis, opens into the prostatic portion of the urethra 

 (Fig. 327). 



The prostate gland is a musculo-glandular mass surrounding the 

 posterior extremity of the urethra. It contains a large number of tubules, 

 more or less branched and convoluted, and lined by columnar epithelium, 

 They secrete a fluid which is poured into the urethra at the time of the ejacu- 

 lation of semen and impart motility to the spermatozoa or spermia. 



The penis consists of three parts: the corpus spongiosum below, through 

 which passes the urethra, and the two corpora cavernosa, one on either side 

 and above. The corpus spongiosum terminates anteriorly in a conic- 

 shaped structure, the glans penis; the corpora cavernosa consist externally 

 of a fibrous investment and internally of a fibrous investment and internally 

 of erectile tissue. These bodies are abundantly supplied with blood, which 

 after entering their substance by the arteries, passes into sinuses or reser- 

 voirs, from which it is carried away by veins. These vessels pass to the 

 dorsum of the penis and unite to form a large vein by which the blood is re- 

 turned to the general circulation. By virtue of the erectile tissue in the cor- 

 pora cavernosa the penis becomes erect and rigid when the blood supply 

 is increased. This takes place in response to peripheral stimulation or 

 emotional states, or both combined. When these conditions are established 

 nerve impulses pass outward through nerves, the nervi erigentes, which have 

 their origin in the lumbar segment of the spinal cord, and bring about an 

 active dilatation of the arteries and a relaxation of the non-striated muscle- 

 fibers in the corpora cavernosa. (See page 380.) With these events there 

 is a rapid influx of blood and a distention and an erection of the organ. 

 This condition is furthered and maintained by a partial compression of the 

 dorsal vein by the fibrous capsule. 



Semen. The semen is a complex fluid composed of the secretions of 

 the testicles, the vesiculae seminales, the prostatic tubules, and urethral or 

 Cowper's glands. It is grayish-white in color, mucilaginous in consistency, 

 characteristic in odor, and somewhat heavier than water. In response to 

 appropriate stimulation the muscle-fibers in the walls of the vasa deferentia, 

 vesiculae seminales, and prostatic tubules contract and discharge their con- 

 tents into the urethra, from which they are forcibly ejected by the rhythmic 

 contraction of the ejaculatory muscles, the ischio- and bulbo-cavernosi. The 

 amount of semen discharged at each ejaculation varies from i to 5 c.c. 



Spermatozoa. The spermatozoa or spermia are peculiar morphologic 

 elements which arise within the seminiferous tubules as a result of com- 

 plex histologic changes in the lining epithelium. An adult spermatozoon 



