REPRODUCTION. 657 



cavernosa consist externally 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 reservoirs, 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 returned to the general circulation. By virtue 

 of the erectile tissue in the corpora 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 nerui erigentes, which have their 

 origin in the lumbar region 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. 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 glands. It is grayish-white in color, mucilaginous in con- 

 sistence, characteristic in odor, and somewhat heavier than water. 

 In response to appropriate stimulation the muscle-fibers in the walls 

 of the vasa deferentia, vesiculae seniinales, and prostatic tubules 

 contract and discharge their contents 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 disr 

 chargcd at each ejaculation varies from i to 5 c.c. -^ "" 



Spermatozoa. The spermatozoa are peculiar morphologic ele- 

 ments which arise within the seminiferous tubules as a result of 

 complex histologic changes in the lining epithelium. An adult sper- 

 matozoon consists of a conoid slightly flattened head, from the pos- 

 terior part of which there projects a short straight rod, provided with 

 a long filamentous tail or cilium and an end-piece (Fig. 323). The 

 head contains a nucleus of chromatin material. The total length of 

 a spermatozoon varies from 50 to 80 micromillimeters. The char- 

 acteristic physiologic feature of spermatozoa is incessant locomotion 

 when in a suitable medium. So long as they are confined to 

 the vas deferens they are quiescent, but with their advent into the 

 vesicula seminalis and dissemination in its contained fluid, they be- 

 come extremely active and move around with considerable rapidity. 

 The power of locomotion depends on the possession of the tail, 

 which, by lashing the surrounding fluid now in this and now in that 

 direction, propels the head from place to place. The vitality of 

 spermatozoa is such as to enable them to retain their physiologic 

 activities in the uterus for more than eight days. 

 42 



