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TEXT-BOOK OF PHYSIOLOGY. 



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 susbtance 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 re- 

 laxation of the non-striated muscle-fibers in the corpora 

 cavernosa. (See page 619.) With these events there is 

 a rapid influx of blood and a distention and an erection 

 of the organ. This condition is furthered and main- 

 tained 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 consistence, 

 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 complex histologic 

 changes in the lining epithelium. An adult spermatozoon 

 consists of a conoid slightly flattened head, from the 

 posterior part of which there projects a short straight 

 rod, provided with a long filamentous tail or cilium and an end-piece 

 (Fig. 350). The head contains a nucleus of chromatin material. The 

 lotal length of a spermatozoon varies from 50 to 80 micro-millimeters. 

 The characteristic 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 become 

 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 



V 



FIG. 350. HUMAN 

 SPERMATOZOON, i. 

 Front view, 2, side 

 view, of the head. 

 k. Head. m. mid- 

 dle piece. /. Tail. 

 e. Terminal fila- 

 ment. (After Ret- 

 zius.} 



