Chap. 18 REPRODUCTION 343 



the body cavity and, joining together, enter the urethra which extends through 

 the introniittent organ or penis to the external opening (Fig. 18.5). 



Other glands, chiefly the seminal vesicles and the prostate gland, also con- 

 tribute to the seminal fluid. This fluid contains salts that act as protective 

 buffers against the acids in the urethra of the male and in the reproductive 

 passages of the female, and glucose, a nutrient. The prostate gland almost 

 completely surrounds the urethra near its exit from the urinary bladder. In 

 elderly men, this gland often enlarges. Since its outer surface is covered by an 

 unyielding capsule it can do nothing else but squeeze the urethra and more or 

 less cut off the passage of urine. The gland was named prostate (Gr., standing 

 before) from its position in front of the urinary bladder and is in nowise 

 "prostrate" as it is sometimes called. 



The urethra extends through the penis to its external opening (Fig. 18.5). 

 It contains sperm cells only when the penis is erected, that is, when the 

 "spongy" tissues surrounding it are stiffened by the blood that floods into them, 

 and the sperm ducts contract spasmodically, forcing the sperms into it before 



Fig. 18.8. The effect of a hormone of the testis on the comb of the cock, a, cas- 

 trated cockerel, otherwise untreated; b, a castrated cockerel after 11 days treatment 

 with extract of testis. Drawn from photographs by Freud and co-workers. (Cour- 

 tesy, Corner: The Hormones in Human Reproduction. Princeton, N.J., Princeton 

 University Press, 1942.) 



or during copulation. At the same time urine is shut out of it. The spurts or 

 ejaculations of seminal fluid, a half-teaspoonful or less in bulk, are estimated 

 to contain about 300 million sperm cells. In the ordinary, somewhat shrunken 

 condition of the penis, the skin is very loose and a fold of it, the foreskin, 

 covers the tip. This is very often removed in babies by a simple operation 

 called circumcision. This is a hygienic measure and a very old religious rite. 

 Testicular Hormone. The testes produce the sperm cells; they also produce 

 fluids. Under stimulation by a hormone (gonadotrophic) of the anterior lobe 

 of the pituitary gland, they secrete a male hormone (androgen) that causes 

 and maintains the development of the secondary sex characters such as voice, 

 form, behavior, and sexual activity (Fig. 18.8). The male hormone is believed 

 to be secreted by interstitial cells lying between the seminiferous tubules. In 

 spite of its name and effects, androgen belongs to the same family of chemical 



