508 THE HUMAN BODY 



The male urethra leads from the bladder to the end of the penis, 

 where it terminates in an opening, the meatus urinarius. It is de- 

 scribed by anatomists as made up of three portions, the prostatic, 

 the membranous, and the spongy. The first is surrounded by 

 the prostate gland and receives the ejaculatory ducts. On its 

 posterior wall, . close to the bladder, is an elevation containing 

 erectile tissues (see below) and supposed to be dilated during 

 sexual congress, so as to cut off the passage to the urinary recep- 

 tacle. On this crest is an opening leading into a small recess, the 

 utricle, which is of interest, since the study of its embryology 

 shows it to be an undeveloped male uterus. The succeeding mem- 

 branous portion of the urethra is about 1.8 cm. ( inch) long; the 

 spongy portion lies in the penis. 



The penis is composed mainly of erectile tissue, i. e., tissues so 

 arranged as to inclose cavities which can be distended by blood. 

 Covered outside by the skin, internally it is made up of three 

 elongated cylindrical masses, two of which, the corpora cavernosa, 

 lie on its anterior side ; the third, the corpus spongiosum, surrounds 

 the urethra and lies on the posterior side of the organ for most of 

 its length; it, however, alono forms the terminal dilatation, or 

 glans, of the penis. Each corpus cavernosum is closely united to 

 its fellow in the middle line and extends from the, pubic bones, 

 to which it is attached behind, to the glans penis in front. It is 

 enveloped in a dense connective-tissue capsule from which nu- 

 merous bars, containing white fibrous, elastic, and unstriped 

 muscular tissues, radiate and intersect in all directions, dividing 

 its interior into many irregular chambers called venous sinuses. 

 Into these blood is conveyed partly through open capillaries, 

 partly directly by the open ends of small arteries; this blood is 

 carried off by veins proceeding from the sinuses. 



The arteries of the penis are supplied with vasodilator nerves, 

 the nervi erigentes, derived from the sacral plexus. Under cer- 

 tain conditions these are stimulated and, the arteries expanding, 

 blood is poured into the venous sinuses faster than the veins drain 

 it off; the latter are probably also at the same time compressed 

 where they leave the penis by the contraction of certain muscles 

 passing over them. Simultaneously the involuntary muscular 

 tissue of the bars ramifying through the erectile masses relaxes. 

 As a result the whole organ becomes distended and finally rigid 



