650 THE HUMAN BODY. 



in childhood ; and atrophies after removal of the testicles later 

 in life. 



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

 penis, where it terminates in an opening, tlie meatus urinarius. 

 It is described 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 eleva- 

 tion containing erectile tissues (see below) and supposed to be 

 dilated during sexual congress, so as to cut off the passage to 

 the urinary receptacle. 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 membranous portion of the urethra 

 is about 1.8 cm. (f 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 spongi- 

 osum, surrounds the urethra and lies on the posterior side of 

 the organ for most of its length; it, however, alone forms 

 the terminal dilatation, or glans, of the penis. Each corpus 

 cavernosiun 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 numerous bars, contain- 

 ing 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 car- 

 ried off by veins proceeding from the sinuses. 



The arteries of the penis are supplied with vaso-dilator 

 nerves, the nervi erigentes, derived from the sacral plexus. 

 Under certain 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. Simul- 

 taneously the involuntary muscular tissue of the bars ran if y- 



