350 REPRODUCTION 



The Penis 



The penis consists essentially of three columns of erectile 

 tissue — the two corpora cavernosa which lie side by side, 

 and the corpus spongiosum which hes inferiorly and sur- 

 rounds the urethra. The three corpora are surrounded 

 by a sheath which contains white, elastic, and plain muscle 

 fibres. Proximally, the corpora are enlarged and are sur- 

 roundod by muscles, the corpora cavernosa by the ischio- 

 cavernosus muscles or erectores penis, and the corpus 

 spongiosum by the bulbo-cavernosus or ejaculator urinse. 

 At the distal end of the penis the corpus spongiosum is 

 dilated to form the glans penis. The erectile tissue of the 

 three corpora consists of a network of trabeculse enclosing 

 venous spaces. 



Erection consists in an engorgement of the venous spaces 

 of the corpora. It is brought about by two factors. There 

 is an active vaso-dilatation of the arterioles, and a compression 

 of the veins by the ischio-cavernosus and bulbo-cavernosus 

 muscles. Erection is essentially a reflex action, for it 

 occurs after section of the spinal cord above the lumbar 

 region. The centre lies in the lumbo-sacral region. The 

 afferent nerves are those arising in the glans penis. The 

 vaso-dilator fibres {nervi erigenies) arise in the first and 

 second sacral nerves. The lumbar nerves, derived from 

 the sympathetic, which also supply the penis, are vaso- 

 constrictor and therefore inhibit erection. 



Erection has been produced by electrical stimulation of 

 the crura cerebri and cord. 



The ejaculation of semen is a reflex induced by the 

 friction of the glans penis against the vulva. Waves of 

 contraction pass along the epididymis and vas. At 

 the same time there occur contractions of the seminal 

 vesicles and prostate. The combined fluid is thus driven 

 into the urethra. It is prevented from entering the bladder 

 by the contraction of the sphincter. The discharge of the 



