THE GENERATIVE ORGANS. 197 



When the urethra is closed the mucous membrane 

 is thrown into irregular longitudinal folds. 



The urethra is enclosed through a part of its 

 length in the corpus spongiosum, which is largely 

 composed of a kind of tissue called erectile tissue ; 

 and before describing this body it will be well to 

 consider for a moment the nature of this kind of 

 tissue. 



Erectile tissue, in certain cases, consists simply of 

 a somewhat circumscribed collection of larger and 

 smaller veins, which, under certain circumstances, 

 may become distended with blood, thus causing the 

 part in which they lie to expand ; in other cases it 

 consists of numerous larger and smaller irregular- 

 communicating cavities, separated from one another 

 by broad or narrow interlacing bundles of connective 

 tissue and smooth muscular tissue, and in communi- 

 cation with arterial trunks or capillary blood-vessels ; 

 the cavities are lined with endothelium, and, while 

 usually containing but a small amount of blood, and 

 in this condition appearing simply as slits or narrow 

 irregular spaces in the tissue, they may, under cer- 

 tain conditions, become distended with blood, when 

 they assume the character of spheroidal, or broad 

 elongated cavities, and thus cause a considerable 

 increase in volume of the part in which they are 

 situated. The corpus spongiosum is composed 

 largely of erectile tissue of the character last 

 described. 



