THE GENERATIVE ORGANS. 195 



urethra is so frequently the seat of surgical opera- 

 tions, and as the corpus spongiosum, which surrounds 

 a portion of it, presents an example of a variety of 

 tissue which we shall have no opportunity to study 

 elsewhere, it is desirable to briefly consider their 

 structure here. 



The urethra, divided into three portions a pros- 

 tatic, a membranous, and a spongy, consists of a 

 mucous membrane which is surrounded by a muscu- 

 lar sheath. The considerable differences in structure 

 which its different parts present are due chiefly to 

 variations in the form of the epithelium and the 

 character and arrangement of the muscular tissue. 

 The mucosa is composed, in all parts alike, of 

 fibrillar connective tissue containing numerous elas- 

 tic fibres and richly furnished with cells ; upon the 

 free surface of this rests the epithelium, which, at 

 the meatus and in the fossa navicularis, is of the flat, 

 laminated variety ; this passes over into a single row 

 of cylindrical cells which line the spongy portion ; 

 and this in turn merges into the spheroidal, pear- 

 shaped, prismatic, and flattened cells of the prostatic 

 portion, which, lying in several layers, are quite 

 similar in form and arrangement to the epithelium of 

 the bladder. 



Outside of the mucosa, and but indistinctly sep- 

 arated from it, is the submucosa, which consists of 

 connective tissue with elastic fibres, and is especially 

 characterized by a dense net-work of veins which re- 



