THE URETHRA. 161 



substance appears cellular, and may be completely distended 

 by it. When air is injected, and the structure becomes dry, 

 the penis may be laid open ; the cellular structure then appears 

 as if formed by a number of lamina and of filaments, >which 

 proceed from one part of the internal surface of the penis to 

 another, and form irregular cells. It has been compared to the 

 lattice-work in the interior of bones ; and it is suggested by M. 

 Roux, that the fibres of which the structure consists resemble 

 those of the strong elastic coat of the penis.* If these cells are 

 filled with colored wax, injected by the artery, and the animal 

 substance is then destroyed by placing the preparation in a 

 corroding liquor, the wax which remains shows that the 

 membranes forming the cells are very thin. 



These cells communicate freely with each other ; and, there- 

 fore, if a pipe be passed through the strong coat of the penis, 

 the whole of them can be filled from it by the ordinary process 

 of injection. 



The Urethra 



Is a membranous canal which extends from the neck of the 

 bladder to the orifice at the extremity of the penis ; and for a 

 very great part of its length is invested by a spongy structure, 

 called the corpus spongiosum urethrae. It proceeds from the 

 neck of the bladder along the upper part of the prostate ; from 

 the prostate it continues between the crura of the penis until 

 their junction : it then occupies the great groove formed by the 

 corpora cavernosa on the lower side of the penis, and continues 

 to the orifice above mentioned. At a small distance from the 

 prostate gland the spongy substance which invests it com- 

 mences, and continues to its termination. After this spongy 

 substance has arrived at the termination of the corpora 



* Mr. John Hunter says on this subject, " That the cells of the corpora ca- 

 vernosa are muscular, although no such appearance is to be observed in men : 

 for the penis in erection is not at all times equally distended. The penis, in a 

 cold day is not so large in erection as in a warm one : which probably arises 

 from a kind of spasm, that could not act if it were not muscular." 



In the horse, the parts composing the cells of the penis appear evidently mus- 

 cular to the eye, and in a horse just killed, they contract upon being stimulated 



TT 



14* 



