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 corpom 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 



— H, 



14* 



