DISSECTION OF THE DOG 109 



Dissection. — Open the bladder by making an incision along the mid-ventral 

 line from the vertex to the neck. 



Til? interior of the urinary bladder is lined by a mucous membrane of 

 distinctly rugose character ; but towards the neck the rugae are generally less 

 prominent than over the rest of the interior. 



In the dorsal part of the neck the openings of the ureters will be found, and 

 if a probe be passed along a ureter, its obUque passage through the wall of the 

 bladder can be demonstrated. The term trigone of the bladder (trigonum 

 vesicae) is apphed to that triangular area whose angles are denoted by the 

 openings of the ureters and urethra. 



Structure of the bladder. — From a naked-eye examination it is evident that 

 the wall of the urinary bladder is composed of the four layers customarily found 

 in connection with hollow viscera of a hke nature. A serous coat is derived 

 from the peritoneum. A muscular tunic contains fibres running in different 

 directions and arranged in rather ill-defined strata. The superficial stratum 

 contains fibres mainly longitudinally disposed. A middle stratum has 

 circular, and a deep stratum oblique and longitudinal fibres. The pbUque 

 and circular fibres form a sphincter vesicce muscle at the neck of the bladder. 



A submucous tela unites the hning mucous membrane to the muscular tunic. 



Peostate. — The prostate is a rounded, lobulated, yellomsh mass completely 

 surrounding the first part of the urethra, and imperfectly divided into two 

 lateral lobes by a shallow median longitudinal groove. The surface is thinly 

 covered by longitudinal muscular fibres continued from the bladder. Generally 

 only a very small portion of the organ Ues within the confines of the pelvis. 



The two deferent ducts insinuate themselves close together between the 

 prostate and the neck of the bladder. 



Urethra. — The male urethra is a tube concerned in the transmission of 

 both urine and semen, and extends from the neck of the bladder to the free end 

 of the penis. In the female it is much shorter and purely urinary in function. 



The tube is divisible into three portions : (1) Pars prostatica surrounded by 

 the prostate ; (2) pars membranacea extending to the bony arch formed by the 

 borders of the ischia ; (3) pars cavernosa within the corpus cavernosum urethrse 

 of the penis. 



The pelvic portion (consisting of pars prostatica and pars membranacea) 

 runs along the middle Hne immediately dorsal to the symphysis of the pelvis 

 and ventral to the rectum. This portion of the urethra is surrounded by 

 the urethral muscle (m. urethrahs). A branch of the pudendal nerve destined 

 for the prostate will be found running along the ventral face of the pars 

 membranacea. 



If the pelvic portion of the urethra be opened by continuing the mid- ventral 

 incision of the bladder backwards to the bony arch formed by the ischia, it wOl 

 be evident that the lumen of the prostatic part is narrow, whereas that of the 



