108 DISSECTION OF THE DOG 



mesentery continuous with the mesocolon, the rectum has its dorsal surface 

 covered by peritoneum to a level shghtly caudal to the acetabulum. The line 

 of peritoneal reflection slopes obliquely across the lateral face of the tube in 

 such a manner that the ventral surface has scarcely any peritoneal investment. 

 The terminal opening of the intestine, the amis, is surrounded by thick skin 

 provided -with a somewhat scanty supply of short fine hairs. Close to the line 

 of junction of skin and mucous membrane is a small opening on each side 

 leading into a sac, from a hazel-nut to a walnut in size, known as the para-mial 

 sinus (sinus para-analis). The sinus lies between the external sphincter muscle 

 of the anus and the longitudinal muscle of the rectum, and has a lining 

 membrane provided with glands which produce a thickish, strong-smelling 

 secretion. 



M. LEVATOR ANI. — The levator muscle of the anus of the dog is relatively large 

 but thin. Triangular in form, it hes over the lateral face of the rectum. Since 

 it is medial to the coccygeus muscle, this has to be turned aside in order that the 

 full extent of the levator may be exposed. The levator ani arises from the 

 symphysis of the ischia and pubes, from the oral border of the pubis, and from 

 the medial surface of the ilium. Some of its fibres blend with the sphincter ani, 

 and some end on the first vertebra of the tail. 



M. SPHINCTER ANI EXTERNUS. — The external sphincter surrounds the anus 

 as a ring. It consists of three portions more or less blended together, and is 

 attached to the aponeurosis of the tail. The oral portion is poorly developed. 

 The middle portion is connected with the m. bulbo-cavernosus, and the aboral 



portion with the retractor muscle of the penis. 



M. SPHINCTER ANI INTERNUS. — The internal sphincter muscle is merely a 



thickening of the circular muscle of the rectum. 



It should be noted here that the longitudinal muscular fibres of the rectum 



send a well-defined band to the ventral aspect of the first few coccygeal 



vertebrae. 



Vesica heinajria. — Although not a pelvic organ in the dog, it is convenient, 



because of its connections, to study the urinary bladder here rather than with 



the abdominal viscera. An ovoid sac Avith a strong muscular wall, the bladder 



is di^^ded, for-descriptive purposes, into a main portion or body (corpus vesicae) ; 



a blunt cranial end, the vertex (vertex vesicae) ; and a narrow pelvic extremity, 



the neck (coUum vesicae), continuous with the urethra. 



Ventrally the bladder lies on the abdominal wall, the great omentum 



sometimes intervening. Dorsally it is in contact partly with the colon, partly 



with coils of the small intestine. Dorsal to the neck are the deferent ducts. 



On each side the bladder is in relation to the small intestine and the ureter. 

 Naturally the relations of the vertex of the bladder depend materially upon 



the amount of distension of the viscus. When the sac is moderately full it 



may touch the ileum. At all times the vertex is in contact with some part of 



the small intestine. 



