THORAX AND ABDOMEN OF THE HORSE 159 



determinable, should receive consideration. Beginning at the dorsal 

 wall of the pelvis, little difficulty will be experienced in demonstrating 

 a mesentery belonging to the first part of the rectum and continuous 

 with the mesocolon. The two layers of the mesorectum separate on 

 reaching the sacrum, and each layer sweeps down the lateral wall of the 

 pelvis. Between the rectum and the urinary bladder the urogenital 

 fold (plica urogenitalis) is produced by a horizontal duplicature of the 

 peritoneum in which are contained the deferent ducts, the seminal 

 vesicles and the unpaired prostatic utricle. The peritoneal pouch 

 (excavatio rectovesicalis), between the rectum and the bladder, is thus 

 incompletely subdivided by the fold. The seminal vesicles are partially 

 covered by the dorsal layer of the urogenital fold. 



The peritoneal covering of the urinary bladder is reflected from this 

 organ in the form of three sheets. One of these, the middle uinhilical 

 fold (plica umbilicalis media) is ventral in position and triangular in 

 outline, with a free cranial edge. The other two folds are lateral in 

 position {lateral umbilical folds : plicae umbilicales laterales) and also 

 triangular in form, with an apex directed towards the neck of the 

 bladder. The base of each is free and contains the rounded cord 

 (ligamentum teres vesicae) which is the remains of the umbilical ^ artery 



It will be noted that the peritoneal covering of the bladder is 

 carried closer to the neck on the dorsal than on the ventral surface. 



Dissection. — Clear the remains of muscles from one hip-bone. 

 Liberate the root of the penis from the ischium of the same side. 

 Make a saw-cut through the pubis and ischium on a level with the 

 most medial part of the obturator foramen. Saw through the ilium on 

 a level with the greater sciatic foramen. In making these sections, 

 every care must be taken that the saw does not go deeper than is 

 absolutely necessary. Now cut through the sacro-spinous and sacro- 

 tuberous ligament close to its attachment to the hip-bone. This will 

 isolate a piece of bone consisting of most of the pubis and ischium and 

 part of the ilium. Remove this carefully, keeping the knife as close to 

 the bone as possible. To do this it will be necessary to sever the 

 origins of the levator ani and ischio-urethral muscles ; but this is of 

 little consequence, as these muscles will be intact on the opposite side of 

 the pelvis. 



Turn the sacro-spinous and sacro-tuberous ligament as far as possible 

 towards the sacrum. 



The next step in the dissection is to clean the lateral surface of the 

 pelvic organs, preserving the vessels and nerves and observing the 

 arrangement of the pelvic fascia and peritoneum. 



M. COCCYGEUS. — The coccygeus - muscle consists of a broad 



1 Umbilicus [L.], the navel, a round peljble, the jirojecting end of the cylinders 

 on which the books of the ancients were rolled. 



" k6kkv^ (coccyx) [Gr.], cuckoo. The bony mass attached to the human sacrum, 

 representing the tail or coccygeal vertebrce of mammals in general, is supposed to 

 resemble the beak of the cuckoo. 



