THORAX AND ABDOMEN OF THE HORSE 171 



tinuous with the ureteral folds of the bladder. In association with the 

 crest is a rounded eminence (coUiculus seminalis), of some size in the 

 stallion but inconspicuous in the castrated animal, on the sides of 

 which are the openings of the ejaculatory ducts, or, if these do not 

 exist, the common openings of the deferent ducts and the excretory 

 ducts of the seminal vesicles. On each side of the urethral crest is an 

 irregular row of openings (12 to 15 in number) of the prostatic ducts. 



It is sometimes possible to detect a small, unpaired opening in the 

 middle line between the ejaculatory duct orifices. This, when present, 

 leads into the prostatic utricle or uterus masculinus. 



The openings of the ducts of the bulbo-urethral glands, on the 

 dorsal wall of the tube, should be sought, as should also the openings, 

 on papilla?, of the small urethral glands which occur at the same level 

 but on the lateral wall. 



The mucous membrane of the urethra is surrounded by a layer of 

 erectile tissue except in the immediate neighbourhood of the neck of the 

 bladder. At the sciatic arch this becomes slightly thicker and forms 

 the btdb of the iirethra (bulbus urethrae), which, however, is very 

 indifferently developed in the horse. 



Dissection. — The fat and loose connective tissue must be removed 

 from that wall of the pelvis that is still intact, and its blood vessels 

 and nerves defined. 



As a matter of convenience, the dissection of the tail should be 

 included in the examination now being conducted. 



M. OBTURATOR INTERNUS. — The internal obturator muscle consists 

 of two parts. The more extensive portion is fan-shaped and 

 covers the greater part of the ventral wall of the pelvis. Its fibres 

 arise from the cranial and medial borders of the obturator ^ foramen, the 

 body of the ischium, and the symphysis between the two hip-bones ; 

 and converge towards the lesser sciatic foramen. 



The other part of the muscle is narrow and semipenniform and 

 arises from the medial surface of the body of the ilium and the trans- 

 verse process of the first sacral vertebra. 



The two portions join a narrow, flattened tendon that leaves 

 the pelvis by the lesser sciatic foramen and is inserted into the 

 trochanteric fossa of the femur. A synovial bursa intervenes between 

 the tendon and the ischium at the lesser sciatic notch. 



The small nerve to the internal obturator muscle, derived from the 

 sciatic, must be sought as it enters the pelvis by the lesser sciatic 

 foramen. 



M. SACROCOCCYGEUS DORSALis MEDiALis. — This is a Strong rounded 

 ' Ohturo [L.], to stop up, to close. The obturator foramen is closed by tissues. 



