62 THE ANATOMY OF THE HORSE. 



from the obturator nerve. The muscle itself may then be removed to 

 expose the obturator vessels and nerve. 



The Obturator Artery (Plates 14 and 46). This vessel begins at the 

 pelvic inlet as one of the terminal branches of the internal iliac. It 

 leaves the pelvis by the obturator foramen, in company with a vein and 

 nerve of the same name. At its point of emergence it is covered by 

 the obturator externus, and it passes backwards between that muscle 

 and the bone, and then curves downwards to terminate in the biceps 

 and semitendinosus. It gives off the artery of the coiyus cavernosum. 



The Obturator Vein passes into the pelvis by the obturator foramen, 

 and aids in forming the internal iliac vein. 



The Obturator Nerve is a branch of the lumbo-sacral plexus. 

 Emerging by the obturator foramen, it divides for the supply of the 

 obturator externus, adductor parvus, adductor magnus, pectineus, and 

 gracilis muscles. 



Directions. — In this stage of the dissection the great sciatic nerve is 

 seen in its course downwards through the thigh. Its examination is 

 more conveniently inidertaken in the dissection of the hip and outer 

 aspect of the thigh, but attention may also be given to it here. 



The Great Sciatic Nerve, which is a branch of the lumbo-sacral 

 plexus, after passing through the hip (see Plate 16), descends in the 

 thigh, behind the femur, where it is deeply enclosed between the 

 biceps and semitendinosus outwardly, and the semimembranosus and 

 great adductor inwardly. Under the name of the intei-nal popliteal, it 

 passes in between the two heads of the gastrocnemius. The following 

 branches whose points of origin are not now visible, being situated in 

 the hip, may be identified by reference to Plate 14: — (1) Branches to the 

 biceps, semitendinosus, and semimembranosus ; (2) the external pop- 

 liteal ; (3) the external saphenous. The last two will be again seen in 

 the dissections of the hip, thigh, and leg. 



Directions. — The vastus internus, situated at the front of the thigh, 

 should now be examined. It is a division of the great muscular mass 

 termed in man the quadriceps extensor cruris, Avhose other divisions — the 

 rectus femoris and vastus extennis — will be dissected with the outer 

 aspect of the thigh. The dissection in this position of the limb will be 

 completed by an examination of the common insertion of the iliflcus 

 and psoas magnus. 



The Vastus Internus (and Crureus*) (Plates 13 and 14) is a thick 

 fleshy muscle whose fibres take origin from the internal surface and inner 

 half of the anterior surface of the femur, meeting along the front of the 

 femur the vastus internus, and with it forming a groove in which the 

 rectus femoris rests. Its fibres are inserted into the inner ligament of 



* This is the name given to the fourth division of the quadricejis in human anatomy. The fibres 

 that represent it in the liorse are in no way separable from tlie inner viistus. Under tlie siime name 

 Percivall describes (inaccurately) the rectus parvus. 



