DISSECTION OP THE POSTERIOK LIMB. 63 



should be traced through the substance of that muscle to their origin 

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

 expose the obturator vessels and nerve. 



The Obturator Artery (Plates 14 and 16). 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 corpus 

 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 maguus, 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 undertaken 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 internal 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 

 popliteal ; (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, whose other divisions — 

 the rectus femoris and vastus externus— 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 iliacus and 



psoas magnus. 



The Vastus Internus (and Cbureus^) (Plates 13 and 14) is a thick 

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



1 This is the name given to the fourth division of the quadriceps in human anatomy. 

 The fibres that represent it in the horse are in no way separable from the inner vastus. 

 Under the same name Percivall describes (inaccurately) the rectus parvus. 



