COMPARATIVE ANATOMY 37 



mal ends. Cut the Insertions of the gluteus maximus and 

 gluteus medius and roll these muscles back out of the way. 

 Much of the origin of the gluteus medius will have to be removed, 

 leaving this muscle attached mainly by a small area of dorsal 

 fascia. There should now be in view a fan-like mass of several 

 small muscles radiating around the great trochanter. 



9. Piriformis. — This is a small triangular muscle. It is 

 now the only muscle left in this region that lies over the sciatic 

 nerve. It originates from the transverse processes of the last two 

 sacral and first caudal vertebrae. The fibers converge and are 

 inserted by a flat tendon on the great trochanter. Loosen the 

 insertion of the piriformis and pin the muscle back. Pull the 

 sciatic nerve to one side to expose the underlying muscles. 



Action: to abduct the thigh. 



10. Gluteus Minimus. — This is an elongated muscle, origi- 

 nating from almost the whole ventral half of the lateral surface 

 of the ilium. It is inserted by means of a strong tendon on the 

 lateral side of the great trochanter. 



Action: to rotate the thigh. 



11. Gemellus Superior. — This is a short, broad, triangular 

 muscle, partly covered by the piriformis, but extending beyond 

 it on each side. It originates from the dorsal border of the 

 posterior half of the ilium and the anterior third of the ischium. 

 The fibers converge rapidly to a tendon that inserts on the great 

 trochanter. 



Action: to abduct the thigh. 



12. Obturator Internus. — This muscle lies caudal to the 

 gemellus superior. Its origin is on the in7ier surface of the 

 ischium near the symphysis. The fibers pass over the dorsal 

 border of the ischium; insertion is by means of a strong tendon 

 in the trochanteric fossa of the femur. 



Action: to abduct the thigh. 



13. Gemellus Inferior. — This is a flat muscle almost 

 entirely covered by the obturator internus. Only a small por- 

 tion shows posterior to the obturator internus. It originates 

 on the lateral surface of the ischium and inserts on the same 

 tendon with the obturator internus. The obturator internus 

 must be cut where it passes over the ischium if much of the 

 gemellus inferior is to be seen. 



Action: to abduct the thigh. 



