692 TEXT-BOOK OF PHYSIOLOGY. 



interims (vi). In the leg, in addition to those already seen from behind, 

 note the tibialis posticus (tp) and the extensor cruris (ec). 



Note in the abdominal cavity the three large spinal nerves, the seventh, 

 eighth, and ninth. 



Dissection of the Sciatic Nerve. The sciatic nerve is composed of 

 the seventh, eighth, and ninth spinal nerves. After its emergence from 

 the pelvic cavity, it passes down the thigh between the semi-membranosus 

 and the biceps muscles, in company with the femoral blood-vessels. Below 

 the knee it divides into the tibialis and peroneus nerves; the former sending 

 branches into the gastrocnemius. In its course, the sciatic sends branches 

 to the muscles of the entire leg. 



Carefully separate the biceps and semi-membranosus by tearing the 

 connective tissue uniting them. The sciatic nerve and femoral blood- 

 vessels come into view; with a bent glass rod gently separate the nerve 

 from its surroundings from the knee to the thigh. Begin at the knee. In 

 order to expose the nerve at the pelvis, it will be necessary to divide the 

 pyriformis and the ileo-coccygeus muscles. Care must here be exercised, 

 so as not to injure the nerve which lies immediately beneath. Lift up 

 the uro-style with the forceps and separate it from the last vertebra. 

 With the scissors cut off the vertebral column above the seventh vertebra. 

 Place the legs on the dorsal surface and then divide the seventh, eighth, 

 and ninth vertebrae lengthwise. With the forceps b'ft up one lateral half 

 of the vertebrae and free the nerve as far as the knee by dividing con- 

 nective tissue and nerve branches. Be careful not to injure the nerve 

 with scissors or forceps. 



The Nerve-muscle Preparation. Divide the tendo Achillis just 

 below its nbro-cartilaginous thickening at the heel, and detach the 

 gastrocnemius up to the knee. Cut through the tibio-fibular bone just 

 below the knee-joint. Cut the femur transversely near its middle and 

 remove the muscles from the lower end, carefully avoiding injury to the 

 nerve. The completed preparation consists of the gastrocnemius muscle, 

 the sciatic nerve, with half of the seventh, eighth, and ninth vertebrae 

 and the lower half of the femur. Place the preparation on a clean glass 

 plate and moisten with normal saline solution, 0.6 per cent. 



