NEEVES OF DISTRIBUTION FROM THE SACKAL PLEXUS. 729 



the first three sacral nerves. These roots unite to form a cord which is closely 

 associated with the tibia! nerve and is placed in front of it and afterwards on its 

 medial side. Extending into the thigh, the trunk is distributed to the hamstring 

 muscles by means of two sets of branches. Just distal to the ischial tuberosity 

 a proximal set of nerves enters the proximal part of the semitendinosus and the 

 ischial head of the biceps. More distally in the thigh the remaining portion of 

 the nerve separates off from the tibial part of the sciatic trunk and supplies 

 branches to the seminiembranosus, the distal part of the semitendinosus, and 

 the adductor magnus. 



Articular branches for the hip-joint arise from the nerve to the quadrat us 

 femoris, and often directly from the anterior surface of the tibial part of the 

 sciatic nerve near its origin. They enter the back of the capsule of the joint in 

 the region of the buttock. 



The posterior branches are : (a) muscular branches a nerve to the piriformis, 

 the superior gluteal nerve, the inferior gluteal nerve, and a nerve to the short head 

 of the biceps ; (6) articular branches (to the knee-joint). 



These nerves all arise from the posterior aspect of those roots of the sacral 

 plexus, which are associated with the origin of the common peroneal nerve. 



The nerve to the piriformis muscle may be double. It arises from the back 

 of the second, or first and second sacral nerves, and at once enters the anterior 

 surface of the muscle. 



N. Glutaeus Superior. The superior gluteal rierve arises from the posterior 

 surface of the fourth and fifth lumbar and first sacral nerves, and is directed 

 backwards and laterally into the buttock, above the piriformis muscle, along with 

 the superior gluteal artery. Under cover of the glutseus maximus and glutseus 

 medius, it passes over the glutseus minimus, along with the inferior branch of the 

 deep division of the superior gluteal artery, to the deep surface of the tensor fasciae 

 latse, in which it ends. On its way it supplies branches to the glutseus medius 

 and glutseus minimus muscles. 



N. Glutaeus Inferior. The inferior gluteal nerve arises from the posterior 

 surface of the fifth lumbar and first two sacral nerves. It appears in the buttock 

 at the lower border of the piriformis muscle, superficial to the sciatic nerve, and at 

 once breaks up into a number of branches for the supply of the glutseus maximus. 

 In its course in the buttock it is closely associated with the posterior cutaneous 

 nerve of the thigh. Its origin is sometimes combined with that of the following 

 nerve. 



The nerve to the short head of the biceps springs from the lateral side of the 

 common peroneal trunk in the proximal part of the thigh. When traced to its 

 origin, it is found to arise (sometimes in combination with the inferior gluteal nerve) 

 from the fifth lumbar and first two sacral nerves. In its course it is closely 

 applied to the common peroneal nerve, from which it separates in the middle third of 

 the thigh, usually in combination with the articular branches of that nerve for the 

 knee-joint. In some cases it has an independent course in the thigh, and it may 

 be associated in the buttock with the inferior gluteal nerve. 



An articular branch for the lateral and anterior aspects of the knee-joint 

 generally arises from the common peroneal nerve in common with the nerve to the 

 short head of the biceps. When traced up to the plexus, it is found to arise from 

 the posterior surface of the fourth and fifth lumbar and first sacral nerves. It passes 

 through the proximal part of the popliteal fossa concealed by the biceps muscle, 

 and separates into proximal and distal branches, which accompany the superior 

 and inferior lateral articular arteries to the knee-joint. 



Terminal Branches. The common peroneal (O.T. external popliteal) and tibial 

 (O.T. internal popliteal) nerves are the two main trunks resulting from the com- 

 bination of the posterior and anterior cords of the sacral plexus respectively. The 

 common peroneal nerve is homologous with the radial nerve of the upper limb ; 

 the tibial nerve represents a medio-ulnar trunk ; and, as already stated, the two 

 nerves, constituting the sciatic nerve, are enveloped in a common sheath for a 

 variable distance before pursuing an independent course in the leg. 



