BACK OF THE THIGH 319 



fascia of the back of the thigh presents no features of special 

 interest, but in it there ramify cutaneous nerves derived from 

 four sources, ( i ) from the posterior cutaneous nerve of the thigh, 

 (2) from the lateral cutaneous nerve of the thigh, (3) from the 

 posterior branch of the medial cutaneous nerve, and (4) from the 

 obturator nerve. 



Dissection. The branches of the posterior cutaneous nerve 

 of the thigh must be sought in the superficial fascia along the 

 middle of the back of thigh ; they vary in number, and they pierce 

 the deep fascia at varying levels (Fig. 127). The search for them 

 will be facilitated if the dissector pulls gently on the trunk of 

 the nerve where it is already exposed in the lower part of the 

 gluteal region. The branches of the lateral and medial cutaneous 

 nerves should be followed from the portions of those nerves 

 which were displayed when the front and medial sides of the 

 thigh were dissected (pp. 227, 248). No time need be lost in 

 seeking for the cutaneous branch of the obturator nerve, for it 

 is not only very variable in size and position, but also it is not 

 uncommonly absent. When present it lies usually in the distal 

 and medial part of the posterior area of the thigh. 



When the cutaneous nerves of the back of the thigh have 

 been studied the remains of the superficial fascia must be re- 

 moved to display the deep fascia. 



Deep Fascia. The deep fascia of the back of the thigh 

 is thin but fairly strong. It consists of longitudinal fibres 

 blended with transverse fibres which serve to bind the ham- 

 string muscles together. 



Dissection. Divide the deep fascia by a longitudinal incision 

 running along the middle of the back of the thigh, and be careful 

 not to injure the posterior cutaneous nerve of the thigh, which 

 lies immediately under cover of the fascia. Turn the two flaps 

 of deep fascia aside, and then follow and clean the posterior 

 cutaneous nerve which is now exposed in the whole of its length. 



After the trunk of the posterior cutaneous nerve of the thigh 

 has been secured clean the posterior surfaces of the hamstring 

 muscles. They are three in number, the biceps femoris, the 

 semitendinosus, and the semimembranosus. The long head of the 

 biceps springs from the posterior part of the tuberosity of the 

 ischium, by means of a tendon common to it and the semi- 

 tendinosus. It is recognised by the fact that it runs distally and 

 laterally, whilst the semitendinosus and semimembranosus run 

 distally and medially. The short head arises from the body 

 of the femur. Pull the long head of the biceps medially to 

 expose the sciatic nerve, which lies deep (anterior) to it ; then 

 follow the lateral border of the sciatic nerve to secure the branch 

 which goes from it to the short head of the biceps. Next pull 

 the sciatic nerve laterally and clean the branches which pass 

 from it to the long head of the biceps, the semitendinosus, the 

 semimembranosus, and the adductor magnus. As a rule there 

 is one branch to the long head of the biceps, a branch which 



