iyo THE NERVOUS SYSTEM 



with the genicular branch of the obturator nerve. The 

 articular branches of the femoral nerve are stimulated on the 

 hip-joint and an "overflow" (p. 191) occurs in the spinal 

 medulla in such a way as to stimulate the cells which receive 

 afferent impulses from the medial and intermediate cutaneous 

 nerves of the thigh. In this way the pain is referred to the 

 region of the knee. 



The Cutaneous Branches of the -obturator nerve supply a 

 small area on the medial aspect of the thigh which is over- 

 lapped by the adjoining medial and posterior cutaneous nerves. 



The Obturator Nerve may be injured in the pelvis by 

 tumours in connexion with the uterus or rectum, or during the 

 passage of the fcetal head at parturition. The first sign of 

 the involvement of the obturator nerve in these cases is pain 

 which is referred to the medial aspect of the thigh. 



When the obturator nerve is completely divided, there is no 

 alteration discoverable in the sensibility of the skin of the 

 thigh, but there is complete paralysis of the adductor muscles, 

 a condition which is best appreciated when the patient is 

 examined lying flat on his back with the lower limbs extended. 

 In this position, the affected limb is maintained in a position 

 of slight abduction by the unopposed abductor muscles. On 

 the other hand, when the patient assumes the erect attitude, 

 the noticeable deformity may be very slight, as the abductor 

 muscles are then opposed by the weight of the limb. 



THE SACRAL PLEXUS 



The Sacral Plexus is formed by a part of the fourth and 

 the whole of the fifth lumbar, the first, second and part of the 

 third sacral nerves. The lumbo-sacral cord, which is formed 

 by the union of the fifth lumbar with a branch of the fourth 

 lumbar nerve, unites with the first and second and a branch of 

 the third sacral nerve. In this way a large band is constituted, 

 which, although mainly continued as the Sciatic Nerve^ gives 

 off several smaller branches, 



