182 THE NERVOUS SYSTEM 



tion in sciatica. In this condition, the sciatic nerve is tender 

 on deep pressure, and subjective symptoms are experienced over 

 the distribution of its sensory branches, usually over the areas 

 supplied by the lateral sural nerve and the nervus suralis 

 (Fig. 74). When the nerve-sheaths are much thickened as 

 the results of the inflammation, the cutaneous branches are 

 nipped as they pierce the deep fascia, giving rise to the 

 "tender points of Valleix." It should be observed that when 

 the condition is due to a localised neuritis of the sciatic nerve, 

 no subjective symptoms are felt in the skin area which is 

 supplied by the posterior cutaneous (small sciatic) nerve (Fig. 

 74). On the other hand, when the condition is due to intra- 

 pelvic pressure, this area is quite likely to be affected. 



When the sciatic nerve is completely paralysed, the motor 

 symptoms are very striking. All the muscles distal to the knee 

 are paralysed, and movements of the ankle, foot and toes are 

 quite impossible. In addition, the hamstring muscles are 

 affected, but this does not produce such a striking change, as 

 the knee can be flexed by means of the gracilis and the 

 sartorius (obturator and femoral nerves, respectively). 



The amount of sensory loss is relatively smaller. The 

 posterior aspect of the leg in its proximal part is supplied by 

 the posterior cutaneous nerve of the thigh (small sciatic), and 

 the medial aspects of the leg and foot are supplied by the 

 saphenous nerve. As a result, the area of sensory loss is 

 restricted to the lateral aspect of the leg, the dorsum of the 

 foot, except near the medial border, and the sole of the foot. 

 Despite the position of the lesion, deep sensibility is only 

 affected in the distal part of the foot (Sherren). 



It is interesting to observe that, when the sciatic nerve is 

 injured as the result of stab or gunshot wounds, the fibres 

 affected are in 90 per cent, of cases (Makins) those of the 

 peroneal portion of the nerve, but no adequate explanation 

 can yet be offered to account for this peculiarity. 



The Posterior Cutaneous (Small Sciatic) Nerve of the thigh 

 (S. i, 2 and 3) arises from the posterior aspect of the 



