136 THE NERVOUS SYSTEM 



cutaneous nerve of the forearm to any extent. As a result, 

 when the whole of the musculo-cutaneous nerve is paralysed, 

 the sensory disturbance is very ill-defined on the back of the 

 forearm, whereas, on the front of the forearm, a fairly sharp 

 line of demarcation can always be found. 



The lateral head of the median will be considered along 

 with the medial head (p. 149). 



(B) The Posterior Cord (C. 5, 6, 7, 8 and T. i) The 

 Upper Subscapular Nerve (C. 5 and 6) is entirely distributed 

 to the subscapularis, which forms the proximal part of the 

 posterior wall of the axilla. 



The siibscapularis arises from the ventral surface of the scapula, crosses 

 the anterior aspect of the capsule of the shoulder-joint, and is inserted into 

 the lesser tubercle of the humerus. It acts as a medial rotator and assists 

 in flexion and adduction of the humerus. Section of the upper subscapular 

 nerve produces little motor disability, but, as the muscle atrophies, the 

 anterior aspect of the capsule of the shoulder-joint becomes seriously 

 weakened, and this condition predisposes to dislocation. 



The Lower Subncapular Nerve supplies a few twigs to the subscapul- 

 aris, but is mainly distributed to the teres major. This muscle arises 

 from the dorsal aspect of the inferior angle of the scapula and passes 

 upwards and laterally on the posterior wall of the axilla to be inserted 

 into the floor of the intertubercular sulcus (bicipital groove). It acts as a 

 medial rotator, adductor and extensor of the humerus. Section of the 

 lower subscapular nerve produces no discoverable disability, as the 

 latissimus dorsi has a precisely similar action to that of the teres major. 



The Thoraco-dorsal (Long Subscapular) Nerve supplies the 

 latissimus dorsi muscle, which is mainly responsible for the 

 formation of the posterior axillary fold. This muscle has a 

 wide origin in the lower part of the back, and it narrows as it 

 passes to its insertion into the medial lip of the intertubercular 

 sulcus (bicipital groove). When both the thoraco-dorsal and 

 the lower subscapular nerves are paralysed, the movement of 

 extension at the shoulder-joint is extremely weak, as it is then 

 performed almost entirely by the posterior fibres of the 

 deltoid, since the infra-spinatus and the teres minor do not 

 act at good mechanical advantage. The posterior fold of the 



