PLATE XIV. 

 THE BACK OF THE ARM AND MUSCULO-SPIRAL NERVE. 



This plate shows a dissection to expose the musculo-spiral 

 nerve as it lies in the musculo-spiral groove. The long and external 

 heads of the triceps have been detached from the common tendon 

 In-low, and turned outwards; the fascial expansion, which comes 

 off from the lower and external part of the triceps, and covers the 

 anconeus, has been partly reflected to expose that muscle. The nerve 

 to the anconeus has been traced through the fibres of the internal 

 head of the triceps. 



The cutaneous nerve-supply of the back of the arm is shown 

 in the adjoining diagram : (a) Acromial branches of cervical 

 nerves ; (b) cutaneous branches from circumflex ; (c) upper, and 



(d) lower external cutaneous branches of the musculo-spiral; 



(e) biternal cutaneous branch of the musculo-spiral ; (/) intercosto- 

 humeral; (g) nerve of Wrisberg; (h) posterior branch of the 

 internal cutaneous. 



The close proximity of the musculo-spiral nerve to the bone, as it lies in the 

 musculo-spiral groove, should be noted, as it is thus rendered liable to pressure- 

 injuries: e.g. pressure of a badly-made crutch may produce paralysis, the so-called 

 ' crutch-palsy.' 



During the dissection of this region, do not forget to examine the course of the 

 ulnar nerve behind the internal condyle, which it grooves, lying against the internal 

 lateral ligament before it passes into the forearm between the two heads of the flexor 

 carpi ulnaris. Whilst in the groove it is covered by a band of fibrous tissue which 

 passes across from the back of the condyle to the olecranon : this ia the degenerate 

 remains of a muscle, the epitrochleo-anconeus, which exists pretty constantly in the 

 lower mammals, including a few of the primates, but is not seen in the anthropoid apes. 



It is also important to remember the relation of the ulnar nerve to the joint in 

 excising the elbow, lest it should be damaged during the operation. 



