FEACTUEES OF THE HUMEEUS. 87 



liability for it to be either injured at the time of the fracture by 

 the broken ends of the bone, or to become involved in the callus 

 which quickly forms around the fragments. The first lesion 

 produces what is called primary paralysis, and the second 

 produces what is termed secondary paralysis, and is the more 

 common of the two. The effect of this paralysis is to produce 

 wrist -drop, and it is therefore readily recognisable. 



The musculo-spiral nerve is best exposed behind the middle of 

 the shaft of the humerus by an incision which is made posteriorly 

 exactly in the middle in all directions, that is, half way between 

 the apex of the shoulder, and the tip of the elbow, and half way 

 between the outer and the inner sides of the arm. If the 

 incision be deepened through the substance of the triceps the 

 nerve is exposed without difficulty. It is important to remember, 

 when the nerve has been freed by operation from the callus, that 

 a layer of muscular tissue derived from the adjacent triceps 

 should be inserted between the nerve and the bed of callus from 

 which it has been lifted, otherwise the nerve may be again 

 caught. When implicated in fractures of the shaft of the 

 humerus below its middle, probably the best incision to expose 

 the nerve is one placed obliquely across the centre of a line 

 drawn from the insertion of the deltoid to the outer condyle of 

 the humerus, when the nerve will be found lying between the 

 brachio-radialis (supinator longus) and the brachialis anticus. 

 If the incision is made too internal, the external cutaneous nerve 

 may be exposed and mistaken for the musculo-spiral. 



It is interesting to note that the humerus is more frequently 

 broken by muscular (indirect) violence than perhaps any other 

 bone, with the exception of the patella. This is commonly the 

 outcome of a great effort in throwing, and the shaft snaps just 

 below the deltoid insertion. The contraction of the muscle 

 suddenly arresting the movement of the bone at this spot, the 

 impetus already gained carries the lower end of the bone and the 

 attached forearm forwards, and under the strain induced, the 

 humerus gives way. 



Fracture of the lower end, Several fractures may occur in 



