44 THE SURGICAL ANATOMY OF THE HORSE 



THE HUMERUS 



This is a massive long bone which extends obhquely downwards and 

 backwards between the shoulder and elbow joints. The upper extremity is 

 made up of an articular head, which is placed posteriorly ; two tuberosities, 

 namely, an outer and inner, each of which has an anterior and a pos- 

 terior division ; and the bicipital groove through which the tendon of 

 origin of the biceps muscle plays, and which is placed between the 

 anterior divisions of the two tuberosities. 



The shaft is peculiar in presenting a twisted appearance, owing to 

 the presence of the musculo-spiral groove, which may be said to 

 commence at the back of the upper extremity just below the articular 

 head. It then winds round the outer surface of the bone, and terminates 

 at the front of the inferior extremity in a depression called the coronoid 

 fossa. The musculo-spiral groove accommodates the brachialis anticus 

 muscle. Above the groove on the outer surface there is a prominent 

 projection which curves slightly backwards. This is the deltoid tubercle, 

 and it gives attachment to the tendon of insertion of the deltoid muscle. 

 From this tubercle a ridge bearing the same name extends upwards to 

 the external tuberosity. Near the junction of the upper and middle 

 thirds, on the inner aspect of the bone, is a slightly projecting piece of 

 bone. This is the internal tubercle which gives attachment to the 

 tendons of insertion ot the teres major and latissimus dorsi muscles. 



Posteriorly, at the lower extremity, there are two thick ridges. 

 These are the outer and inner condyles, and they blend with one 

 another superiorly. As we descend, however, they diverge, for whilst 

 the inner one is disposed parallel to the long axis of the shaft, the outer 

 runs obliquely downwards and outwards. There is placed between them 

 a deep fossa, to which the name olecranon is given, and which accom- 

 modates the olecranon process of the ulna during extreme extension of 

 the elbow joint. 



