184 THE OS HUMERt. 



is in the most natural posture, is equally broad, and has an 

 obtuse smooth head rising from it forwards. From the rough 

 part of the condyle, several muscles arise ; and on the smooth 

 head the upper end of the radius plays. The internal condyle 

 is more pointed and protuberant than the external, to give origin 

 to the flexor muscles of the wrist and hands, &c. Between 

 the two condyles, is the trochle^i, or pulley ; which consists of 

 two lateral protuberances and a middle cavity that are smooth, 

 and covered with cartilage. When the forearm is extended, the 

 tendon of the internal brachialis muscle is lodged in the forepart 

 of the cavity of this pulley. The external protuberance, which 

 is less than the other, has a sharp edge behind : but forwards, 

 this ridge is obtuse, and only separated from the little head, 

 already described, by a small fossa, in which the adjoining edges 

 of the ulna and radius move. The internal protuberance of the 

 pulley is largest and highest ; and therefore, in the motions of 

 the ulna upon it, that bone would be inclined outwards, were 

 it not supported by the radius on that side. Between this in- 

 ternal protuberance and condyle, a sinuosity may be remarked, 

 where the ulnar nerve passes. 



The substance and the internal structure of the os humeri 

 are the same, and disposed in the same way, as in the other long 

 bones. 



The round head, at the upper end of this bone, is articulated 

 with the glenoid cavity of the scapula ; which being superficial 

 and having long ligaments, allows the arm a free and extensive 

 motion. These ligaments are, however, considerably strong. 

 For, besides the common capsular ligament, the tendons of the 

 muscles perform the office, and have been described under the 

 name of ligaments. Then the acromion and coracoid process,* 

 with the strong broad ligaments stretched between them, secure 

 the articulation above ; where the greatest and most frequent 

 force is applied, to thrust the head of the bone out of its place. 

 It is true, that there is not near so strong a defence in the lower 

 part of the articulation ; but, in the ordinary postures of the 

 arm, that is, so long as it is an acute angle with the trunk of 



