THE OS HUMERI. 183 



inside, about its middle, by the belly of the biceps flexor cubiti. 

 In the middle of this plane surface, the entry of the medullary 

 artery is seen slanting obliquely downwards. At the fireside 

 of this plane, the bone rises in a sort of ridge, which is rough, 

 and often has a great many small holes in it, where the strong 

 deltoid muscle is inserted ; on each side of which the bone is 

 smooth and flat, where the brachialis internus rises. The 

 exterior of these two flat surfaces is the largest : behind it is a 

 superficial spiral channel, formed by the muscular nerve, and the 

 vessels that accompany it ; it runs from behind forwards and 

 downwards. 



The body of the os humeri is flattened behind by the exten- 

 sors of the forearm. 



Near the lower end of this bone, a large sharp ridge is 

 extended on its outside ; from which the musculus supinator 

 radii longus, and the longest head of the extensor carpi radi- 

 alis, arise. Opposite to this there is another small ridge to 

 which the aponeurotic tendon, that gives origin to the fibres of 

 the internal and external brachial muscles, is fixed ; and from 

 a little depression on the foreside of it, the pronator radii teres 

 arises. 



The body of the os humeri becomes gradually broader 

 towards the lower end, where it has several processes ; at the 

 roots of which there is a cavity before, and one behind, called 

 sigmoid. The anterior is divide'd by a ridge into two ; the 

 external, which is the least, receives the end of the radius ; and 

 the internal receives the coronoid process of the ulna, in the 

 flexions of the forearm ; while the posterior deep triangular 

 cavity lodges the olecranon in the extensions of that limb. 

 The bone between these two cavities is pressed so thin by the 

 processes of the ulna, as to appear transparent in many sub- 

 jects. The sides of the posterior cavity are stretched out into 

 two processes, one on each side. These are called condyles ; 

 from each of which a strong ligament goes out to the bones of 

 the forearm. The external condyle, which has an oblique 

 direction forwards with respect to the internal, when the arm 



