68 OSTEOLOGY. 



HUMERUS. 

 (PL. I. K.) 



The humerus is a long bone extending from the scapula to the 

 radius, in an oblique direction, downwards and backwards. Like 

 all long bones it possesses a shaft and two extremities. 



The shaft or body is characterised in most animals by a 

 more or less twisted appearance, most apparent externally and 

 anteriorly. We will describe it as presenting four surfaces. The 

 anterior surface, somewhat triangular in shape, with the apex 

 below, is roughened between its middle and inferior third, for 

 the insertion of the coraco-humeralis muscle. The posterior 

 surface, round and smooth, and not clearly divided from the 

 internal and external surfaces, terminates inferiorly in two 

 prominent ridges, the epitrochlea and epicondyle. The external 

 surface contains the musculo-spiral groove, which winds obliquely 

 downwards and forwards through the entire length of the shaft. 

 This groove, which is occupied by the humeralis obliquus 

 muscle, gives the twisted appearance to the bone ; it is 

 separated from the anterior surface by the deltoid ridge, which 

 extends from the outer part of the proximal end of the shaft 

 to the coronoid fossa, a depression in its antero-distal end. A 

 roughened prominence, the external tuberosity, on the upper 

 third of this ridge, bends backwards over the spiral groove, and 

 gives attachment to the teres externus muscle. The internal 

 surface has no distinct separation from the anterior and posterior 

 surfaces ; a rough prominence about its middle, the internal 

 tuberosity, gives insertion to the teres major and latissimus dorsi 

 muscles. The medullary foramen is in its inferior third. 



The proximal extremity presents the head and the trochanters. 

 The head is convex, and considerably larger than the glenoid 

 cavity with which it articulates, allowing extensive and varied 

 motion ; it is surrounded by a roughened border for the insertion 

 of the capsular ligament, below which is the constricted cervix or 

 neck of the bone. A rough depression in front of the head 

 contains several small foramina for the blood-vessels which supply 

 the cancellated tissue within. 



The external troclianter has two prominences, the anterior, or 

 summit of the troclianter, which forms the external boundary of 

 the bicipital groove, and gives insertion to the outer tendon of 



