PRACTICAL CONSIDERATIONS : THE HUMERUS. 



271 



is apt to be slight, the muscles inserted into the bicipital groove and acting on 

 the lower fragment being antagonized by those mserted into the greater tuberosity. 

 That tuberosity may be torn off as a rare accident. The displacement — theoreti- 

 cally — will depend upon the action of the muscles inserted into that portion of the 

 bone (page 590). 



The large upper epiphysis of the humerus (made up of centres for the head and 

 the tuberosities which begin to coalesce about the sixth year) is fully formed by the 

 age of puberty. It includes then the two tuberosities, the upper fourth of the bicipi- 

 tal groove, all of the head, the anatomical neck, and a little of the shaft just below it. 

 A line nearly horizontal and crossing the bone beneath the great tuberosity, and 

 therefore considerably below the anatomical neck, represents the epiphyseal line at 

 the twentieth year, when the epiphysis and shaft become united. It is within a half 

 inch of the so-called surgical neck (Fig. 286). 



The lower surface of the epiphysis is concave and the upper surface of the 

 diaphysis convex or conical (Fig. 287). 



Fig. 287. 



Fig. 286. 



Upper end of humerus, showing epiphyseal line. A, on surface; 

 £, in section. 



Upper end of humerus, showing cupping 

 01 epiphysis to receive the pointed end of 

 diaphysis. 



The traumatic separation of this epiphysis is a not infrequent accident of child- 

 hood and adolescence. It is commonly caused by forcible traction of the arm 

 upward and outward. In such cases three anatomical factors probably enter into 

 the production of the lesion, (i) The partial fixation of the epiphysis by the sub- 

 scapularis, supra- and infraspinatus, and the upper fibres of the teres minor. Even 

 on the dead subject, rotation outward with abduction will most readily produce the 

 disjunction. (2) The ease with which the periosteum, strongly attached to the 

 epiphysis but very loosely to the diaphysis, may be separated from the latter. This 

 is illustrated by the fact that in cases of detachment the teres minor, though inserted 

 below the epiphyseal line, is apt to retain its connection with the periosteum covering 

 the epiphysis. (3) The powerful muscles resisting abduction and inserted into the 

 diaphysis just below the epiphyseal line. 



There may be only separation with little or no displacement ; but if displace- 

 ment occurs, the muscles just alluded to (the latissimus, pectoral, and teres) tend to 



