270 



HUMAN ANATOMY. 



become one, making a cap for the top of the shaft, which latter extends into the 

 head. The largest centre for the lower end is that for the capitellum, which is seen 

 by the end of the first half-year. It forms also a part of the outer side of the 

 trochlea. A centre for the tip of the inner condyle is evident by the fifth year. One 

 or more minute points of ossification for the trochlea appear in the tenth year, and 

 one for the tip of the external condyle in the fourteenth. Although all these 

 epiphyses are originally in the same strip of cartilage, they do not unite into one 

 piece of bone. The capitellum is joined by the ossification for the trochlea, and joins 

 the shaft at from fourteen to fifteen. We are not sure whether the insignificant centre 

 for the outer condyle, which fuses at about the same time, joins the epiphysis or the 

 shaft. Rambaud and Renault seem to believe the latter. The centre for the internal 

 condyle remains separate after the rest are fused and joins the shaft at about eighteen. 

 The upper end joms at about nineteen, the line of union being lost at twenty or 

 twenty-one. It is usually lost earlier in the female. 



Surface Anatomy. — The external and internal condyles are the only points 

 that are truly subcutaneous. The outer is easily recognized under normal conditions, 

 but is quickly obscured by swelling. The internal is so prominent that it can always 

 be recognized, unless the joint has been utterly broken to pieces. The fact that the 

 inner condyle joins the shaft after the rest of the lower end exposes it to the danger of 

 being broken off before the union has occurred, or while it is still weak. The upper 

 end of the humerus is everywhere covered by muscle, but much of its outline can be 

 explored. The amount of its forward projection varies much ; but it always projects 

 outward beyond the acromion. The lesser tuberosity and the bicipital groove can 

 be recognized on rotating the bone, but indistinctly. The groove is filled by the 

 tendon and still further obscured by the capsule and muscles. The surgical neck is 

 best felt in the axilla, whence, the arm being extended, the head can be examined, 

 though imperfectly. 



PRACTICAL CONSIDERATIONS. 



The humerus occasionally fails to develop, either alone or together with the 

 other bones of the extremity. The bone of one arm may be shorter and thicker 



than the normal bone. Lengthening beyond normal 

 limits is much rarer. 



The shallowness of the glenoid cavity obviates the 

 necessity for projecting the head of the bone from the 

 shaft, as in the femur ; the "neck" is, therefore, merely 

 a very narrow and superficially shallow constriction of an 

 inward prolongation of the shaft between the tuberosities 

 below and the joint surface above. Both its shortness 

 and its shallowness render it far less liable to fracture than 

 the femoral neck. When, in old age, absorption and 

 fatty degeneration of the cancellous tissue have occurred, 

 fracture does take place, as a result usually of falls upon 

 the shoulder. It is often accompanied by impaction, the 

 head being driven into the broad surface of cancellated 

 tissue on the upper end of the lower fragment (Fig. 285 ). 

 This results in a lessening of the bulk of the upper end, 

 or subacromial portion, of the humerus, and thus in a 

 little flattening of the deltoid and a little increased promi- 

 nence of the acromion. If impaction does not occur, 

 and the capsule of the joint is completely torn through its 

 entire circumference, necrosis of the upper fragment 

 must follow. Usually, through untorn periosteum and 

 through portions of capsule reflected from the inner side 

 of the shaft below the anatomical neck to the edge of 

 the articular cartilage on the head, the blood-supply is 

 maintained so that necrosis is prevented and union results. 

 There is no direct blood-supply to the head of the humerus corresponding to that 

 received by the femoral head through the ligamentum teres. The displacement 



Head 



Shaft 



Fracture of anatomical neck of 

 humerus, showing impaction. 



