150 THE SKELETON. 



nection with these fractures arc (1) that the musculo-spiral nerve maybe injured as it lies in the 

 groove on the bone, or may become involved in the callus which is subsequently thrown out ; 

 and (2) the frequency of non-union. This is believed to be more common in the humerus than 

 in any other bone : and various causes have been assigned for it. It would seem most probably 

 to be due to the difficulty that there is in fixing the shoulder-joint and the upper fragment, and 

 possibly the elbow-joint and lower fragment also. Other causes which have been assigned 

 for the non-union are: (1) that in attempting passive motion of the elbow-joint to overcome 

 any rigidity which may exist, the movement does not take place at the articulation, but at the 

 seat of fracture ; or that the patient, in consequence of the rigidity of the elbow, in attempting 

 to flex or extend the forearm moves the fragment and not the ioint. (2) The presence of small 

 portions of muscular tissue between the broken ends. (3) Want of support to the elbow, so 

 that the weight of the arm tends to drag the lower fragment away from the upper. An import- 

 ant distinction to make in fractures of the lower end of the humerus is between those that 

 involve the joint and those which do not ; the former always serious, as they may lead to 

 impairment of the utility of the limb. They include the T-shaped fracture and oblique frac- 

 tures which involve the articular surface. The fractures which do not involve the joint are the 

 transverse above the condyles and the so-called epitrochlear fracture, when the tip of the 

 internal condyle has broken off, generally from direct violence. 



Under the head of separation of the epiphysis two separate injuries have been described. 

 One where the whole of the four ossific centres which form the lower extremity of the bone are 

 separated from the shaft ; and secondly, where the articular portion is alone separated, the two 

 condyles remaining attached to the shaft of the bone. The epiphysial line between the shaft 

 and lower end runs across the bone just above the tips of the condyles, a point to be borne in 

 mind in performing the operation of excision. 



Tumors originating from the humerus are of frequent occurrence. A not uncommon place 

 for a chondroma to grow from is the shaft of the bone somewhere in the neighborhood of the 

 insertion of the deltoid. Sarcomata frequently grow from this bone. 



THE FOREARM. 



The Forearm is that portion of the upper extremity which is situated between 

 the elbow and the wrist. Its skeleton is composed of two bones, the ulna and 

 radius. 



The Ulna. 



The Ulna (Figs. 100, 101), so called from its forming the elbow (/ei^), is a 

 long bone, prismatic in form, placed at the inner side of the forearm, parallel with 

 the radius. It is the larger and longer of the two bones. Its upper extremity, of 

 great thickness and strength, forms a large part of the articulation of the elbow- 

 joint ; it diminishes in size from above downward, its lower extremity being very 

 small, and excluded from the wrist-joint by the interposition of an interarticular 

 fibro-cartilage. It is divisible into a shaft and two extremities. 



The Upper Extremity, the strongest part of the bone, presents for examination 

 two large, curved processes, the Olecranon process and the Coronoid process ; and 

 two concave, articular cavities, the greater and lesser sigmoid cavities. 



The Olecranon Process (ibUvy, elbow ; x^oawov, head) is a large, thick, curved 

 eminence situated at the upper and back part of the ulna. It is curved forward 

 at the summit so as to present a prominent tip which is received into the olecranon 

 fossa in extension of the forearm ; its base being contracted where it joins the shaft. 

 This is the narrowest part of the upper end of the ulna, and, consequently, the 

 most usual seat of fracture. The posterior surface of the olecranon, directed back- 

 ward, is triangular, smooth, subcutaneous, and covered by a bursa. Its upper 

 surface is of a quadrilateral form, marked behind by a rough impression for the 

 attachment of the Triceps muscle ; and in front, near the margin, by a slight trans- 

 verse groove for the attachment of part of the posterior ligament of the elbow-joint. 

 Its interior surface is smooth, concave, covered with cartilage in the recent state, 

 and forms the upper and back part of the great sigmoid cavity. The lateral 

 borders present a continuation of the same groove that was seen on the margin of 

 the superior surface ; they serve for the attachment of ligaments ; viz., the back 

 part of the internal lateral ligament internally, the posterior ligament externallv. 

 lo the inner border is also attached a part of the Flexor carpi ufnaris, while to the 

 outer border is attached the Anconeus. 



