PRACTICAL CONSIDERATIONS: THE ULNA. 



285 



attachment of the fibro-cartilage. The styloid process is a short, slender process 

 running down from what may be called the posterior internal angle of the lower 

 end. There is a distinct groove between the styloid process and the head on the 

 posterior aspect, and sometimes a faint one in front, transmitting respectively the 

 tendons of the extensor and the flexor carpi ulnaris. 



Structure. — There is much solid bone in the shaft, and altogether the ulna is 

 a strong-walled bone. Many plates near together from the anterior surface pass 

 upward under the coronoid process to the middle of the greater sigmoid notch. 

 The best-marked system of plates in the coronoid is in the main parallel to these. 

 The greater sigmoid notch is bounded by compact substance. Sagittal sections 

 show plates radiating from it, some of which form arches near the top of the 

 olecranon with others from the posterior surface. The ]iead is composed of spongy 

 tissue within thin walls. 



Development. — The centre for the shaft appears in the eighth week, from 

 which practically all the bone except the lower end is developed. At about five. 



Ossification of ulna. A, at birth ; B, at five years; C, at ten years ; £>, at about sixteen years, a, centre for 



shaft ; d, c, cartilaginous epiphyses ; d, centre for lower epiphysis ; e, for upper epiphysis. 



one appears for the head and styloid process ; and at about ten, one for the top of 

 the olecranon. This fuses at about sixteen ; the lower end joins the shaft at eighteen. 



PRACTICAL CONSIDERATIONS. 



The ulna may be absent, or may be more or less defective in size or shape. 

 Such deformities are not common. Fracture of the olecranon at its junction with 

 the shaft, where it is narrowed, is frequent. The degree of displacement is largely 

 determined, as in the parallel case of the patella, by the amount of laceration of the 

 enveloping fibrous structure (Fig. 585). If this is great, the triceps strongly elevates 

 the fractured process. Occasionally the mere tip of the olecranon, or even a thin 

 portion of the superficies only, may be separated either by muscular action or by 

 direct violence. 



The epiphyseal line is above the constriction that marks the union of the olec- 

 ranon with the shaft. The epiphysis is small and includes the upper part of the 

 olecranon with the insertion of the triceps, a part only of the attachment of the pos- 

 terior ligament, and a very small portion of the posterior triangular subcutaneous 

 surface. The epiphyseal line runs from the upper part of the sigmoid cavity in 

 front downward and backward. The epiphysis enters but little into the elbow-joint ; 

 it is largely within the limits of strong periosteal and tendinous and Hgamentous 

 expansions, is of small size, and before the fourteenth or fifteenth year is on a 



