THE ULNA. ISH 
on the lower and outer part of the great sigmoid notch, so as to narrow it 
considerably. Separated from it by a rectangular curved edge, it displays a surface 
which is plane from above downwards, and concave from before backwards. Its 
anterior extremity is narrower and more pointed than its posterior, and becomes 
confluent with the anterior edge of the coronoid process, at which point the 
orbicular ligament, which retains the head of the radius in position, 1s attached in 
front. Its | posterior border, wider and more outstanding, lies in line, and is con- 
tinuous with the interosseous margin of the shaft. Behind this border, the 
orbicular ligament is attached posteriorly. 
The shaft of the ulna (corpus ulne), which is nearly straight, or but slightly 
curved, is stout and thick above, gradually tapering towards its lower extremity. 
It may be divided into two surfaces, a flexor and an extensor, by two well-defined 
borders, an external or interosseous (crista interossea), und a posterior (argo 
dorsalis), which latter is subcutaneous throughout its whole length. 
The outer, or interosseous border (crista interossea), 1s crisp and sharp in the 
upper three-fourths of the shaft, but becomes faint and ill-defined in the lower 
fourth. To this, with the exception only of the part which forms the posterior 
boundary of the bicipital hollow, is attached the interosseous membrane which 
connects the two bones of the forearm. The posterior border (margo dorsalis), of 
sinuous outline, curving outwards above, and slightly inwards below, is continuous 
superiorly with the triangular subcutaneous area on the back of the olecranon, 
being formed by the confluence of the borders which bound that surface; well 
marked above, it becomes faint and more rounded below, but may be traced down- 
wards to the posterior surface of the base of the styloid process. To this border is 
attached an aponeurosis common to the flexor carpi ulnaris, extensor carpi ulnaris, 
and flexor profundus digitorum muscles. A noteworthy feature in connexion with 
this part of the shaft is the fact that it is subcutaneous, and can easily be felt 
beneath the skin throughout its whole length. 
The flexor surface corresponds to the front and inner side of the shaft. It is 
frequently described as consisting of two surfaces, an anterior and an internal, 
which are separated by a rounder anterior border (margo volaris), which extends 
from the tubercle above towards the styloid process below. The prominence of 
this ridge varies in different bones, being well marked in bones of a pronounced 
type, but corresponding merely to the rounding of the surfaces in poorly developed 
specimens. ‘The flexor aspect of the bone affords an extensive origin to the flexor 
profundus digitorum muscle, which clothes its anterior and inner sides in its upper 
three-fourths, reaching as far hack as the posterior border, and extending upwards 
as high as the inner side of the coronoid process. immediately below the small 
sigmoid cavity there is a hollow triangular area, limited behind by the upper part 
of the interosseous crest, and defined in front by an oblique line which extends 
downwards and backwards from the outer margin of the coronoid process. In this 
hollow the bicipital tubercle of the radius rests when the forearm is in the prone 
position, and to its floor are attached the fibres of origin of the supinator brevis 
muscle. The lower fourth of the shaft is crossed by the fibres of the pronator 
quadratus muscle, which derives its origin from a more or less well defined crest, 
which winds spirally downwards and backwards towards the front of the root of the 
styloid process, and is continuous above with the so-called anterior border. 
The extensor aspect of the shaft lies posteriorly between the posterior border 
and the interosseous crest. At its upper part it is placed behind the great and 
small sigmoid cavities, extending on to the outer side of the olecranon. Here an 
area corresponding to the upper third of the length of the bone is marked off 
inferiorly by an oblique ridge which leaves the interosseous crest about an inch or 
more below the hinder edge of the small sigmoid cavity. Into this somewhat 
triangular surface the fibres of the anconeus are inserted. Below this the 
posterior surface is subdivided by a faint longitudinal ridge, the bone between 
which and the interosseous crest furnishes origins for the extensor ossis metacarpi 
pollicis, extensor longus pollicis, and extensor indicis muscles, in order from above 
downwards. The surface of bone between the posterior border and the afore-men- 
tioned longitudinal line is smooth and overlain by the extensor carpi ulnarié muscle. 
