256 THE SKELETON. 



the ulna, for the attachment of the oblique ligament. Its outer surface presents a 

 narrow, oblong, articular depression, the lesser sigmoid cavity. The inner surface, 

 by its prominent, free margin, serves for the attachment of part of the internal 

 lateral ligament. At the front part of this surface is a small, rounded eminence 

 for the attachment of one head of the Flexor sublimis digitorum ; behind the 

 eminence, a depression for part of the origin of the Flexor profundus digitorum ; 

 and, descending from the eminence, a ridge which gives attachment to one head 

 of the Pronator radii teres. Generally, the Flexor longus pollicis has an origin 

 from the lower part of the coronoid process by a rounded bundle of muscular fibres. 



The Greater Sigmoid Cavity, so called from its resemblance to the old shape of 

 the Greek letter 2", is a semilunar depression of large size, formed by the olecranon 

 and coronoid processes, and serving for articulation with the trochlear surface of 

 the humerus. About the middle of either lateral border of this cavity is a notch 

 which contracts it somewhat, and serves to indicate the junction of the two 

 processes of which it is formed. The cavity is concave from above downward, 

 and divided into two lateral parts by a smooth, elevated ridge whioiwruns from the 

 summit of the olecranon to the tip of the coronoid process. Of these .two portions, 

 the internal is the larger, and is slightly concave transversely; the external por- 

 tion is convex above, slightly con-cave below. The articular surface, in the recent 

 state, is covered with a thin layer of cartilage. 



The Lesser Sigmoid Cavity is a narrow, oblong, articular depression, placed on 

 the outer side of the coronoid process, and serving for articulation with the head 

 of the radius. It is concave from before backward, and its extremities, which 

 are prominent, serve for the attachment of the orbicular ligament. In the recent 

 state it is covered with a thin layer of cartilage. 



The Shaft, at its upper part, is prismatic in form, and curved from behind 

 forward and from without inward, so as to be convex behind and externally ; its 

 central part is quite straight ; its lower part rounded^ smooth, and bent a little 

 outward ; it tapers gradually from above downward, and presents for examination 

 three borders and three surfaces. 



The anterior border commences above at the prominent inner angle of the coro- 

 noid process, and terminates below in front of the styloid process. It is well marked 

 above, smooth and rounded in the middle of its extent, and aifords attachment to 

 the Flexor profundus digitorum : its lower fourth, marked off from the rest of the 

 border by the commencement of an oblique ridge on the anterior surface, serves 

 for the attachment of the Pronator quadratus. It separates the anterior from the 

 internal surface. 



The posterior border commences above at the apex of the triangular subcuta- 

 neous surface at the back part of the olecranon, and terminates below at the back 

 part of the styloid process ; it is well marked in the upper three-fourths, and 

 gives attachment to an aponeurosis common to the Flexor carpi ulnaris, the 

 Extensor carpi ulnaris, and the Flexor profundus digitorum muscles ; its lower 

 fourth is smooth and rounded. This border separates the internal from the 

 posterior surface. 



The external or interosseous border commences above by two lines, which con- 

 verge one from each extremity of the lesser sigmoid cavity, enclosing between them 

 a triangular space for the attachment of part of the Supinator brevis, and terminates 

 beloAv at the middle of the head of the ulna. Its two middle fourths are very pro- 

 minent, its lower fourth is smooth and rounded. This border gives attachment to 

 the interosseous membrane, except along its upper fourth, and separates the anterior 

 from the posterior surface. 



The anterior surface, much broader above than below, is concave in the upper 

 three-fourths of its extent, and affords attachment to the Flexor profundus digi- 

 torum ; its lower fourth, also concave, to the Pronator quadratus. The lower 

 fourth is separated from the remaining portion of the bone by a prominent ridge, 

 directed obliquely from above downward and inward ; this ridge (the oblique or 

 Pronator ridge] marks the extent of attachment of the Pronator quadratus above. 



