THE BONES OF THE UPPER LIMB 199 



The shaft diminishes in size from above downwards, and is 

 triangular in its upper three-fourths, where it is slightly curved 

 with the convexity directed backwards. In the lower fourth it 

 is slender and subcylindrical, being flattened in front. It presents 

 three borders and three surfaces. The anterior border extends 

 from the flexor sublimis tubercle on the inner margin of the 

 coronoid process to the front of the styloid process. Over its 

 upper three-fourths it is round, and gives origin to fibres of the 

 flexor profimdus digitorum. Over its lower fourth it is sharp, 

 and gives origin to the pronator quadratus. The posterior border, 

 which is subcutaneous, extends from the apex of the triangular 

 subcutaneous surface on the back of the olecranon to the back of 

 the styloid process. Over its upper two- thirds it gives attachment 

 to a strong aponeurosis, which gives common origin to the flexor, 

 and extensor, carpi ulnaris, and flexor profundus digitonmi. The 

 external or interosseous border extends from the apex of the bicipital 

 hollow, about two inches below the small sigmoid cavity, to the 

 outer aspect of the head. Over the middle three-fifths of the 

 shaft it is sharp and prominent, but over the lower fifth it is very 

 faint. It gives attachment to the interosseous membrane. 



The anterior surface is situated between the anterior and inter- 

 osseous borders. It is concave over its upper three- fourths, and 

 gives origin to part of the flexor profundus digitorum. The 

 lower fourth is flat, and gives origin to the pronator quadratus. 

 This surface presents the medullary foramen a little above the 

 centre, the direction of the canal to which it leads being upwards. 

 It gives passage to the medullary branch of the anterior inter- 

 osseous artery. The internal surface is situated between the 

 anterior and posterior borders. Over its upper two-thirds it 

 gives origin to part of the flexor profundus digitorum, the lower 

 portion being subcutaneous. The posterior surface is situated 

 between the posterior and interosseous borders, and is directed 

 backwards and outwards. It presents superiorly the oblique line, 

 which extends from the supinator ridge on the posterior margin 

 of the bicipital hollow to the posterior border at the junction of 

 the upper and middle thirds. The triangular portion above this 

 line is called the anconeal surface, which extends over the outer 

 surface of the olecranon. It gives insertion to the anconeus. The 

 posterior surface, below the oblique line, is divided into two lateral 

 parts by a vertical ridge. The inner portion supports the extensor 

 carpi ulnaris, and the outer gives origin, from above downwards, to 

 the extensor ossis metacarpi pollicis, extensor longus pollicis, and 

 extensor indicis. On the outer aspect of the shaft superiorly there 

 is a triangular depression, which commences immediately below 

 the small sigmoid cavity, and extends downwards for about 2 inches. 

 It is bounded in front and behind by prominent lips, the anterior 

 of which passes above into the outer margin of the coronoid process, 

 and the posterior into the posterior margin of the small sigmoid 

 cavity. The upper part of the posterior lip, which is prominent. 



