138 



OSTEOLOGY. 



Fig. 92. Plan of the Development of the Ulna. 

 By 3 Centres. 



Agpea rs at lO^y^ 



etc /ft.* y' 



outwards, is narrow, convex, and received into the sigmoid cavity of the radius. 

 The stylo id process projects from the inner and back part of the bone, and descends 

 a little lower than the head, terminating in a rounded summit, which affords 

 attachment to the internal lateral ligament of the wrist. The head is separated 

 from the styloid process by a depression for the attachment of the triangular 

 interarticular fibro-cartilage ; and behind, by a shallow groove for the passage 

 of the tendon of the Extensor carpi ulnaris. 



Structure. Similar to that of the other long bones. 



Development. By three centres: one for the shaft, one for the inferior extremity, 

 and one for the olecranon (fig. 92). Ossification commences near the middle of 



the shaft about the fifth week, and soon 

 extends through the greater part of the 

 bone. At birth, the ends are cartilaginous. 

 About the fourth year, a separate osseous 

 nucleus appears in the middle of the head, 

 which soon extends into the styloid pro- 

 cess. About the tenth year, ossific matter 

 appears in the olecranon near its extremity, 

 the chief part of this process being formed 

 from an extension of the shaft of the bone 

 into it. At about the sixteenth year, the 

 upper epiphysis becomes joined, and at 

 about the twentieth year the lower one. 



Articulations. With the humerus and 

 radius. 



Attachment of Muscles. To the olecra- 

 non : the Triceps, Anconeus, and one head 

 of the Flexor carpi ulnaris. To the coro- 

 noid process; the Brachialis anticus, Pro- 

 nator radii teres, Flexor sublimis digito- 

 rum, and Flexor profundus digitorum. To 

 the shaft: the Flexor profundus digitorum, 

 Pronator quadratus, Flexor carpi ulnaris, 

 Extensor carpi ulnaris, Anconeus, Supi- 

 nator brevis, Extensor ossis metacarpi 

 pollicis, Extensor secundi internodii polli- 

 cis, and Extensor indicis. 



Afrpeart at 4 



-Joint Slirrft at 



THE EADIUS. 



The Eaclius, so called from its fancied resemblance to the spoke of a wheel, is 

 situated on the outer side of the forearm, lying parallel with the ulna, which 

 exceeds it in length and size. Its upper end is small, and forms only a small part 

 of the elbow-joint; but its lower end is large, and forms the chief part of the 

 wrist. It is 'one of the long bones, having a prismatic form, slightly curved 

 longitudinally, and presenting for examination a shaft and two extremities. 



The Upper Extremity presents a head, neck, and tuberosity. The head is of 

 a cylindrical form, depressed on its upper surface into a shallow cup, which 

 articulates with the radial or lesser head of the humerus in flexion of the .joint. 

 Around the circumference of the head is a smooth articular surface, coated with 

 cartilage in the recent state, broad internally where it rotates within the lesser 

 sigmoid cavity of the ulna, narrow in the rest of its circumference, to play in the 

 orbicular ligament. The head is supported on a round, smooth, and constricted 

 portion of bone, called the neck, which presents, behind, a slight ridge, for the 

 attachment of part of the Supinator brevis. Beneath the neck, at the inner and 

 front aspect of the bone, is a rough eminence, the tuberosity. Its surface is 

 divided into two parts by a vertical line ; a posterior rough portion, for the 



