172 THE ARTICULATIONS OF THE UPPER LIMB. 



posterior ligament is supported, and drawn up during extension of the joint, by the 

 triceps muscle which is closely adhc. ent to it near the insertion into the ulna. 



The synovial membrane extends upwards on the humerus so far as to line 

 the fossae for the coronoid and olecranon processes, and is loose and vascular in the 

 latter positions. It is also prolonged round the neck of the radius, and lines the 

 annular ligament. A small crescentic fold projects between the marginal part of 

 the head of the radius and the capitellum. Between the posterior ligament 

 and the synovial membrane is a mass of fat projecting into the olecranon fossa, 

 and two smaller pads are placed beneath the anterior ligament over the coronoid 

 and radial fossae. 



Movements. Flexion and extension are the chief movements which take place between 

 the humerus and ulna ; and in the perfect limb these are arrested, flexion by the meeting- of 

 the soft parts of the arm and forearm, extension by the tightening of the ligaments and 

 muscles on the front of the joint, before the coronoid and olecranon processes meet the bottom 

 of their respective fossae on the humerus. The movements take place upon an axis which is 

 inclined at an angle of about 84 with the shaft of the humerus above and that of the ulna 

 below, so that while in the extended position of the joint these bones form an obtuse angle 

 open outwards, they are placed nearly opposite one another when the elbow is bent. 1 The 

 inner lip of the trochlea, being prominent in front, forms an expansion which corresponds to 

 an inward projection of the coronoid part of the ulnar surface ; while the outer lip of the 

 trochlea, being enlarged at the upper and back part, forms a surface which is only in use in 

 complete extension, and which then corresponds to a surface on the outer aspect of the 

 olecranon, which comes into contact with no other part of the humerus. Owing to a slight 

 degree of incongruence, the surface of the sigmoid cavity and the opposed portion of the 

 trochlear surface of the humerus are not in accurate contact over their whole extent, and a 

 very limited amount of lateral motion on the part of the ulna is allowed. In flexion and 

 extension the radius moves by its cup-shaped head upon the capitellum, and on the groove 

 between that eminence and the trochlea by a ridge internal to the cup. It is most completely 

 in contact with the humerus in the position of semi-flexion and semi-pronation. 



THE WRIST-JOINT AND ARTICULATIONS OF THE CARPUS. 



The RADIO-CARPAL ARTICULATION, orwrist- joint, is formed between the radius and 

 triangular fibro-cartilage above, and the scaphoid, lunar and pyramidal bones below. 

 The superior surface is concave both transversely and from before backwards ; the 

 inferior surface is correspondingly convex, and is prolonged farther down upon the 

 carpal bones behind than in front. The articular surface of the radius is subdivided 

 into two parts by a linear elevation ; an outer, triangular, for the scaphoid, and an 

 inner, quadrilateral, which, together with the triangular fibro-cartilage, corresponds 

 to the lunar bone. The small articular surface of the pyramidal is in most cases in 

 contact with a portion of the capsule of the joint. The latter is rather loose, and is 

 divided into two lateral, anterior and posterior ligaments. 



The internal lateral ligament is a rounded cord passing downwards from, 

 the styloid process of the ulna to the pyramidal bone, and by its anterior fibres to 

 the pisiform bone. 



The external lateral ligament extends from the styloid process of the radius 

 to a depression on the scaphoid bone between the radial articular surface and the 

 tuberosity. 



The anterior ligament, broad and membranous, consists partly of fibres which 

 have a nearly transverse direction, partly of others which diverge as they descend 

 from the anterior border of the radius to the scaphoid, lunar, and pyramidal bones ; 

 some of them are continued to the os magnum. On the inner side a strong bundle 

 springs from the root of the styloid process of the ulna and passes to the pyramidal 

 and lunar bones. 



1 Braune and Kyrklund, Arch. f. Anat., 1879. 



