96 THE CAT. [CHAP. iv. 



rather sharp inner or radial margin, to which a membrane (the 

 interosseous membrane) is attached, which connects the shaft of the 

 ulna with that of the radius. 



The upper end of the ulna presents a deep concavity (gs) for 

 articulation with the trochlea of the humerus. This fossa is called 

 from its shape, the great sigmoid cavity, and is divided unequally 

 by an ill-defined vertical prominence which extends between the two 

 processes which bound the fossa above and below respectively. 



The lower of these two processes is called the coronoid process (c), 

 and is received into the corresponding fossa on the front of the 

 humerus. The inferior surface of its apex is rough, and serves for 

 the insertion of the brachialis anticus muscle. 



The higher and much larger process is termed the olecranon (o), 

 and passes into the olecranal cavity on the back of the humerus 

 when the limb is stretched out. The olecranon forms the prominence 

 of the elbow, and terminates in a rough process or " tuberosity," 

 into which the triceps muscle is inserted. 



On the outer side of the coronoid process is a small, elongated 

 concave, articular surface called the lesser sigmoid cavity (Is), destined 

 for the border of the head of the radius which turns upon it. 



The lower end of the ulna presents a small, rounded, convex 

 surface or head, which articulates with the adjacent surface of the 

 radius. On the opposite side there is developed a large, laterally 

 compressed prominence called the styloid process (s), which directly 

 articulates with the wrist. 



6. The CARPUS consists of seven small bones arranged in two 

 transverse series. 



The bones of the upper or proximal row are : (1) The scapho- 

 lunar bone ; (2) the cunei/orme ; (3) and the pisiforme. 



The first two together form an upper convex surface which fits 

 into the distal articular cup of the fore-arm. The carpus has a 

 convex dorsal surface, while its palmar surface is concave from side 

 to side. 



The scapho-lunar bone (Fig. 57, si) is the largest of all. Its long 

 axis is transverse. Above, it is smooth and mainly convex, and joins 

 the radius. Below, it fits into a depression formed by the four distal 

 carpals. From the hinder part of the outer side of the bone a 

 tubercle projects outwards and backwards. To this tubercle there 

 may be attached a small bone which is found within the tendon of 

 a muscle here inserted. Such a bone is called a " sesamoid bone/' 



The cuneiforme (c) is somewhat wedge-shaped, and articulates 

 below with the innermost, or most ulnar, carpal only. 



The pisiforme (p) is a bone which projects freely backwards and 

 downwards from the palmar surface, having a long, compressed and 

 curved process, which ossifies separately as an epiphysis (see Fig. 60, 

 pe). It is mainly supported by the cuneiform, but also articulates 

 with the unciform. 



The bones of the distal row are: (1) the trapezium; (2) the 

 trapezoidcs; (3) the os magnum; and (4) the unciforme. 



