192 PHYLUM CHORDATA 



The proximal division, or upper arm, is composed of a single 

 bone, the humerus. The head of it, which is cartilaginous, fits 

 in the glenoid cavity and forms the shoulder joint. At the distal 

 end is a large round projection, on each side of which is a ridge 

 forming the articular surface for the bone of the next division. 



The middle division, or forearm, is composed of a single bone, 

 the radioulna. It is formed by the fusion of the radius and ulna, 

 the two bones which are present in the forearm of most verte- 

 brates. The larger part of this bone is the radius. Its proximal 

 end is concave, the projecting process on it being the olecranon, 

 or elbow. Its distal end has two articular surfaces. 



The distal division is composed of the carpus, or wrist, and the 

 hand. The carpal bones are six in number, arranged in two rows, 

 a proximal and a distal row. The hand is made up of five digits, 

 of which the first digit, or thumb, is very small and rudimentary. 

 Each of the other four digits is composed of two parts : the 

 metacarpus, the long proximal bone which articulates with the 

 carpus ; and the phalanges, two or three small bones which form 

 the finger. The thumb contains a metacarpus alone. 



Exercise 24. Draw an outline sketch of the pectoral girdle and breast- 

 bone, representing them in one plane, also the foreleg in outline, 

 showing accurately all the bones and cartilages; label them all 

 carefully. 



The Posterior Appendages. These consist of the hind legs, and 

 the pelvic girdle which joins them with the trunk. 



The pelvic girdle, like the pectoral, is composed of a right and 

 a left half which meet ventrally and form an arch. The dorsal 

 ends of the arch articulate with the last vertebra of the spinal 

 column, and at its ventral end, on each side, is the acetabulum, 

 the articular surface of the hind leg. Extending backward from 

 the last vertebra between the two sides of the pelvic girdle is the 

 long bone called the urostyle, which forms the hinder part of the 

 spinal column. 



Each half of the pelvic girdle is composed of two portions, a 

 dorsal and a ventral portion. The former consists of the long, 

 slightly arched ilium, which forms the side of the arch and 



