PHYLUM OHORDATA 



321 



division composed of two bones — the radms and ulna, and a distal 

 division or mamos. In the natural position of the parts the humerus 

 is directed, from the glenoid cavity with which it articulates, 

 backwards, upwards and outwards ; the radius and ulna pass 

 from their articulation with the humerus downwards and slightly 

 forwards, while the manus has the digits directed forwards 

 and outwards. When the limb is extended at light angles 

 to the long axis of the trunk, it presents, like that of the Frog, 

 dorsal and ventral surfaces, and pre-axial and post-axial borders. 

 In this position the radius is seen to be pre-axial, the ulna post- 

 axial. In the natural position the pre-axial border of the humerus 



Fig, 9()3.— Pectoral arch and sternum of Iiacerta agilis. (■; clavicle ; cor. coracoid ; 

 ep. cm: epicoracoid ; epuL episternuni ; fjhn. glenoid cavity for head of humerus ; 

 2yi'- cor. procoracoid ; — ri. — i-^. fii-st to fourth sternal ribs ; ^c. scapula ; st. sternum ; 

 supra, sc. suprascapula. (After Hoffmann.) 



is external, and the distal end of the forearm is rotated in such a 

 way that, while the pre-axial border looks forwards and out- 

 wardp at the proximal end, it faces directly inwards at its distal 

 end, the manus being rotated so that its pre-axial border looks 

 inwards. 



The humerus is a long bone consisting of a shaft and two ex- 

 tremities, each of the latter being formed of an epiphysis of calcified 

 cartilage, the proximal rounded, the distal (trochlea) pulley-like with 

 two articular surfaces, one for the radius and the other for the ulna. 

 The racZms is a slender bone consisting, like the humerus, of a shaft 

 and two epiphyses; the distal extremity has a concave articular 

 surface for the carpus, and is produced pre-axially into a radial 

 styloid process. The proximal end of the ulna is produced into an 

 upwardly directed process — the olecranon : the distal end bears a 



