xv.] GENERAL CHARACTERS, 269 



side of the shaft of the bone, and indicating the course of 

 the musculo-spiral nerve. 



The whole of the shaft of the humerus is developed 

 ectosteally from a single centre of ossification, but at each 

 extremity there is a large epiphysis : the upper one includes 

 the head and both tuberosities, and is usually formed by the 

 coalescence of two distinct endosteal nuclei ; the lower one 

 includes the whole inferior articular surface with the condyles, 

 and is formed of three or four originally distinct centres of 

 ossification. The lower epiphysis unites to the shaft before 

 the upper one. 



The skeleton of the second segment of the upper limb, 

 the fore-arm or antibrachium, consists of two bones called 

 radius and ulna, placed side by side, articulating with the 

 humerus at their proximal, and with the carpus at their 

 distal, extremity. 



In their primitive or unmodified condition, these bones* 

 may be considered as placed one on each border of the 

 iimb, the radius being preaxial, and the ulna postaxial. 1 

 The radius articulates above with the preaxial (external) 

 side of the humerus, the ulna with the postaxial (internal) 

 side of the humerus. 



This position is best illustrated in the fore-limb of the 

 Cetacea (see Fig. 104), where the two bones are fixed side by 

 side and parallel to each other, the preaxial border being 

 external, and the postaxial border internal, in their whole 

 extent. 



In the greater number of Mammals, the bones assume a 



1 So termed (by Prof. Huxley) in relation to the assumed central axis 

 of the limb, when it is extended out from the body in a certain stage of 

 its embryonic position, the extensor surface of the arm and back of the 

 hand being upwards or dorsal, and the flexor surface of the arm and palm 

 of the hand being downwards or ventral. 



