CHAPTER IV 



THE SKELETON OF THE F0RE-LDII3 



What exactly we are to regard as the most simple con- 

 dition of the actual skeletal structures of the fore-limb 

 of the primitive land-living VertelDrate is, of course, open 

 to some doubt; that we shall not be very far wrong in 

 assuming it to be generally similar to that which is present 

 in the most generalized Amphibians and Reptiles is 

 certain. At the time of mammalian divergence from the 

 Therapsid ancestor, we may assume the limb to be of 

 this primitive Reptilian t^q^e, with the added tendency 

 to a general lengthening, to which Broom has attached 

 so much importance. Tn such a primitive limb there is 

 a proximal humerus free to move upon the pectoral 

 girdle at the shoulder-joint. The next segment consists 

 of a pre-axial radius and a post-axial ulna, both bones 

 articulating with the humerus at the elbow-joint, and at 

 that joint both are flexed and extended on the humerus. 

 Each bone is free of the other, so that movements of 

 rotation can take 2^1ace between them. Both bones of 

 the second segment articulate with the first row of the 

 carpal bones, so that although the hand is flexed and 

 extended on the forearm, it is rotated with the forearm. 



Discussions as to w^hat is the primitive condition of the 

 carpus, and how this j^i'iii^itive condition has been de- 

 parted from in different types, open up the possibilities 

 for widely divergent views. We will here ado2:)t the 

 oldest and simplest teaching — that of Gegenbaur, which 

 is backed by the greatest weight of evidence. 



Three bones enter into the formation of the first row 



23 



