XI INTRODUCTION TO TETRAPODA . AMPHIBIA 405 



tip of the middle finger is taken as the axis of the Hmb. The head- 

 ward margin of the hmb in this position is said to be its pre-axial 

 margin while the opposite margin is said to be post-axial. The thumb 

 is thus the pre-axial digit : the little finger the post-axial. When 

 the digits are indicated by numbers the numbering starts from the 

 pre-axial side : thus the first digit is the thumb, the fifth the little finger. 



Each half of the limb-girdle has the form of an inverted Y with a 

 more or less rounded joint surface at the junction of the three branches 

 for the attachment of the limb-skeleton. The single dorsal branch of 

 the girdle is termed in the case of the fore-limb the scapula, in the hind- 

 limb the ilium. Of the two ventral branches the anterior is termed in 

 the fore-limb the precoracoid, in the hind-limb the pubis : the posterior 

 branch is termed in the fore-limb the coracoid, in the hind-limb the 

 ischium. 



The pelvic girdle is given firmness by the dorsal end of the ilium 

 bearing against one or more (" sacral ") vertebrae, while in the case of 

 the pectoral girdle the tips of the coracoid and precoracoid are attached 

 to the sternum or breast-bone. 



The limb of the tetrapod is differentiated into three regions represented 

 in the human being respectively by (i) the upper arm or thigh, (2) the 

 forearm or leg and (3) the hand or foot (cf. Figs. 177 and 180) and each 

 of these has its characteristic skeletal supports, the whole constituting 

 what has been termed the cheiropteryglum (Huxley) in contradistinction 

 to the " ichthyopterygium " or type of skeleton found in the paired fins 

 of fish. 



Articulating directly with the limb-girdle is a long bone — the humerus 

 (fore-limb) or femur (hind-limb). To the end of this (elbow- or knee- 

 joint) are attached two other long bones — the radius and ulna in the case 

 of the fore-limb, the tibia and fibula in the case of the hind-limb. These 

 bones are primitively parallel to one another but the primitive splay- 

 footed condition of the fore-limb tends to become modified by its foot 

 being rotated inwards so as to be more directly underneath the body, 

 and this causes the radius to be twisted round in front of the ulna so as 

 to lie across it in X-fashion. 



In the foot itself there is first a group of small bones forming the 

 carpus ( = wrist) or tarsus (= ankle). Beyond these are the metacarpals 

 or metatarsals and beyond these in turn are the phalanges contained 

 within the digits — usually two in number in the case of the first digit 

 and three in the others. 



