XXI] SKELETON OF LIMBS 521 



those of the antebrachium are called radius and ulna, those of 

 the crus tibia and fibula (Fig. 251). 



The skeletons of the pes and manus are typically exactly the 

 same. Situated proximally close to the middle segment of the limb 

 is a transverse row of three small bones, the central one being called 

 the intermedium in both limbs, whilst the outer and inner are 

 named after the bones of the middle segment of the limb adjacent 

 to them. Thus we find in the wrist a radiale and ulnare and 

 in the ankle a tibiale and fibulare. Distal to this row of bones 

 there is a single central bone which probably belongs to the middle 

 ray, and still more distally situated a row of five small bones cor- 

 responding to the digits. This last row are denominated carp alia 

 in the wrist and tarsalia in the ankle. The individual bones 

 are called carpale (or tarsale) 1 5 in accordance with the digits 

 opposite which they are situated. 



In almost every case this typical skeleton of nine bones has 

 undergone some modification, owing either to the absence of some 

 bones or the fusion of others, but in the hind-limb of the lower 

 Amphibia it is exactly typical. In the higher Amphibia not only 

 has great reduction of the elements taken place but the radius and 

 ulna in the fore-limb and the tibia and fibula in the hind-limb have 

 coalesced, a groove only being left to show their primitive distinct- 

 ness. 



The primitive position of the limbs with reference to the trunk 

 is, from the study of development, assumed to be one in which they 

 are stretched out at right angles to it, with the inner surface of the 

 hand and the sole of the foot directed ventrally and in such a 

 position that a line joining the tips of the fingers is parallel to the 

 long axis of the body. If we suppose an imaginary line or axis to 

 run down the centre of each limb, we shall be able to distinguish a 

 pre-axial from a postaxial side. In the lower Amphibia the only 

 change from this position that has taken place in the hind-limb is 

 that each segment of the limb is bent at right angles on the one 

 which follows it. The fore-limb is bent similarly, but it is also 

 rotated backwards so that its upper segment is almost parallel to 

 the axis of the body, and the elbow points backwards. If this 

 position were maintained the first digit would become external; but 

 the manus in most cases is at the same time twisted forwards so 

 that the lower end of the radius lies internal to that of the ulna, 

 and the radius thus crosses the ulna in its course. In the higher 

 Vertebrata this twisting can be undone and the hand reverted to 



