290 AN INTRODUCTION TO ZOOLOGY 



distally with the groups of bones forming the carpus or tarsus as 

 the case may be. These groups consist primitively of ten bones 

 arranged in three rows. The first or proximal row comprises three 

 bones, the radiale and ulnare or the tibiale and fibulare, articulating 

 with the correspondingly named limb bones, and between them 

 in each case is the third element, an intermedium. The second 

 row consists of two central bones, the centralia ; in most animals 

 but one of these is represented either freely or fused, but there is 

 evidence to show that primitively there were two. In the distal 

 row are five bones, the distal carpalia or tarsalia, and again there is 

 frequently loss or fusion of these. Each of them is followed by 

 a metacarpal or metatarsal, which are longer bones contained within 

 the palm of the hand or sole of the foot. Following these are the 

 five digits composed of a series of phalanges. In each case the first 

 digit, the thumb or pollex and the big toe or hallux, is shorter than 

 the remainder and composed of but two phalanges, whereas the others 

 are each composed of three. Ihis, then, is the structure of the 

 primitive pentadactyl limb, but it also has definite relations to the 

 body as a whole. In its original position, when extended, it was 

 related to the main or vertebral axis in the same way as our own 

 arm would be if it were held straight out to the side at right angles 

 to the body with the thumb uppermost. An imaginary line drawn 

 lengthwise through the limb, i.e. through the proximal bone, the 

 intermedium and the middle digit, is regarded as the axis of the limb. 

 The parts of the limb on the same side as the radius and pollex or 

 tibia and hallux are termed pre-axial, those on the opposite side 

 e.g. ulna or fibula, etc., are post-axial. The digits are numbered one 

 to five from the preaxial side, and so we have in both limbs a 

 phalangeal formula of 2.3.3.3.3. In the primitive animals this was 

 the position of the limbs which were, however, bent or flexed at 

 right angles at the elbow or knee so as to allow the palm of the hand 

 or sole of the foot to reach the ground. The surface on the inner 

 side of the bend is termed the flexor surface, while that on the outer 

 side is the extensor surface. However, in the rabbit and in mammals 

 generally the limbs have undergone a certain amount of rotation. 

 Taking the arm from the position just described and swinging it to 

 the front, still at right angles to the body, it will be seen that in 

 order for the palm of the hand to touch the ground it is necessary 

 to turn the hand, and with it the forearm, through an angle of 90 

 so that the thumb, instead of pointing upwards, points inwards and 

 the palm downwards. In this position the hand is said to be in 

 pronation, while in the original position it is in supination. The fore 

 limb of Lepus therefore is permanently in a position of pronation, 

 i.e. the forearm and hand are twisted with regard to the arm. 



