232 ELEMENTARY ANATOMY. [LESS. 



of one bone only, as in the pectoral arch, we have the con- 

 currence of two (the pubis and ischium), separated from each 

 other by an interspace the obturator foramen. 



We have seen, however, that both a coracoid and a pre- 

 coracoid may exist on the ventral aspect of the pectoral girdle, 

 so that this difference also disappears on a more extended 

 survey. Such a survey nevertheless reveals some other differ- 

 ences, as nothing in the pectoral girdle clearly corresponds 

 with the marsupial bones, large ilio-pectineal processes, and 

 os cloacae, all of these being parts which are occasionally 

 developed in the pelvic girdle of different animals. 



The femur is constantly a single bone in all Vertebrates. 

 In man it differs from the humerus in that the radial (or 

 pre-axial) tuberosity is the larger of the two, while in the femur 

 the tibial (or pre-axial) trochanter is the smaller of the two. 

 But this difference disappears when we extend our view, as 

 the relative size of the trochanters varies in different forms, 

 as is the case with the tuberosities. 



22. The knee-joint is directed forwards, and thus markedly 

 differs from the elbow-joint. When, however, the rudi- 

 mentary pelvic limb first appears, the leg is so placed that 

 the knee-joint is directed outwards, thus agreeing with the 

 primitive position of the thoracic limbs. Later, the knee 

 becomes rotated forwards. 



In this latter position there is no necessity for any pronation 

 of the leg-bones to enable the sole of the foot to be applied to 

 the ground, and the absence of this necessity constitutes a 

 difference between the arm and the leg. We have seen, how- 

 ever, that a sort of rudimentary pro- and supi-nation becomes 

 possible in some Marsupials ; and in some other animals (as 

 the Flying Lemur, and especially the Tortoises) the position 

 of both the pectoral and pelvic limbs is similar through 

 the elbow, knee, and the extensor surfaces being all directed 

 outwards in both limbs. 



In Bats the thigh is turned backwards, so that the knee 

 bends backwards like an elbow ; and here, were it necessary 

 to apply the sole to the ground with the digits forwards, a 

 pronation of the leg-bones would be required. 



23. The skeleton of the leg, like that of the arm, is divisible 

 into a tri- and bi-digital series, placed side by side. 



Thus there is, first, the tibia ; the astragalus and naviculare ; 

 the three cuneiform bones ; the first, second, and third meta- 

 tarsals ; and the corresponding digits forming the tri-digital 

 series. 



