viii.] THE MUSCLES. 361 



seating normally certain special peculiarities, though such 

 differences are not absolutely constant, owing to individual 

 variation. Thus he is peculiar in the following points : He 

 has an extensor primi internodii pollids and a peronens ter- 

 tius. He has the flexor longus pollids disconnected from 

 the flexor proj c undus digitorum. His soleus arises both from 

 the tibia and the fibula, and his flexor brevis digitorum 

 springs entirely from the calcaneum. Moreover, the pero- 

 neal interosseus muscle of the second toe arises on the 

 dorsal side of the origin of the tibial interosseus of the 

 middle toe. 



29. In comparing the myology of the pectoral and pelvic 

 limbs of man, we find that owing to the diverse flexure of the 

 two limbs, the extensors are placed forwards in the lower 

 limb, backwards in the upper one. The flexors of the upper 

 limb easily bend the hand upon the arm, but those of the 

 lower cannot bend the foot upon the leg ; at the most they 

 straighten it a motion called " extension," but which is really 

 an imperfect " flexion." 



The serial homology of the appendicular muscles can only 

 be understood by imagining the limbs in their primitive 

 position, with the extensors outwards, the flexors inwards. 

 Then we have the supra- and infra-spinatus and the iliacus, 

 the extensor carpi radialis and tibialis anticus, all pre-axial ; 

 the sub-scapularis and glutei, the peronei and the extensor 

 ulnaris, all post-axial. 



The muscles inserted into the post-axial tuberosity of the 

 humerus lie in man on the inner side of the thoracic girdle 

 (sub-scapular surface), those inserted into the post-axial tro- 

 chanter of the femur lie on the outer side of the pelvic girdle 

 (gluteal surface). Their positions may, however, correspond 

 unmistakably, as in the Echidna. 



Muscles inserted into the pre-axial trochanter arise within 

 the ribs. No muscles so arising (except in Chelonians) go to 

 the pre-axial tuberosity. 



In man there are no common flexors or extensors taking 

 origin in the hand in the upper limb, and none such springing 

 from the femur in the lower one ; but we have seen that both 

 these conditions may exist in other animals. 



In the arm, the long flexors of the thumb and digits arise 

 on the same side of the limb as that to which they are 

 distributed. 



In the leg, they arise from the opposite side to that of their 

 distribution, their tendons crossing each other. 



