MYOLOGY OF REPTILES. 221 



tively, to which the muscles conform in relative size and posi- 

 tion. These opposite bends are shown in the skeleton of the 

 Crocodile, fig. 57. 



When the leg, 66, is brought forward (protracted), widening the 

 angle between it and the thigh, v, and when the fore-arm, 55, is 

 brought forward, contracting the angle between it and the arm, 

 53, the motions are the same, or homologous in both limbs. But 

 in one case such motion is called ' flexion ;' in the other ' exten- 

 sion :' these terms relating; not to the absolute line or direction of 

 motion of the limb, but to the resulting relative position of one 

 segment of the limb to another. The protractor muscles draw- 

 ing forward the second segment of the limb to an angle with 

 the first, are called ' flexors ;' those drawing forward the second 

 segment from an angle with the first, are called ' extensors.' The 

 same distinction is made with the ' retractors,' according as, in 

 drawing back the second segment, they rotate it from an angle 

 to a straight line with the first segment, or from a straight line to 

 an angle. Thus the homologous movements are signified by dif- 

 ferent terms, and the homotypy of the muscles has been masked 

 by the same artificial verbal distinctions. The ' flexors ' of the 

 fore-arm answer to the ' extensors ' of the leg in serial homology. 

 The ' biceps flexor cubiti,' with the ' brachialis anticus,' is the 

 homotype of the l triceps extensor cruris,' not of the ' biceps flexor 

 cruris ;' while this muscle, with the semitendinosus, is the homotype 

 of the ' triceps extensor cubiti.' Much of the difficulty of com- 

 prehending the true serial homology of the parts of the fore and 

 hind limb has arisen from regarding the flexors in the one limb to 

 be the homotypes of the flexors of the other, and vice versa. The 

 pertinacity with which the idea of the patella being the homotype 

 of the olecranon is maintained, depends in a great degree upon the 

 error of supposing the ' triceps extensor cubiti ' and the ' biceps 

 extensor cruris' to be homotypes or serial homologues. 1 



Returning to the Newts, we find the chief retractor or extensor, 

 fig. 141, 39, of the foot answering to the retractors or flexors of 

 the carpus, 25 and 26. But, as regards the toes, since their joints 

 are so arranged as to allow them to be most easily and extensively 



1 Some anatomists assuming this to bo a matter determined and unquestionable, 

 make it the basis for impugning the opinion that the patella answers serially to the 

 tendon of the biceps brachii, and especially to the sesamoid sometimes developed 

 therein. " Unless we are entirely to disregard the guidance of muscular relations 

 in determining homology, we must admit that the ossicle upon the olecranon is the 

 homotype of the patella," &c. CLXxr. p. 21, and clx. passim. The muscular 

 concur with the osseous relations in showing that the ossicle upon the olecranon is 

 the homotype of that upon the peronecranon, or produced head of the fibula in 

 certain marsupial and other mammals, clxi. pi. 1, fig. 16. 



