234 ELEMENTARY ANATOMY. [LESS. 



The second digit of the foot never aborts (the other digits 

 being well developed), as we have seen to be the case with 

 the second digit of the hand in the Potto. 



Certain resemblances occur which might not be expected. 

 Thus when, as in Birds, the distal part of the tarsus 

 anchyloses with the metatarsus, the distal part of the carpus 

 also anchyloses with the metacarpus, and this in spite of the 

 greatly different use of the parts in the two limbs respectively. 



But the greatest degree of resemblance between the pectoral 

 and pelvic limbs is shown in the existing Tortoises, and in 

 the extinct Ichthyosauria and Plesiosauria, where we find a 

 concomitant shortening of the long limb bones, and multi- 

 plication of phalanges. 



FIG. 200. SKELETON OF FIG. 201. SKELETON OF 



PLESIOSAURUS. ICHTHYOSAURUS. 



Certain discrepancies, however, should be noted. Thus the 

 pelvic member is never elongated or concentrated to such 

 excess, as is the pectoral member in the Bats and the Moles. 

 Again, we may have, as in Chameleons, three pre-axial digits 

 opposed to two post-axial ones in the hand, and two pre-axial 

 digits opposed to three post-axial ones in the foot. 



Though in rare instances present alone, yet, when both 

 are present, the pectoral limbs never show so persistent and 

 considerable an inferiority of development in air-breathing 

 animals as do the pelvic limbs in Fishes. 



On the contrary, there is never a highly developed pelvic 

 girdle without a rudiment of a pelvic limb ; but we may, as 

 we have seen in Anguis, have a well-developed pectoral 

 girdle without any rudiment of a pectoral limb. 



Again, we may have (as in Lialis) leg-bones without a 

 foot (fig. 1 68), but we never meet with arm-bones without a 

 hand. 



27. It is obvious that the importance of the axial skeleton 

 related as it is to the brain, the spinal marrow, and the 

 nerves which pass out from these is far greater than that of 

 the appendicular skeleton. 



Accordingly we have found that the latter may be entirely 

 absent (as in the Lamprey, the Ophiomorpha, and many 

 Serpents), but that the former is constantly present in man's 

 sub-kingdom. 



