xn. 6 



(3°3) 



6. Vertebral column of Amphibia 



The general build of the body is essentially fish-like in stegocepha- 

 lian and urodele amphibians. Such forms have two means of locomo- 

 tion. When they are frightened and move fast they wriggle along with 

 the belly on the ground, the effective agent being serial contraction 

 of the segmentally arranged myotomal musculature, by means of which 

 the animal as it were 'swims on land', with the legs hardly touching 

 the ground (Fig. 175). When moving deliberately, on the other hand, 



Fig. 176. Drawings made from photographs of a newt (Triturus) 

 in slow locomotion. (After Evans, Anat. Rec. 95.) 



a newt raises up its body on the legs, which then propel it along as 

 movable levers, the main part of the action being produced by drawing 

 back the humerus or femur, the more distal muscles of the limbs 

 serving to maintain the digits pressed against the ground (Fig. 176). 

 The carrying of the weight on four legs places an entirely new set 

 of stresses on the vertebral column. Instead of being mainly a com- 

 pression member as it is in fishes it comes to act as a girder, carrying 

 the weight of the body and transmitting it to the legs. This new 

 function produces a column whose parts are largely bony and articu- 

 lated together, flexibility becoming less important than strength. The 

 new types of strain involve new muscle attachments and the develop- 

 ment of special processes and parts of the vertebrae (p. 307). These 

 changes, however, have not proceeded very far in the amphibians; 

 many urodeles spend much time in the water and their vertebrae often 

 show a lack of ossification, parts of the notochord persist and provide 

 the main compression member required for swimming. 



