THE FROG AND VERTEBRATES IN GENERAL 181 



each ; in digits III and V, there are three phalanges each ; in digit 

 IV, there are four phalanges ; thus making in all a total of fourteen 

 phalanges in the foot as compared with ten in the hand. Lying 

 near digit I in the foot of the Frog is an accessory digit, the calcar, 

 which is a spur-like structure formed from two small bones. (W. 

 f. 103.) ' • 



The Vertebrate Limb. The pentadactyl limbs of the Frog, 

 the structure of which has just been considered, are essentially the 

 same as, and are homologous with, those found in all the higher 

 Vertebrates, including Man. The greatest amount of variation 

 between the limbs of various Vertebrates takes place in the wrist 

 and hand bones of the fore limb, and in the ankle and foot bones of 

 the hand limb ; the larger bones lying nearer the body being some- 

 what more constant throughout the Vertebrate series. Thus in 

 the fore limb, or wing, of a Bird the humerus, radius, and ulna 

 are all typical, but in the wrist and hand there has been a marked 

 reduction and modification in the bones. 



In the hoofed Mammals the structure of the limbs has departed 

 more widely from the pentadactyl type than in most other groups 

 of Vertebrates. Thus in the fore limb of the Horse the humerus 

 is about the only bone that has retained the usual structure. 

 Only a small portion of the ulna is present, and it is fused with the 

 radius. Digits I and V have entirely disappeared. Digits II and 

 IV are present as rudimentary structures, known as splint bones. 

 Digit III remains the sole functional digit, so that the Horse walks 

 on the tips of the third digits. In certain Vertebrates, such for 

 example as Snakes or Whales, the hind limbs and girdles have 

 almost disappeared and are no longer functional. In Man, the 

 limbs have remained true to the pentadactyl type, but there has 

 been a fusion of certain bones in the wrist and ankle. ( W. fs. 

 105, 227, 230, 234.) 



C. Internal Structure 



In considering the internal structure of the Vertebrate animal, it 

 will be well to recall the "tube within a tube" condition which was 

 seen to best advantage in the Earthworm. Here we found (a) the 

 outer tube, or body wall, composed largely of muscle tissue ; (6) the 

 endodermal lined inner tube, or alimentary canal, also largely 

 muscular, running through the body from the anterior to the pos- 

 terior ends ; (c) the many chambered body cavity, or coelom, 



