90 MEMBERS, AND ORGANS OF NUTRITION, IN VERTEBRATA. 



The extremities attain tlieir greatest development in Birds and 

 terrestrial Mammals ; whose motion is almost entirely due to 

 them. In these cases, the spinal column requires more firmness ; 

 in order that a sufficient support may be given to the limbs, and 

 that the muscles that move them may have fixed points from 

 which to act. Hence the vertebrae are often found to be securely 

 locked together by processes, or projections of bone, which fit 

 into each other, and render displacement difficult, even when the 

 ligaments have been removed. This is most the case in Birds, 

 in which class the whole bony framework of the trunk is so 

 constructed, as to be very little capable of movement ; the powers 

 of motion being delegated, as it were, to the limbs, which can 

 act more advantageously in proportion to the fixity of the points 

 from which their muscles act. They stand in remarkable con- 

 trast, therefore, to the groups of Fishes and Serpents, just now 

 adverted to ; in which, from the absence or low development of 

 the extremities, the movements are performed altogether by the 

 trunk, and the vertebral column is proportionally flexible. In 

 no instance is the number of members greater than four. 



69. The Vertebrata are distinguished from Invertebrated 

 animals, by the possession of red corpuscles (commonly termed 

 globules) in their blood ; and a marked difference hence exists 

 between their circulating fluid and that of the lower tribes, which 

 is colourless or nearly so, containing only the white or colourless 

 corpuscles, ( ANIM. PHYSIOL. 235). They have, moreover, a separ- 

 ate system of vessels for the absorption of nutrient fluid, both 

 from the alimentary canal, and from the substance of the body 

 itself; to which we find nothing analogous in the Invertebrata. 

 These vessels are termed absorbents (ANIM. PHYSIOL. 217); 

 and they empty the fluid which they have taken up, and which 

 bears a strong analogy to the blood of the Invertebrated animals, 

 into the current of the circulating blood. This current is regu- 

 larly kept up, in Vertebrata, by the action of a heart, endowed 

 with considerable muscular power ; and the blood is sent, by its 

 means, not only to the body in general, but also to the respiratory 

 organs, in which it is to be exposed to the influence of the air. 

 These organs are always restricted to some particular portion of 



