90 MEMBERS, AND ORGANS OF NUTRITION, IN VERTEBRATA. 



The extremities attain their 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 vertebra 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. All the Yertebrata are distinguished from Invertebrated 

 animals, by the possession of red corpuscles (commonly termed 

 globules) in their blood ; and this gives to their circulating fluid 

 a marked difference from that of the lower tribes, which is colour- 

 less or nearly so, containing only the white or colourless corpuscles, 

 (ANIM. PHYSIOL. 235). There is, moreover, a separate sys- 

 tem 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 regularly 

 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 



