PART II. 



CLASS II. : BIRDS (AVES). 



VERTEBRATE animals with warm blood, breathing by means of lungs, with 

 a covering of feathers, the fore-limbs forming organs of flight, or wings ; 

 oviparous, laying hard-shelled eggs. 



Birds live under the most varying conditions, necessitating special 

 bodily arrangements. (Thus, in swimming birds the feet are adapted 

 for swimming webbed ; in cursorial birds, for running, etc.) All, 

 however, agree in their most essential features, for all, with few 

 exceptions (cursorial birds and penguins), possess in common one faculty, 

 viz., the poiver of traversing the air by flight. 



1. Body. The body of a bird, which in its flight has to overcome the 

 resistance of the air, like a ship which ploughs its way through the 

 waters, has the form of a spindle. (What is the import of this 

 particular shape in the case of moving objects ?) 



In flying the bird has to make active movements with its wings, the 

 organs of flight. If the muscles by which the wings are moved were 

 attached to bones capable of free motion upon one another, their action 

 would be considerably weakened ; for in order to execute their work 

 efficiently the muscles require firm points of attachment. Therefore 

 the skeleton of the trunk in birds is firm and rigid, like the hull of a 

 ship. This general compactness and rigidity of the body results from the 

 following skeletal arrangements : 



(a) The vertebra of the trunk form one complete, continuous, and 

 almost rigid whole, differing in this respect from those of mammals, 

 which are more or less movable inter se, and divisible into three distinct 

 sets, viz., the dorsal, lumbar, and sacral portions of the spinal column. 

 The dorsal vertebrae, which carry ribs, are but slightly movable upon 

 one another ; the lumbar and sacral vertebrae are anchylosed into one 

 continuous portion, the so-called " sacrum." Accordingly, muscles 

 serving to move these vertebras are almost entirely absent. (This effects 

 a diminution in the body weight.) 



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