2 CASSELLS BOOK OF BIRDS. 



clear notions concerning the machinery employed before its adaptation to its intended uses can 

 be made manifest ; and further, because in the study of ornithology, as in every other branch of 

 natural history, there are certain conventional terms that may require explanation before the words 

 used in describing an object are intelligible to the uninitiated. 



The Bird is an inhabitant of the air in the fullest sense of the expression. The atmosphere is 

 emphatically the sphere of its activity ; it mounts it as it would a ladder ; it sails through it in 

 triumph, and rides upon the winds as upon a fleet steed. Moreover, it is the atmosphere itself 

 which endows the feathered Ariel with such capabilities, and it is in the perfection of his respiration 

 that we must search for an explanation of his wonderful achievements. 



The muscular activity of every animal is intimately dependent upon the efficiency of its 

 breathing apparatus, upon the freedom with which the vital element finds admission to the blood 

 which it is destined to renovate, and upon which it confers those qualities so inseparably connected 



Fig. I. — RESPIRATORY APPARATUS OF A FOWL. 



a, the Lungs, immovably fixed ; c, d t the Breast-bone, moving as upon a hinge at //, so that it can be raised to the position indicated by dotted 

 lines at h. I, 2, 3, 4, 5, 6, Cells, with membranous walls, into which the air is freely admitted during the act of inspiration. 



with the elimination of increased temperature, and the vigour of muscular action. In this respect, 

 as we shall see immediately, the feathered races surpass all living creatures, with the exception, 

 perhaps, of the members of the insect creation. 



The lungs of a bird are not suspended, like those of a quadruped, within a circumscribed chest 

 or thoracic cavity, in such a manner as to become inflated by each inspiration ; they are rather to be 

 described as soft, porous, and highly vascular organs, through which the air passes as through the 

 interior of a sponge. The movements of the chest, upon which depend the inspiration and expiration 

 of the atmospheric fluid, may be compared to those of a bellows continually employed in taking in and 

 expelling the surrounding element by a mechanism represented in the accompanying figure (Fig. i). 

 The framework of the chest, consisting of the ribs and of the breast-bone, is so put together that at 

 each inspiration it can be raised, as shown in the drawing, from the position d to the position h, thus 

 materially enlarging the thoracic chamber, just as the upper board of an ordinary bellows is raised for 

 the purpose of taking in the air ; but, in this case, the surrounding element, instead of entering through 

 a valve-defended orifice, rushes down the windpipe, and through the immovable, sponge-like lungs, 

 permeating the wide passages with which they are perforated, and not only filling the entire thorax, but 

 penetrating into the interior of the very bones, which are left marrowless for its reception. 



