Before, however, we permit our author to speak for himself, it may perhaps be desirable to preface his remarks by a few general observations concerning the structure of the beautiful creatures that form the subjects of his teaching, inasmuch as it is obviously desirable to have 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. [Illustration: Fig. 1.--RESPIRATORY APPARATUS OF A FOWL. _a_, the Lungs, immovably fixed; _c_, _d_, the Breast-bone, moving as upon a hinge at _b_, so that it can be raised to the position indicated by dotted lines at _h_. 1, 2, 3, 4, 5, 6, Cells, with membranous walls, into which the air is freely admitted during the act of inspiration.] 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 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.