_c_, the External Horny Sheath, slit open; _d_, _d_, Web of the Feather produced in successive layers from the central stem, _e_.] The power of inflating their whole body with air, and the possession of feathers, are therefore the most distinctive endowments of a bird, inasmuch as these attributes are quite peculiar to the class. To creatures thus gifted with strength and activity so extraordinary, it is manifest that perceptions of great acuteness are requisite, corresponding with the rapidity of their movements and the intelligence necessary for the performance of the important duties entrusted to their charge; and in this respect, as will be made manifest by a perusal of their history, they occupy a position in the economy of nature fully equal or even superior to that enjoyed by the most favoured quadrupeds. The mental faculties of the parrots correspond with those of the monkeys, whom in their habits and capabilities these birds closely resemble; in cunning they are quite upon a par with their four-handed neighbours, with which, in the forests of tropical countries, they are so generally associated; and when removed from their native woods, and made, as they often are, the companions of mankind, the facility with which they can be taught to imitate human actions--nay, to mimic our very speech--bears ample testimony to the exalted character of their mental capacities. On examining the brain of a bird, the anatomist is therefore by no means surprised to find that, both in its development and in the perfection of its structure, it surpasses that of many quadrupeds. The proportionate volume of the brain of some of our singing birds, as compared with the dimensions of their body, is astonishing, and reveals to us at a glance the reason why these little favourites are so sagacious and so eminently susceptible of education. (See Fig. 4.) [Illustration: Fig. 4.--HEAD OF A SWAN (_Cygnus olor_). The upper part of the skull has been removed to show the brain and eyes _in situ_. _a_, _a_, the Cerebral Hemispheres; _b_, the Cerebellum; _c_, the Spinal Cord. All the above parts are represented covered by their investing membranes. _e_, Sinuses of the Dura Mater; _g_, _g_, Walls of the Skull; _h_, _h_, Base of the Beak; _m_, Optic Nerve of the Left Eye; _p_, _p_, Large sentient Nerves supplying the Bill; _v_, Bony Ring in front of the Eye-ball; _x_, _x_, Transparent Cornea; 1, 3, 5, 7, Muscles moving the Eye-ball.]