The organs of respiration in birds, as well as their sexual organs, are, according to Purkinje and Valentin, supplied with cilia on their surface.
Brain, Nervous System, and Senses.
The brain of birds possesses the same characters which are to be found in other oviparous vertebrated animals, but its proportional volume is its distinguishing peculiarity ; and this volume often sur passes the development of that organ in mammifers. Indeed, in some birds, and more particularly in some of the songsters, the brain has been said to exceed that of man when considered in reference to the size of the head and of the whole body. The following scale has been given as an example of the size of the brain in relation to that of the body :—Eagle, 1.264th of the body ; sparrow, 1-25th ; chaffinch, 1-27th; redbreast, 1-32nd; blackbird, 1-6Sth ; canarybird, 1-14th; cock, 1-25th ; duck, 1-257th ; goose, I-360th. In man the basin forms from 1.22nd to I-33rd of the body ; in some apes, 1-22nd; in the elephant, 1-500th; in the horse, 1-100th ; in the dog, 1-161st; and in the mt, 1-94th.
The size of the brain in birds arises principally from tubercles analogous to the corpora striata of mammifers, and not from the hemispheres, which are small, smooth, and without convolutions. The cerebellum is large, almost without lateral lobes, and formed principally by the vermiform process. Several parts found in the brain of marnmifers are absent in birds, and among these are the corpus callosinn and pone Varolii.
Of the five senses, sight, smell, and hearing are most acute in binla.
Sight —We have seen that the bony orbits are of great magnitude, and tho organs of sight which are contained therein are proportionately large. In the bin's of prey the orbits have the shape of a "chalice," says Illumentrach, "or cup used in the communion service. The cornea, which is very convex, forms the bottom of the cup, and the posterior segment of the nelerotica resembles its cover. This peculiar form arises from the curvature and length of the bony plates, which, as in all other birds, occupy the front of the selerotiea, lying close together and overlapping each other. There bony plates form in general a flat or slightly convex ring ; being long and curved in the A ecipitres (Hawks) they form a concave ring, which gives the whole eyeball the above-mentioned form." By means of this ring the eye
becomes a kind of self-adjusting telescope, so as to take in both near and very distant objects.
A representation of the sclerotic plates, forming the bony ring in the eye of the Penguin (A ptesodytes), is here given. They remind us forcibly of the eye-plates in some of the reptiles, particularly of those belonging to the eyes of the Enaliosauriana, or fossil marine lizards. The penguin has to adjust its eye for vision both on land and under water. This contrivance must greatly assist the adjustment necessary for seeing clearly in such different media.
' The cryntalline humour is flat in birds; and the vitreous humour is very smalL Tho colour of the iris varies in different species, and in many cases is very brilliant. The marsupium, which arises in the back of the eye, and the use of which is not very clearly ascertained, is a peculiarity in the eye of birds. They have three eye-lids, two of which, the upper and lower, are closed iu most of the race by the elevation of the lower one, as may be frequently seen in our domestic poultry. The owl, the goat-sucker, and a few others, have the power of depressing the upper eye-lid. Of these birds the upper only is furnished with eye-lashes generally ; the ostridh, secretary vulture, some parrots, and a few other birds, have them in both lids. But the third eye-lid, or nictitating membrane, forms the most curious apparatus. When at rest, this, which is a thin semi-transparent fold of the tunics. conjunctiva, lies in the inner corner of the eye, with its loose edge nearly verticaL By the combined action of two muscles which are attached towards the back of the sclerotica, it is capable of being drawn out so as to cover the whole front of the eye-ball like a curtain, and its own elasticity restores it to the corner in which it rested. This, it is said, enables the eagle to look at the sun. The peculiar movements of this organ may be seen amongst the fine collection of eagles at the gardens of the Zoological Society in the Regent's Park.