NERVOUS SYSTEM. The nervous system of birds is highly developed, but presents few sali ent differences from the vertebrate type. There is a perceptible improvement of the brain over that of reptiles, more especially in the increased proportional development of the cerebral hemi spheres; but the surfaee-eonvolutions and other features that play so important a part in the brains of the higher mammals are absent or merely indicated. In this, and in the way the cerebral nerves originate, the brain remains near ly reptilian. The sympathetic system is highly developed, as Would be expected in creatures of such activity and sensitiveness. See NERVOUS SYSTEM, EVOLUTION OF.
7'he Senses of Birds, as a whole, equal those of most mammals, and in some directions are supe rior to those of other animals. Taste is, perhaps, feeblest, the horny nature of the jaws and the character of the food not favoring the develop ment of this sense in birds; yet a certain amount of discrimination is undoubtedly exercised by them, apart from the guidance of touch and smell. Odors affect birds also, but to a less degree probably than is frequent among even reptiles, not to say mammals; and it is probable that even the carrion-eating species depend little upon their sense of smell in guiding them even to odoriferous food. Birds owe less to the sense of touch than many other creatures, for they have few tactile organs, the bristles about their beaks having other purposes, mainly. The bills of some mud-hunting species, however, are spe cialized in this direction (see DUCK and SNIPE). In respect to hearing, birds are well endowed, having the internal apparatus of the ear (see EAR) highly perfected, and exposed to the vi brations of the air through a cartilaginous meatus with a fairly wide opening. One function of the ears of birds, as well as of other creatures, seems to be to inform the animal as to its re lations with space, balance, direction, etc. Whether or not this is to be regarded as a sense, is undecided. There is no external ear, the near est approach being in the nocturnal owls (where the organ presents other peeuliarities—see OWL), though the so-called 'ears' of some of those birds are merely tufts of feathers which have nothing to do with audition. Moreover, this faculty in birds extends beyond mere quick perception of sounds to 'the power of distinguishing or under standing' pitch. notes and melodies, or music; and it has been an important factor in the develop ment of bird-singing.
The vocal performances of birds are one of their foremost and most pleasing characteristics; yet of only a few, in the most highly organized classes. can it be said that they sing. Even in the order called 'singers' (Oscines), many make little melody, though, like the crow, they may utter a long list of significant notes—a real lan guage. which is developed among birds generally to a degree realized by few. The vocal media nism of this class is not in the larynx, but at the lower or pectoral end of the windpipe, in an organ called the syrinx, which consists of several stretched and vibratory membranes, tracheal rings and controlling muscles. This complicated organ, peculiar to birds, presents almost un limited modifications, and is quite unlike the arrangement of laryngeal vocal cords in mam mals, for the tongue has nothing to do with a bird's speech or music. (See Plate, Figs. 7. 8, 14.) Further consideration of the singing of birds leads into the domain of their evolution.
The Eyes of Birds are large, clear, highly per fected in structure, and, in many species, en dowed with a power of vision which' is beyond anything known elsewhere. The way in which vultures descend upon carrion from a height where they are invisible to us, or sea-birds swoop down upon fishes, or flycatchers dart long dis tances upon insects so small we can barely per ceive them at close range, are examples of their sharp-sightedness, with which is combined re markable ability in rapidly changing the focus, as must be necessary to a bird darting swiftly upon prey it must keep constantly in view. There are no movable eyelids, but in place of them a nictitating membrane is frequently drawn over the eyeball to clean and lubricate it, or shield it from a glare. The owls and other noc turnal birds have great power of contractility not only in the pupil, but in the whole case of the eye. See EYE.
A 'sense' of Direction.—The so-called 'sixth sense' of orientation or direction, that is, the power of instinctively knowing and maintaining a course of flight through the air independent of guidance of landmarks, is manifested by birds in a most surprising way; for no other explana tion (little as we understand it) seems to ac count for their ability to cross vast spaces of ocean straight toward small islands or restricted areas of coasts, as some birds habitually do in their migrations. See MIGRATION OF ANIMALS.