* The membrana nictitans is simply a fold of the conjunctiva on the inner side of the eye. It occurs in some Fishes (e.g., some Sharks), in some Reptiles and Amphibians, in Birds, in Monotremes and Marsupials, and in some of the higher Mammals. In Man, however, in Monkeys, and in most of the higher Mammals, it is rudimentary, and constitutes the so-called "plica semilunaris."
The last anatomical peculiarity of Birds which requires notice is the peculiar apparatus known as the "inferior larynx," or "syrinx," by which the song of the singing birds is conditioned. "The air-passages of birds commence by a simple superior larynx, from which a long trachea extends to the anterior aperture of the thorax, where it divides into the two bronchi, one for each lung. At the place of its division, there exists in most birds a complicated mechanism of bones and cartilages, moved by appropriate muscles, and constituting the true organ of voice; this part is termed the inferior larynx" (Owen). The inferior larynx may be developed from the trachea only, before the division of this tube into the bronchi; or, it may be developed wholly from the bronchi; or, lastly, and more commonly, it may be developed at the junction of the trachea and bronchi and out of both. The structure of the vocal apparatus is extremely complicated, and there is no necessity for entering upon it here. It is to be remembered, however, that those modifications of the voice which constitute the song of birds, are produced in a special and complex cavity placed at, or near, the point where the trachea divides into the two bronchi, and not in a true larynx situated at the summit of the windpipe. The syrinx is wanting in a few birds (e.g., the Ratitae). Lastly, the trachea of birds is always of considerable proportionate length, and it is often twisted or dilated at intervals, this structure, doubtless, having something to do with the production of vocal sounds.
Before passing on to the consideration of the divisions of Birds, a few words may be said as to the migration of birds. In temperate and cold climates comparatively few birds remain constantly in the same region in which they were hatched. Those which do so remain, are called "permanent birds" (aves manentes). Other birds, such as the Woodpeckers, wander about from place to place, without having any fixed direction. These are called "wandering birds " (aves erraticae), and their irregular movements are chiefly conditioned by the scarcity or abundance of food in any particular locality. Other birds, however, at certain seasons of the year undertake long journeys, usually uniting for this purpose into large flocks. These birds - such as the swallows, for instance - are properly called "migratory birds" (aves migratoriae). The movements of these birds are conditioned by the necessity of having a certain mean temperature, and consequently they leave the cold regions at the approach of winter, and return again for the warmer season.