9. The organs of deglutition and digestion begin with the tongue, which is subject to much variation of structure, according to the different groups of Birds, and is of course correlative with their habits. It has little connexion with taste, though often of assistance in obtaining nutriment. To this follows the gullet (_oesophagus_), which in many cases has an enlargement forming the crop (_ingluvies_), wherein the food may be temporarily retained before passing into the stomach, the last-named always having an antechamber (_proventriculus_) where digestion is largely accomplished, in front of the gizzard (_ventriculus_). This has frequently strong muscular walls, and its action is often assisted by the mechanical process of comminution performed by stones, grit or sand, swallowed for that purpose. The stomach is succeeded by the intestines, which in most cases have a pair of blind-sacs (_caeca_) attached to them, often acting as aids to digestion, though these are not always functional, and are absent in many Birds, while in others they attain a very large size, their condition being in consequence of some importance as a systematic character. 10. The organs of voice in Birds have long attracted special interest from the loud cries which some utter, and the melody with which others are gifted. Setting aside the part played by {13}the _trachea_ or windpipe in supplying air to the lungs, its formation is worthy of attention. Its upper end consists of the _larynx_, and it passes down the neck as a flexible tube, formed by a continuous succession of bony rings connected by membrane, until it bifurcates into two _bronchi_, which open into the lungs. A common feature, found in many groups not nearly allied, is the dilatation of a portion, generally near the middle, while a remarkable modification is exhibited by the males of many of the Duck-tribe, some of the lowest rings being fused together and forming what is known as the _bulla ossea_ or "labyrinth." In other _Anatidae_ (some of the Swans), and some of the Cranes, the trachea enters the keel of the sternum; but a not unfrequent modification, usually confined to the male sex, often occurs elsewhere, when the windpipe is looped back upon itself. All these arrangements, however they may affect the sounds uttered by Birds, do not in themselves constitute the voice organ of most. That is reserved for the _syrinx_, a peculiarity of the Class _Aves_, consisting of the lower end of the trachea and the adjoining part of the bronchial tubes; and the varied modulations are effected by means of muscles attached thereto. These voice-muscles may be wholly absent or of the simplest character, but they attain their highest perfection in the _Passeres_, and especially in the large group of them known as _Oscines_, where there are often five or seven pairs. In this group the lowest four or five tracheal rings are solidly fused into a little bony box communicating with the bronchi; the first and