The _middle ear_ is divided into an outer part, the _tympanic cavity proper_, and a part next to the labyrinth, the _recessus cavi tympani_. Within the tympanic cavity, besides blood-vessels and nerves, is found the _columella_ with its appendage (found in all Reptilia), the _recessus scalæ tympani_. The tympanic cavity is formed mainly by the quadrate, though the exoccipital and squamosal bones take some part. In outline it might be compared to a truncated, four-sided pyramid, with its base below, its truncated apex above, and with an anterior, a posterior, a mesial, and a lateral side. From the floor of each tympanic cavity a _Eustachian tube_ leads towards the throat. These tubes unite and connect with the throat by a single small opening just behind the posterior nares, as shown in the figures of the skull. The _semicircular canals_ with their _ampullæ_ lie in the usual positions as seen in other vertebrates: the anterior vertical, posterior vertical, and horizontal. The details in structure of the inner ear cannot be given here. The nervous epithelium is said to have the same characteristics as in other vertebrates. CHAPTER V THE DIGESTIVE SYSTEM THE ORAL CAVITY The _mouth_ in the Crocodilia is large, as is well known, and may be opened very wide. It is bounded anteriorly and laterally by the teeth of the two jaws; these teeth were described in connection with the skull. The mucous membrane of the roof of the mouth and of the dorsum of the tongue, especially the former, exhibits numerous small papillæ (see page 160), and among these, in the posterior region of the mouth, are the ducts of mucous glands. [Illustration: FIG. 33. INTERIOR OF THE MOUTH OF A. MISSISSIPPIENSIS _f_, transverse fold at the base of the tongue; _v_, _v_, velum palatinum.] The _tongue_ extends from just back of the mandibular symphysis to the glottis. It is attached throughout its entire ventral side except for a short distance at its tip, so that it may be elevated and depressed but not protruded. Among the papillæ on its dorsal surface are sense organs, said to be tactile and gustatory corpuscles (see page 165). The posterior margin of the tongue is elevated as a transverse fold that meets a corresponding fold, the _velum palatinum_, from the lower side of the palate and completely shuts off the mouth from the openings of the trachea and gullet (Fig. 33). Into this hinder chamber open the posterior nares, so that the animal can open its mouth under water without getting water into its trachea; or it may, while holding its prey in its mouth, come to the surface to breathe, without danger of letting water into its trachea. The nasal passages, leading from the nostrils to the posterior nares, are, of course, completely inclosed by bone, as described in connection with the skull. Ventral to the larynx and posterior part of the mouth is the large, shield-shaped hyoid apparatus, Fig. 25, h, also described in connection with the skull. THE ŒSOPHAGUS