Page:EB1911 - Volume 23.djvu/183

Rh teeth are enlarged and furnished with a deep groove for the reception of poison. In the Solenoglypha or Viperidae the 38.—Two Aspects of a Tooth of Heloderma horridum (after Bocourt). 1, antero-internal aspect of the tooth, showing a very deep longitudinal groove; 2, postero-external aspect of the same tooth, showing a very faint longitudinal groove. enlarged teeth of the Opisthoglypha have moved to the front, owing to reduction of the anterior portion of the maxilla. The latter, much shortened, moves with the firmly anchylosed poison fang upon the prefrontal as its pivot, being pushed forward, or “erected,” by the ectopterygoid bone, which connects it with the pterygoid, and this in turn can be moved forwards and backwards, together with the quadrate. (See fig. 24, skull of Vipera nasicornis and the diagram of the mechanism in article .) In the still unfinished fang the furrow is open, later the edges close together and the end of the duct of the gland itself is surrounded by the substance of the growing basal portion of the tooth, so that the furrow is converted into a canal continuous with that of the gland. The poison is now sure to be projected into the very deepest part of the wound with the precision of a surgical instrument. The Proteroglypha, with their long, non-erectile maxillae, bite, or, like Elaps, deliberately chew their victim; the Viperidae rather strike with the mouth widely open. The teeth of snakes and lizards are often of irregular size; but it is rare that a kind of differentiation into incisors, canines and molars occurs. In many lizards, especially in Iguanidae, some teeth are multi-cuspid, trilobed, or somewhat serrated; in Tiliqua, universally known as Cyclodus, most of the hinder teeth are roundish crushers.

Lizards and snakes are born with an “egg-tooth” which is lost a day or two after hatching. Its function is the filing through of the eggshell. This tooth, always unpaired, is in Tropidonotus natrix one millimetre long and half a millimetre broad at its base, which rests upon a middle depression of the premaxillary bone; it stands forward above the mouth and is curved upwards. In crocodiles and tortoises the same effect is produced by another organ, which, as in birds, lies well outside the mouth on the top of the end of the snout and consists of a little cone of calcified epidermis.

Tongue.—The tongue of the crocodiles is very broad and flat, and with nearly its whole broad base attached to the floor of the mouth; however, in its whole circumference its edge is well marked, and it arises on its hinder border as a transverse fold which meets a similar fold descending from the palate in front of the posterior nares. By these folds the mouth can be completely shut off from the nasal passages into the trachea. The upper surface of the tongue contains several dozen large flat papillae, each with a central pit-like opening; it is not known whether they are gustatory organs. Besides scarce mucous glands on the tongue, there is an absence of salivary glands in the mouth. The tongue of tortoises is likewise short, broad, and not protractile, and there appears to be only a sublingual gland; the surface of the tongue is covered with velvety papillae in the terrestrial, with larger folds in the marine Chelonians. In the Lacertilia the tongue presents a number of variations which have been referred to 'as diagnostic characters of the various families of (q.v.). The chief modifications are the following: Either flat and broad, not protractile, e.g. Agamidae; or the body of the tongue is somewhat cylindrical, elongated, and the whole organ can be protruded; lastly, the anterior half of the tongue, which can be protruded, is retractile or telescoped into the posterior portion, e.g. Anguidae. In nearly all cases the posterior dorsal end of the body of the tongue is well marked off by a margin raised above the root, a character which does not occur in any snake. The upper surface is either smooth or curved with velvety, flat, or scaly, always soft; papillae. In the majority the tip of the tongue

is bifid, either slightly niched or deeply bifid. The tips contain tactile corpuscles, although sometimes covered with a horny epithelium. The most specialized is the tongue of the chameleon. The body of this tongue is very thick, club shaped, fleshy and full of large mucous glands which cover it with a sticky secretion. The base or root is very narrow, composed of extremely elastic fibres and supported by a much elongated copular piece of the hyoid. This elastic part is, so to speak, telescoped over the style-shaped copula, and the whole apparatus is kept in a contracted state like a spring in a tube. A pair of wide blood vessels and elastic bands extend from the base into the thick end, which in an ordinary chameleon can be shot out to a distance of about 8 in.

The tongue of the snakes is invariably slender, smooth and almost entirely retractile into its posterior sheath-like portion. It is always bifid and contains many tactile and other sensory corpuscles by which these creatures seem to investigate. The tongue is always protruded during excitement. How this is done is not very obvious, since the hyoid apparatus itself is much reduced. There is a niche in the middle of the rostral shield to permit protrusion of the tongue whilst the mouth is shut, and probably herewith is correlated the almost universal absence of teeth in the premaxilla. The tongue and the larynx are placed very far forwards in the mouth and, during the act of swallowing, the larynx approaches the chin, or it may even protrude out of the mouth to secure breathing during the often painfully protracted act.

Of Glands, sublingual glands are of general occurrence in reptiles; they open near the root or in the sheath of the tongue. Labial glands seem to be absent in crocodiles and tortoises, but upper and lower labial glands exist in lizards and snakes, generally in considerable numbers. Heloderma is the only lizard in which some of these glands—those along the lower jaw—produce a poisonous secretion, each small gland conducting its secretion towards the base of one of the somewhat furrowed teeth. In the snakes, upper and lower labial glands are well developed for salivation. It is the upper series which attracts our interest by its eventual modification into the deadly poison glands. Probably the saliva of most snakes, like their serum, possesses toxic properties. In most of the harmless Colubrine snakes the glands extend in a continuous series from behind the premaxilla along the whole of the upper jaw, with numerous openings. In the Opisthoglypha a gradual differentiation takes place into an anterior, middle and posterior portion; the middle, extending from below and behind the eye backwards, is the thickest and yellowish in colour; behind it follows a small portion, reddish grey like the anterior portion, with which it is more or less continuous below the middle complex. Thus, still rather indifferent, is Dryophis. In Dipsas, e.g. D. fusca, the middle portion has become predominant; some of its enlarged ducts lead to the pair of posterior, enlarged and well-grooved, maxillary teeth. It is this middle portion which becomes the characteristic poison gland with one long duct. The gland itself retains its position; all the other upper labials, except the anterior series, abort. In the Viperidae the poison duct opens near the base of the perforated fangs, which, owing to the shortening of the anterior portion of the maxilla with its teeth, have come to be the only teeth in the upper jaw. In the Elapine, still more in the Hydrophine snakes, the position of the gland and its duct is the same, but the duct has been carried past the smaller harmless teeth which stand in the maxilla and open at the base of the anterior maxillary teeth. The effect is the same, although the poison fangs are not homologous, in the one case the most posterior, in the other the most anterior, of the maxillary series. In Doliophis, one of the Malay genera of Elapine snakes, each poison gland sends an enormously elongated recess far into the body-cavity. (For some other details see ; The best account of the buccal glands and teeth of poisonous snakes is that by G. S. West, P.Z.S., 1895, pp. 812-826.)
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Stomach, &c.—In lizards and in Sphenodon the wide pharynx and oesophagus passes gradually into the stomach, which is