The trachea is long, and the tracheal rings may be complete in front and incomplete behind, or incomplete throughout. The bronchus opens at once into the more or less elongate, usually single lung, with or without a rudiment of a second, which seems to be constantly the left; in some snakes the lung extends nearly to the cloacal region. In most of the Boidæ there are two well-developed lungs, the left shorter than the right. The lung has highly cellular walls in front, and becomes thin-walled, smooth, or but little vascular, behind, where it may receive its blood from the systemic and not from the pulmonary circulation. In the Typhlopidæ and Viperidæ, as well as in some of the Boidæ, Colubridæ, and Amblycephalidæ, the posterior end or the greater part of the trachea may have its wall enlarged and provided with air cells, resembling the normal lung, with which it is usually continuous; this has been called the "tracheal lung," but, although serving as an accessory breathing organ, it is not a prolongation of the true lung, nor does it represent the missing left lung, as has been believed by some authors. The glottis has a longitudinal slit, and can be projected forwards when the pharynx is obstructed by a voluminous prey. An epiglottis is usually absent, or represented by a rudiment. It is, however, present in some large American species of _Coluber_ (_Pityophis_), said to produce, when hissing, a loud and hoarse sound which has been compared to the bellow of the bull--hence the popular name of Bull-snakes by which they are known. It has also been found in a few allied species from Mexico, for which the genus _Epiglottophis_ has been proposed. This epiglottis is a narrow, thin flap, erect in front of the glottis; it is not hinged, and therefore not capable of falling down to cover the opening of the windpipe during the process of swallowing, its function evidently being to increase the sound produced by the escape of the air from the windpipe. The larynx is represented by two longitudinal bands of cartilage, united by transverse bands; it is extremely long in some snakes (_Leptognathus_). The oesophagus, which may be extremely elongate, sometimes measuring almost one-third of the digestive canal, passes into the tubular or sac-like stomach, often with thickened walls, which itself gradually or abruptly merges into the narrower intestine. The windings of the small intestine are connected by ligamentous tissue, and enclosed in a common sheath of peritoneum. In several of the Glyphodont Water-snakes (Homalopsinæ and Hydrophiinæ), the intestine is much convoluted; in _Herpeton_ it is even longer than the body, although when coiled occupying only one-fourth of that length. The rectum is sometimes very short, sometimes rather long, and its anterior portion may have a short cæcum; it may be divided by transverse septa, with median or lateral perforation.