In most snakes there is a very marked asymmetry of the viscera and their blood-supply, the organs of the right side being anterior to, as well as larger than, those of the left.

The heart in most cases is situated between the anterior seventh and the anterior fourth of the body; it may be much farther back, beyond the anterior third, in Doliophis, Platurus, and some ViperidÆ and AmblycephalidÆ, in the middle in Chersydrus. It is of rather elongate form, enclosed in a pericardium in which it lies freely, and has a sinus venosus, two auricles, and a single ventricle divided by a septum. Three arteries leave the ventricle, the pulmonary and two systemic arches. The right systemic arch gives off the carotid artery, which in many snakes, the common Grass-snake for instance, may branch into two, or in others be double from its origin. The anterior abdominal vein is single in most snakes, double in some BoidÆ, and conveys blood from the ventral body-wall to the liver. The caudal vein is continued as the renal portal. Veins which have been regarded as remains of the two posterior cardinal of lower Vertebrates have been found in some of the BoidÆ.

The bifurcate transverse processes of the vertebrÆ at the limit between the body and tail enclose the lymph-hearts, which are large and more or less elongate, metamerically divided into several chambers, the right often more developed than the left. The thymus gland lies on each side of the trachea, near the heart, and the thyroid gland is in the middle line, close to the base of the carotid artery.

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.

In snakes which swallow hard-shelled snails, the anterior part of the intestine has its inner wall furnished with zigzag muscular folds producing a reticulate appearance, followed farther down by transverse and then longitudinal folds. In these snakes the intestine is abruptly constricted behind the stomach, at which point the shells are broken or crushed after their contents have been digested; whilst in the egg-eating snakes, in which the eggshell has to be broken previous to its contents reaching the stomach, the oesophagus is narrowed in front of the latter, at the point where the tooth-like ventral processes of the vertebrÆ project and pierce the wall of the oesophagus in order to aid in this function, after which the broken shell is rejected through the mouth.

The more or less elongate, feebly-lobed kidneys are placed in the posterior part of the body, often extending nearly to the cloaca; the right is usually a little longer than the left, or extends a little farther forward, or even may commence where the other ends. The suprarenal bodies are narrow and elongate, placed on the renal veins or on the vena cava inferior.

The ureters leave the hind ends of the kidneys, and open through the side-walls of the cloaca on a papilla which in the males contains also the opening of the vas deferens. There is no urinary bladder. The genital organs will be mentioned in the next chapter. The liver is usually long and narrow, measuring one-fifth to one-fourth the length of the body, on the right side of the alimentary canal, commencing just behind the heart or farther back. It is exceptionally short in Chersydrus. It is sometimes divided by transverse furrows. Its posterior extremity is bilobate, and the left lobe usually extends beyond the right, although the reverse has been observed in some snakes. The gall-bladder, which may be absent, is remarkable for its distance from the liver. The pancreas, elongate but comparatively small, is located near the spleen, on the left side of the alimentary canal, at a considerable distance from the liver.

The peritoneal part of the body-cavity is subdivided into a number of spaces or coelomic compartments enclosed in serous capsules—viz., a posterior or intestino-genital, a gastric on the left side, and a pair round the liver, corresponding to its two lobes.

Fat-bodies are much developed, either in the form of small separate lobes, or as a continuous, much folded band, on each side of the body.