VISCERA
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.
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