The vertebral column consists of many, often nearly three hundred vertebrae, and these skeletal segments correspond in number with those of the ventral and transverse scales of the skin. The vertebrae are procoelous; in addition to the anterior and posterior zygapophyses they have a pair of accessory articulations on the neural arches, dorsally to the zygapophyses;–the "zygantrum" carried by the posterior end of the neural arches, its articular surfaces looking upwards; and the "zygosphene" carried by the anterior end and looking downwards. Such accessory articulations occur also in a few Lizards, e.g. Iguanidae. The vertebrae of many Snakes have unpaired vertical, blade-like haemapophyses on their centra for the more effective attachment of the muscles. All the vertebrae, except the atlas, carry ribs. These articulate by their capitular portions only, and are very movable in a head- and tail-ward direction. The ribs being long, and fitting with their ventral ends into the connective tissue of the sides of the ventral transverse scales, are the principal agents in pushing the body forwards, the posterior edges of these scales being sharp and imbricating.
The skin is covered with scales, absolutely devoid of osteoderms. When the scales are enlarged they are called shields. The keel, a common feature, is caused by a slight ridge of the cutaneous part of the scale. The whole skin is covered with a thin layer of horny epidermis, which is shed frequently, at least several times in one year; the shedding begins at the lips, and the whole outer skin is turned inside out from head to tail, retaining every minute detail of the cutaneous scales; even the watch-glass-like covering of the eyes is preserved.
The eyes are peculiar in so far as they possess no lids. The latter are still present in a vestigial condition in the embryo, but their place is taken by what is probably a modification of the nictitating membrane, which is drawn over the eye and covered with a single transparent scale of the horny skin, like a watch-glass. The eyes themselves are quite movable. The "tears," which of course cannot appear on the outside, are drained off into the nasal cavities by the naso-lacrymal ducts.
The ear is likewise peculiar. There is a long columellar rod with a fibrous or cartilaginous pad at the outer end, which plays against the middle of the shaft of the quadrate, an arrangement which, we must assume, produces a thundering noise in the internal ear, since every motion of the quadrate during the act of swallowing conveys the vibrations directly to the fenestra ovalis. The tympanic cavity, the Eustachian tubes, and the tympanum are abolished, and no external traces of the ear are visible. However, in spite of all this, Snakes can hear very well.
The nose is well developed, and many Snakes, for instance the Grass-Snake, are guided to their prey as much by the sense of smell as by the eyes and ears. The tongue is slender, very protractile and bifid, always moist, and furnished with many sensory corpuscles. It acts entirely as an additional sense-organ, hence the incessant play of the tongue of a snake which wants to investigate anything. In spite of the protractility of the tongue, the hyoid apparatus is very small; the hyoid arches themselves are reduced to mere vestiges near the base of the first and only branchial arches, which are thread-like and extend backwards down the throat.
The trachea is very long, and opens far forwards in the mouth; it can be slightly protruded between the two halves of the lower jaw so as not to be blocked during the act of swallowing. This is a laborious process. The snake, having got hold of its prey with its teeth, generally shifts it into the most convenient position, in order to swallow the head first. One half of the mandible is then pushed forwards, then the other half; the recurved teeth afford the necessary hold, and the snake, little by little, draws its mouth-cavity, and later on itself, over the prey. In fact, it literally gets outside it. Sometimes with a large victim this process may last for hours; the whole mouth and head become painfully distended and the veins swollen almost to bursting. The snake pushes the prey against a stone or other obstacle, rests awhile quite exhausted, and begins afresh. At last the bulk of the prey has passed the mouth, the skin of the neck is stretched to the utmost, the scales being separated by wide interstices, the ribs work spasmodically, the victim is pressed into the shape of a sausage, and the deed is done. In order to assist deglutition there is a great amount of salivation, but the often-heard story that Snakes cover their prey with saliva before they swallow it, is a fable, or based upon faulty observation, snakes sometimes being forced to disgorge the half-swallowed prey, which, in such a case, is covered with slime. One of my tame snakes had swallowed a frog on my table when a friend entered the room. The snake was frightened, jumped on to the ground, striking it with its full belly, and thereby hurting the frog, which squeaked loudly, whereupon the snake reversed its mechanism and the frog hopped away, none the worse for its terrible experience.
In correlation with the elongated narrow space of the body-cavity the lungs are not equally developed, the right being much smaller than the left. The latter is a very thin-walled, hollow bag, and the posterior half or third scarcely contains any of the honey-comb-like respiratory "cells," but acts merely as a reservoir of air.
The cloacal arrangement is essentially the same as that of the Lacertilia, but Snakes possess no urinary bladder. The copulatory organs are stowed away beneath the skin in recesses of the posterior lateral corners of the shallow cloacal vestibulum. Each organ is generally bifurcated at the free end, and furnished with little spike-shaped, but scarcely horny, excrescences. On each side of the outer cloacal chamber, in both sexes, lies a roundish gland with an offensive, strongly-scented secretion; that of various Boas smells disagreeably sweet and musky. The majority of Snakes lay eggs, but most of the Viperidae and the thoroughly aquatic kinds, besides a few terrestrial forms, are viviparous. The egg-shells are like parchment, with very little or no calcareous deposit, so that they are always soft; many embryos are, however, provided with a little "egg-tooth" on the tip of the snout.
Fig. 153.–Map showing the distribution of dangerously poisonous snakes.