The second order of Reptiles is that of the Ophidia, comprising the Snakes and Serpents, and distinguished by the following characters:
The body is always more or less elongated, cylindrical, and worm-like, and whilst possessing a covering of horny scales, is always unprovided with a bony exoskeleton. The dorsal vertebra are concave in front (procoelous), with rudimentary transverse processes. There is never any sternum, nor pectoral arch, nor fore-limbs, nor sacrum, and as a rule there are no traces of hind-limbs. Rudimentary hind-limbs, however, are occasionally present (e.g., in Python and Tortrix). There are always numerous ribs. The two halves or rami of the lower jaw are composed of several pieces, and the rami are united anteriorly by ligaments and muscles only, and not by cartilage or suture. The lower jaw further articulates with the skull by means of a quadrate bone (fig. 288, a) which is always more or less movable, and is in turn united with the squamous portion of the temporal bone ("mastoid bone"), which is also movable, and is not firmly united with the skull. The superior maxillae are united with the praemaxillae by ligaments and muscles only, and the palatine arches are movable and armed with pointed recurved teeth. Hooked conical teeth are always present, but they are never lodged in distinct sockets or alveoli. Functionally they are capable of performing nothing more than merely holding the prey fast, and the Snakes are provided with no genuine masticatory apparatus. The heart has three chambers, two auricles and a ventricle, the latter imperfectly divided into two cavities by an incomplete septum. The lungs and other paired organs are mostly not bilaterally symmetrical, one of each pair being either rudimentary or absent. There is no urinary bladder, and the cloacal aperture is transverse.
Of these characters of the snakes, the most obvious and striking are to be found in the nature of the organs of locomotion. The front limbs, with the scapular arch and sternum, are invariably altogether absent; and the hind-limbs, if not wholly wanting, are never represented by more than an imperfectly-developed series of bones concealed within the muscles on each side of the anal aperture, and never exhibiting any outward evidence of their existence beyond the occasional presence of short horny claws or spurs ("calcaria"). In the entire absence, then, or rudimentary condition of the limbs, the Snakes progress by means of the ribs. These bones are always extremely numerous (sometimes amounting to more than three hundred pairs), and in the absence of a sternum, they are, of course, extremely movable. Their free extremities, in fact, are simply terminated by tapering cartilages, which are attached by muscular connections to the abdominal scales or "scuta" of the integument. By means of this arrangement the Serpents are enabled to progress rapidly, walking, so to speak, upon the ends of their ribs; their movements being much facilitated by the extreme mobility of the whole vertebral column, conditioned by the cup-and-ball articulation of the bodies of the vertebrae with one another.
The body in the Snakes is covered with numerous scales, developed in the dermis, and covered by a thin, translucent, superficial epidermic pellicle, which is periodically cast off and renewed. Usually the scales are flat and overlap one another; but sometimes they are tubercular and do not overlap. On the head and along the abdomen these scales are larger than over the rest of the body, and they constitute what are known as the "scuta" or shields.
The only other points in the anatomy of the Ophidia which demand special attention are the structure of the tongue, teeth, and eye.
The tongue in the Snakes is probably an organ more of touch than of taste. It consists of two muscular cylinders, united towards their bases, but free towards their extremities. The bifid organ, thus constituted, can be protruded and retracted at will, being in constant vibration when protruded, and being in great part concealed by a sheath when retracted.
As regards the eye of Serpents (fig. 296, A) the chief peculiarity lies in the manner in which it is protected externally. There are no eyelids, and hence the stony unwinking stare of all snakes. In place of eyelids, the eye is surrounded by a circle of scales (e e), to the circumference of which is attached a layer of transparent epidermis, which covers the whole eye (d), and is termed the antocular membrane. This is covered internally by a thin layer of the conjunctiva, which is reflected forwards from the conjunctiva covering the ball of the eye itself. In this way a cavity or chamber is formed between the two layers of conjunctiva, and the lachrymal secretion, by which the eye is moistened, is received into this. The outer epidermic layer (antocular membrane) covering the ball of the eye in front, is periodically shed with the rest of the epidermis, the animal being rendered thereby blind for a few days. The pupil of the eye is round in most Snakes, but forms a vertical slit in the venomous Serpents and in the Boas.
Fig. 295. - The Rattlesnake (Crotalus horridus).
As regards the dental and maxillary apparatus of the Serpents, the following points require notice : Firstly, in consequence of the articulation of the lower jaw with a movable quadrate bone, which is often directed backwards, in consequence of the quadrate bone being connected with a movable squamosal bone, and in consequence of the rami of the jaw being united in front by ligaments and muscles only, the mouth in the Snakes is capable of opening to an enormous width, and the most astonishing feats in the way of swallowing can be performed. Secondly, this structure of the jaws accords exactly with the structure of the teeth, both concurring to render the Snakes wholly incapable of anything like mastication, and at the same time capable of swallowing immense morsels entire. The teeth, namely, are simply fitted for seizing and holding the prey, but not in any way for dividing or chewing it. In the non-venomous and most typical Snakes, the jaws and palatine bones carry continuous rows of solid conical teeth, so that there are four rows above and two below; and the superior maxillae are very long and are not movable. Thirdly, in the Viperine Snakes, and the Crotalidae, the ordinary teeth are wanting upon the superior maxillae, whilst these bones are themselves very much shortened, and are capable of being raised and depressed at will. In place of the ordinary teeth, each maxilla carries a "poison-fang," in the form of a long, conical, curved fang, which is concealed in a fold of the mucous membrane when not in use, and has numerous germs or reserve-fangs behind it (figs. 296, 13, and 297). Each tooth is perforated by a tube, opening by a distinct aperture at the apex of the tooth, and conveying the duct of the so-called poison-gland. (In reality the poison-duct of the fang is formed by an inflection of the tooth upon itself, and not by its actual perforation.) This is a gland (fig. 298), probably produced by a modification of one of the buccal salivary glands, situated behind and under the eye on each side, and secreting the fluid which renders the bite of these snakes dangerous or fatal. When the animal strikes its prey, the poison-fangs are erected by the elevation of the movable maxillae (to which they are anchylosed), and the poison is forced through the tube which perforates each, partly by the contractions of the muscular walls of the gland, and partly by the muscles of the jaws. In most poisonous Snakes the superior maxillae carry no other teeth except the poison-fangs and their rudimentary successors, but in some cases there are a few teeth behind the fangs; whilst the palatine teeth are always present, as in the harmless species. In some other venomous Snakes, again (e.g., Naja and the Hydrophidae), the jaws and teeth agree in most characters with those of the non-venomous Snakes, but the first maxillary teeth are larger than the others, and form canaliculated fangs. Lastly, in a few forms the terminal maxillary teeth are deeply canaliculated, but are not connected with the duct of any poison-gland.