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Poisonous snakes are divided by naturalists into two great Families, the Colubridæ and Viperidæ, distinguished from each other by certain anatomical characters, and especially by the dentition.

The Colubridæ resemble harmless snakes, which renders them all the more dangerous.

They are divided into two groups: Opisthoglypha (ὄπισθεν, behind; γλυφὴ, a groove) and Proteroglypha (πρότερον, before; γλυφὴ, a groove).

The Opisthoglypha have the upper jaws furnished in front with smooth or non-grooved teeth, but behind with one or several rows of long, canaliculate teeth.

This group includes three Sub-families:—

A. The Homalopsinæ, having valved nostrils, placed above the snout.

B. The Dipsadomorphinæ, in which the nostrils are lateral in position, and the dentition is highly developed.

C. The Elachistodontinæ, which have but rudimentary teeth only on the posterior portion of the maxillary, on the palatine and on the pterygoid bones.

Almost all the snakes belonging to these three sub-families are poisonous, but only slightly so. They are not dangerous to man. Their venom merely serves to paralyse their prey before deglutition takes place; it does not afford them an effective means of defence or attack.

All the Homalopsinæ are aquatic; they bring forth their young in the water, and are met with commonly in the Indian Ocean, starting from Bombay, and especially in the Bay of Bengal, on the shores of Indo-China and Southern China, from Singapore to Formosa, in the Dutch Indies, in Borneo, the Philippines, New Guinea and the Papuan Archipelago, and as far as the north of Australia.

The Dipsadomorphinæ comprise a large number of highly cosmopolitan genera and species, found in all the regions of the earth except the northerly portions of the Northern Hemisphere. None of these reptiles is capable of causing serious casualties among human beings, owing to the peculiarly defective arrangement of their poison-apparatus. I therefore do not think it worth while to linger here over their description.

The Elachistodontinæ are of even less importance; at the present time only two species are known, both of small size and confined to Bengal.

The Proteroglypha group of the Colubridæ is of much greater interest to us, since all the snakes belonging to it are armed with powerful fangs, in front of the upper maxillaries. These fangs, which are provided with a channel in the shape of a deep groove, communicate at the base with the efferent duct of poison glands, which are often of very large size.

The group is composed of two Sub-families:—

A. The Hydrophiinæ (sea-snakes), provided with a flattened oar-shaped tail. The body is more or less laterally compressed; the eyes are usually small, with circular pupils; the scales of the nose have two notches on the upper labial border.

The normal habitat of all the members of this sub-family is the sea, near the shore, with the exception of the genus Distira, which is met with in the fresh water of a lake in the Island of Luzon, in the Philippines. They are frequently found in very large numbers in the Indian seas and throughout the tropical zone of the Pacific Ocean, from the Persian Gulf to the west coast of the American Continent, but they are entirely absent from the West Coast of Africa.

B. The Elapinæ (land-snakes), with a cylindrical tail, and covered with smooth or carinate scales. These serpents are frequently adorned with brilliant colours. Some of them (belonging to the genus Naja) have the faculty of expanding the neck in the shape of a parachute, by spreading out the first pairs of ribs when they are alarmed or excited: the breadth of the neck then greatly exceeds that of the head. They are distributed throughout Africa, Asia, and North and South America, and are also found in Australia, where almost all the snakes that are known belong to this sub-family.

The Family Viperidæ is characterised by a triangular head, which is widened posteriorly, and by the general aspect of the body, which is usually thick-set and terminated by a short tail. The bones of the face are movable. The præfrontal bone is not in contact with the nasal; the maxillary is greatly shortened and may be articulated perpendicularly to the ectopterygoid; it bears a pair of large poison-fangs, one on each side, and these are always accompanied by several teeth to replace them, folded back in the gum; these latter teeth come in succession to take the place of the principal tooth, when this is broken or falls out of itself when the snake sheds its skin.

The poison-fangs are not grooved, as in the Proteroglyphous Colubridæ; they are pierced by a perfectly formed canal, the upper end of which inosculates with the efferent duct of the corresponding poison-gland, while its lower extremity opens to the exterior a little above and in front of the tip. The latter is always very sharp.

The palate and lower jaw are furnished with small hooked teeth, which are solid and non-venomous.

With the exception of the species of Atractaspis, these snakes are all ovoviviparous. The majority are terrestrial; a few lead a semi-aquatic existence, while others are arboreal.

Their distribution includes Europe, Asia, Africa (with the exception of Madagascar), and North and South America. They do not exist in Australia.

They are divided into two Sub-families:—

A. The Viperinæ, in which the head, which is very broad and covered with little plates and scales, has no pit between the nose and the eyes;

B. The Crotalinæ (κρὁταλον, a rattle), in which the head is incompletely covered with scales, and exhibits a deep pit on each side, between the eye and the nostril.

Among snakes, the characters that serve as a basis for the determination of genera and species are the general shape of the body, especially that of the head, the arrangement of the cephalic scales, the cranial skeleton, and the dentition.

Cranial Skeleton.—The cranium is composed of a certain number of bones, the homologues of which are found in the mammalian skeleton; but the bones are complex, and subject to modifications according to the structure and habitat of each species.

The special arrangement of the bones of the face is above all characteristic of the poisonous snakes. Those forming the upper jaw, the palate and the mandibles or “inter-maxillaries” are movable upon each other and on the cranium. The upper and lower maxillaries are united by an extensile ligament and articulated with the tympanic bone, which permits the mouth to be opened very widely when the animal swallows its prey.

Dentition.—The non-poisonous snakes have two rows of teeth in the upper jaw—one external, the maxillary, usually composed of from 35–40 small, backwardly curved teeth; the other internal, the palatine, which only numbers from 20–22 teeth, having the same curvature (fig. 1, A).

Fig. 1.—A, Cranial skeleton of one of the non-poisonous Colubridæ (Ptyas mucosus); B, cranial skeleton of one of the poisonous Colubridæ (Naja tripudians); C, cranial skeleton of one of the poisonous Colubridæ (Bungarus fasciatus); D, cranial skeleton of one of the Viperidæ (Vipera russellii); E, cranial skeleton of one of the Viperidæ Crotalinæ (Crotalus durissus); F, cranial skeleton of one of the Colubridæ Hydrophiinæ (Hydrophis pelamis).

In the poisonous snakes the maxillary bones are shorter, and the outer row is represented by a single long and tubular or grooved tooth (the fang), fused with the maxillary bone, which is itself movable (fig. 1, B, C, D, E, F).

Fig. 2.—A, Maxillary bone and fangs of one of the Viperidæ (Vipera russellii); B, maxillary bone and fangs of one of the Colubridæ (Naja tripudians); C, maxillary bone and fangs of one of the Colubridæ (Bungarus fasciatus); D, maxillary bone and teeth of one of the non-poisonous Colubridæ (Ptyas mucosus). (After Sir Joseph Fayrer.)

Certain species (Dipsas) have maxillary teeth which increase in size from front to rear; the longest teeth are grooved and serve for the better retention of prey, and also to impregnate it with saliva; but they are not in communication with the poison-glands.

The poison-fangs are normally covered with a fold or capsule of mucous membrane, in which they are sheathed. This fold conceals a whole series of reserve teeth in different degrees of development, which eventually become attached to the extremity of the maxillary when the principal tooth falls out or is broken (fig. 2).

Fig. 3.—A, Fang of one of the Viperidæ (Vipera russellii); D, transverse section of the fang.

Poison Apparatus.—The grooved or furrowed teeth in the Proteroglypha and the canaliculate teeth in the Solenoglypha are arranged, not for the purpose of seizing prey, but in order to deal it a mortal blow by injecting the venom into its flesh.

In the normal position they lie almost horizontally, and exhibit no mobility of their own. But, when the animal prepares to bite, their erection is effected by the snake throwing its jaw back; and this movement, which is always very sudden, enables it at the same time to compress its poison-glands, by the aid of special constrictor muscles.

On examining the various species of poisonous snakes, we observe very sharply marked differences in the arrangement and dimensions of the teeth. Thus, in the Viperidæ they are long, extraordinarily sharp, and capable of producing deep wounds (fig. 2, A, and fig. 3). They are traversed by an almost completely closed canal, from the base, which communicates with the poison-duct, to the neighbourhood of the point, where it opens very obliquely on the convex surface (fig. 3, A and D).

Fig. 4.—B, Fang of one of the Colubridæ (Naja tripudians); E, transverse section.

Fig. 5.—C, Fang of one of the Hydrophiinæ (Hydrophis pelamis); F, transverse section.

Fig. 6.—Three transverse sections of a poison-fang of one of the Colubridæ, through A A, B B, C C; P P, pulp cavity; V V V, poison-canal (groove). (After C. J. Martin.)

In the Elapinæ sub-family of the Colubridæ, and especially in the Hydrophiinæ, the teeth are much shorter and simply grooved or canaliculate; that is to say, the canal communicates with the exterior throughout its extent by a narrow slit, which traverses the entire convex surface of the tooth (figs. 4, 5, and 6).

It does not follow from this that the bites of these reptiles are less dangerous; the contrary, indeed, is the case, for their venom is infinitely more active.

These differences, as well as the particular mode of arrangement of the other little non-poisonous teeth in both jaws, enable us in many cases to recognise, by the mere appearance of the bite, the species of snake by which the bite has been inflicted.

Fig. 7.—Marks produced on the Skin by the Bites of different Species of Snakes.

I. Non-venomous Colubrine.—The bite is marked only by the imprint of from 35–40 small palatine or pterygoid teeth, and 20–22 upper maxillary teeth (on the outside of the foregoing) on each side.

II. Venomous Colubrine (Naja tripudians, Proteroglypha).—The bite exhibits 25 or 26 punctures from the pterygoid or palatine teeth, and, on each side, one or two, rarely three, circular wounds produced by the principal poison-fangs and by the reserve teeth.

III. Viperidæ (Solenoglypha).—The sole indication of the bite consists of 8 or 10 punctures from the palatine or pterygoid teeth, and one little round wound, on each side, produced by the poison-fangs.

(After Sir Joseph Fayrer.)

Fig. 7, above, shows how it is possible to distinguish the marks left by a non-venomous reptile, and by one of the Proteroglypha or Solenoglypha respectively.

Poison-glands.—The poison-glands occupy an extensive inter-muscular space behind the eyes, on each side of the upper jaw. They are oval in shape, and may, in Naja tripudians for example, attain the size of a large almond (fig. 8).

Their structure is the same as that of the salivary glands of the larger animals. The poison that they secrete accumulates in their acini and in the efferent duct that opens at the base of the corresponding fang.

Each gland is surrounded by a capsule, to which are partly attached the fibres of the masseter muscle, which violently compresses it and drives the poison, just as the piston of a syringe would do, into the canaliculi or groove of the fang.

Fig. 8.—Poison-gland and Fangs of a Venomous Snake (Naja tripudians, Colubridæ). (Natural size.)

L, Lobe of the gland; D, poison-duct; F, fang attached to the maxillary bone; G G, gland; M, capsule of mucous membrane surrounding the fangs; R, reserve fangs; A A, muscular fascia covering the gland.

(After Sir Joseph Fayrer.)

In a few venomous snakes the gland is developed to such an extent that it extends as far as the first ribs.

The species, on the contrary, that have the poison-teeth placed in the hinder part of the mouth (Opisthoglypha) have glands but little developed.

Muscular Apparatus of the Head.—Figs. 9 and 10, 11 and 12 show the arrangement of the principal muscles that work the jaws and glandular organs in Vipera russellii and Naja tripudians, which respectively represent the most formidable types of Viperidæ and venomous Colubridæ.

It is not necessary to give a detailed description of each of these muscles. Let it suffice to point out that all contribute in giving the greatest elasticity to the jaws, and at the same time strength sufficient to retain the prey and to cause it to pass from front to rear towards the œsophagus, by a series of alternate antero-posterior movements and analogous lateral ones. By means of these movements, which are participated in by the upper and lower maxillary bones, the palatines, mandibles or inter-maxillaries, and the pterygoids, the animal in a manner draws itself over its prey like a glove, since the arrangement of its dentition does not admit of mastication.

A A, Fascia covering the anterior and posterior temporal muscles; B, small gland; C, tendinous insertion of the fascia; D, poison-duct; E, poison-fang; F, reserve fangs; G, mandible; H, ectopterygoid muscle; J, poison-gland covered by the masseter; K, masseter inserted in the mandible; L, insertion of the temporal muscle; M, digastric muscle.

A A, Ectopterygoid muscle; B, præ-spheno-pterygoid muscle; C, intermandibular muscle; D, præ-spheno-palatine muscle; E præ-spheno-vomerine muscle; F, capsule of mucous membrane surrounding the fangs; G, long muscle of the neck.

Figs. 9 and 10.—Muscular Apparatus and Poison-gland of Vipera russellii (Viperidæ). (After Sir Joseph Fayrer.)

A, Tracheo·mastoid muscle; B B, digastric muscle; C C, posterior temporal muscle; D D, anterior temporal muscle; E E, masseter; F, poison-gland, covered by the masseter and fascia; G, poison-duct; H, maxillary bone; I, neuro-mandibular muscle; J, costo-mandibular muscle.

A, Entopterygoid muscle; B, poison-gland; C, poison-duct; D, poison-fangs; E, præ-spheno-palatine muscle; F, præ-spheno-vomerine muscle; G, capsule of mucous membrane surrounding the fangs; H, præ-spheno-pterygoid muscle (which erects the fangs); J, inter-mandibular muscle; K, ectopterygoid muscle; L, long muscle of the neck (longus colli).

Figs. 11 and 12.—Muscular Apparatus and Poison-gland of Naja tripudians (Colubridæ). (After Sir Joseph Fayrer.)

Fig. 13.—Arrangement of the Scales of the Head in one of the Non-poisonous Colubridæ (Ptyas mucosus). (After Sir Joseph Fayrer.)

A, Rostral scale; B, anterior frontals; B¹, posterior frontals; C, vertical; D, occipitals; E, supra-ciliaries; F, temporals; L, M, nasals; N, loreals, or frenals; O, anterior oculars, or præ-orbitals; P, posterior oculars, or post-orbitals; Q, supralabials; G, median infralabial; H H, lateral infralabials; I K, mentals.

The enormous extensile power of the mouth and œsophagus thus enables snakes to swallow animals, the size of which is several times in excess of their own diameter.

Deglutition is slow and painful, but the gastric and intestinal juices are so speedy in action, that the digestion of the most resistant substances rapidly takes place. The very bones are dissolved, and the fæces, which are voided some days later, contain only a few osseous remains and a felt-like material composed of hair or feathers.

Scales.—The skin of snakes, which is very elastic and extensile, is covered with scales, small on the back, and in great transverse plates on the entire ventral surface.

Fig. 14.—Arrangement of the Scales of the Head in one of the Poisonous Colubridæ (Naja tripudians, OR Cobra-di-Capello). (After Sir Joseph Fayrer.)

The shape and dimensions of the scales of the head are highly characteristic in each species. It is therefore necessary to know their names and the arrangement that they exhibit: these details are shown with sufficient clearness in figs. 13 and 14.

Coloration.—The colouring exhibited by the scales of snakes is governed generally by the biological laws of mimicry. It is therefore not a character of specific value, and may be modified several times in the course of the existence of the same reptile, according to the surroundings in which it is obliged to live.

“Nature,” write Dumeril and Bibron, “seems to have caused the tints and colours of snakes to vary in accordance with their habits and modes of life. Generally speaking, the colours are greyish or dull in species that are wont to live among sand, or which bury themselves in loose earth, as also in those that lie in wait on the trunks or large boughs of trees; while these hues are of a bluish-green, resembling the tint of the leaves and young shoots of plants, in snakes that climb among bushes or balance themselves at the end of branches. It would be difficult to describe all the modifications revealed by a general study of the colours of their skins. Let us imagine all the effects of the decomposition of light, commencing with white and the purest black, and passing on to blue, yellow, and red; associating and mixing them together, and toning them down so as to produce all shades, such as those of green, of violet, with dull or brilliant tints more or less pronounced, and of iridescent or metallic reflections modified by spots, streaks, and straight, oblique, undulating, or transverse lines. Such is the range of colours to be found in the skin of snakes.”

This skin is covered by a thick epidermis, which is periodically detached in its entirety, most frequently in a single piece. Before effecting its moult, the reptile remains in a state of complete repose for several weeks, as if asleep, and does not eat. Its scales grow darker and its skin becomes wrinkled. Then one day its epidermis tears at the angle of the lips. The animal thereupon wakes up, rubs itself among stones or branches, divests itself entirely of its covering as though it were emerging from a sheath, and proceeds forthwith in quest of food.

The moult is repeated in this way three or four times every year.