Venoms: Venomous Animals and Antivenomous Serum-therapeutics. A. Calmette
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