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Snake venom is a highly modified saliva that is produced by special glands. The gland which secretes the toxin is a modification of the parotid salivary gland of other vertebrates, and is usually situated on each side of the head below and behind the eye, invested in a muscular sheath. It is provided with large alveoli in which the venom is stored before being conveyed by a duct to the base of the channelled or tubular fang through which it is ejected. Snake venom is a combination of many different proteins and enzymes. Many of these proteins are harmless to humans, but some are toxins.
Note that snake venoms are generally harmless when ingested, and are therefore not technically poisons.
The chemistry of snake venoms
Snake venom is a mixture of toxsins and different enzymes used for other purposes like increasing the prey's uptake of toxins.
Phosphodiesterases are used to interfer with the prey's cardiac system, mainly to lower the blood pressure.
Snake venom inhibits cholinesterase to make the prey lose control of its muscles.
Hyaluronidase increases tissue permeability to increase the rate that other enzymes are absorbed into the prey's tissues.
Amino acid oxidases and proteases are used for digestion. Amino acid oxidase also triggers some other enzymes and is responsible for the yellow color of the venom of some species.
Snake venom often contains ATPases which are used for breaking down ATP to disrupt the prey's energy supply.
Injection
In the vipers, which furnish examples of the most highly developed venom delivery apparatus, although inferior to some in its toxic effects, the venom gland is very large and in intimate relation with the masseter or temporal muscle, consisting of two bands, the superior arising from behind the eye, the inferior extending from the gland to the mandible. A groove or duct can be located traveling from the modified salivary glands where venom is produced down the length of the fang and out to the tip. In some species, notably the vipers and cobras, this groove is completley closed over. In other species, such as the adders and mambas, this groove is not covered, or only covered partially. From the anterior extremity of the gland the duct passes below the eye and above the maxillary bone, where it makes a bend, to the basal orifice of the venom fang, which is ensheathed in a thick fold of mucous membrane, the vagina dentis. By means of the movable maxillary bone hinged to the prefrontal, and connected with the tranverse bone which is pushed forward by muscles set in action by the opening of the mouth, the tubular fang is erected and the venom discharged through the distal orifice in which it terminates. When the snake bites, the jaws close up, causing the gland to be powerfully wrung, and the venom pressed out into the duct.
In some of the proteroglyphous colubrids, the venom fangs are not tubular, but only channelled and open along the anterior surface; and as the maxillary bone in these snakes is more or less elongate, and not or but slightly movable vertically, the venom duct runs above the latter, making a bend only at its anterior extremity, and the tranverse bone has not the same action on the erection of the fangs. Otherwise the mechanism is the same.
In the opisthoglyphous colubrids, with grooved teeth situated at the posterior extremity of the maxilla, a small posterior portion of the upper labial or salivary gland is converted into a venom-secreting organ, distinguished by a light yellow colour, provided with a duct larger than any of those of the labial gland, and proceeding inward and downward to the base of the grooved fang; the duct is not in direct connection with the groove, but the two communicate through the mediation of the cavity enclosed by the folds of mucous membrane surrounding the tooth, and united in front.
The reserve or successional teeth, which are always present just behind or on the side of the functional fang of all venomous snakes, are in no way connected with the duct until called upon to replace a fang that has been lost. It could not be otherwise, since the duct would require a new terminal portion for each new fang; and as the replacement takes place alternately from two parallel series, the new venom-conveying tooth does not occupy exactly the same position as its predecessor.
Two genera, Doliophis among the Elapine Colubrids, and Causus among the Viperids, are highly remarkable for having the venom gland and its duct of a great length, extending along each side of the body and terminating in front of the heart. Instead of the muscles of the temporal region serving to press out the venom into the duct, this action is performed by those of the side of the body.
When biting, a Viperid snake merely strikes, discharging the venom the moment the fangs penetrate the skin, and then immediately leaves go. A proteroglyph or opisthoglyph, on the contrary, closes its jaws like a dog on the part bitten, often holding on firmly for a considerable time. The venom, which is mostly a clear, limpid fluid of a pale straw or amber colour, or rarely greenish, sometimes with a certain amount of suspended matter, is exhausted after several bites, and the glands have to recuperate.
It must be added that the venom can be ejected otherwise than by a bite, as in the so-called spitting cobras of the genera Naja and Hemachatus. The fact that some of these deadly snakes when irritated are in the habit of shooting venom from the mouth, at a distance of 4 to 8 feet, even apparently aiming at a man’s face, has been too often witnessed in India and Malaysia, and especially in Africa, from the days of the ancient Egyptians, for any doubt to subsist as to their being endowed with this faculty, but the mechanism by which this action is produced has not been satisfactorily explained. In all probability, the venom escapes from the sheath of mucous membrane surrounding the base of the fangs, and is mixed with ordinary saliva, the membranes of the mouth perhaps acting as lips, in which case the term “spitting” would not be incorrect. The spitting, which may take place three or four times in succession, has been observed to be preceded by some chewing movements of the jaws. If reaching the eye, the poisonous fluid causes severe inflammation of the cornea and conjunctiva, but no more serious results if washed away at once.
Effects
There are three distinct types of venom that act on the body differently. Hemotoxic venoms act on the heart and cardiovascular system. Neurotoxic venom acts on the nervous system and brain. Cytotoxic venom has a localized action at the site of the bite.
The effect of the venom of proteroglyphous snakes (Hydrophids, cobras, Bungarus, Elaps, Pseudechis, Notechis, Acanthophis) is mainly on the nervous system, respiratory paralysis being quickly produced by bringing the venom into contact with the central nervous mechanism which controls respiration; the pain and local swelling which follow a bite are not usually severe.
Viper venom (Vipera, Echis, Lachesis, Crotalus) acts more on the vascular system, bringing about coagulation of the blood and clotting of the pulmonary arteries; its action on the nervous system is not great, no individual group of nerve-cells appears to be picked out, and the effect upon respiration is not so direct; the influence upon the circulation explains the great depression which is a symptom of Viperine envenomation. The pain of the wound is severe, and is speedily followed by swelling and discoloration. The symptoms produced by the bite of the European vipers are thus described by the best authorities on snake venom (Martin and Lamb):
The bite is immediately followed by local pain of a burning character; the limb soon swells and becomes discoloured, and within one to three hours great prostration, accompanied by vomiting, and often diarrhoea, sets in. Cold, clammy perspiration is usual. The pulse becomes extremely feeble, and slight dyspnoea and restlessness may be seen. In severe cases, which occur mostly in children, the pulse may become imperceptible and the extremities cold; the patient may pass into coma. In from twelve to twenty-four hours these severe constitutional symptoms usually pass off; but in the meantime the swelling and discoloration have spread enormously. The limb becomes phlegmonous, and occasionally suppurates. Within a few days recovery usually occurs somewhat suddenly, but death may result from the severe depression or from the secondary effects of suppuration. That cases of death, in adults as well as in children, are not infrequent in some parts of the Continent is mentioned in the last chapter of this Introduction.
The bite of all the proteroglyphous elapids, even of the smallest and gentlest, such as the Elaps or coral snakes, is, so far as known, deadly to man. The Viperidae differ much among themselves in the toxicity of their venom. Some, such as the Indian Vipera russelli and Echis carinatus, the American vipers, Crotalus, Lachesis mutus and lanceolatus, the African Causus, Bitis, and Cerastes, cause fatal results unless a remedy be speedily applied. On the other hand, the Indian and Malay Lachesis seldom cause the death of man, their bite in some instances being no worse than the sting of a hornet. The bite of the larger European Vipers may be very dangerous, and followed by fatal results, especially in children, at least in the hotter parts of the Continent; whilst the small Vipera ursinii, which hardly ever bites unless roughly handled, does not seem to be possessed of a very virulent venom, and, although very common in some parts of Austria-Hungary, is not known to have ever caused a serious accident.
It is noteworthy that the size of the venom fangs is in no relation to the virulence of the venom. The comparatively innocent Indo-Malay Lachesis alluded to above have enormous fangs, whilst the smallest fangs are found in the most justly dreaded of all snakes, the Hydrophids.
Little is known of the physiology of the venom of the opisthoglyphous colubrids, except that in most cases it approximates to that of the proteroglyphs. Experiments on Coelopeltis, Psammophis, Trimerorhinus, Dipsadomorphus, Trimorphodon, Dryophis, Tarbophis, Hypsirhina, and Cerberus, have shown these snakes to be possessed of a specific venom, small mammals, lizards, or fish, being rapidly paralyzed and succumbing in a very short time, whilst others (Eteirodipsas, Ithycyphus) do not seem to be appreciably venomous. Man, it is true, is not easily affected by the bite of these snakes, since, at least in most of those which have a long maxillary bone, the grooved fangs are placed too far back to inflict a wound under ordinary circumstances.
There are, however, exceptions. A case was reported a few years ago of a man in South Africa nearly dying as a result of the bite of the Boomslang, Dispholidus tytus, the symptoms, carefully recorded, being those characteristic of Viperine envenomation, an important fact to oppose to the conclusions, based on the physiological experiments on Coelopeltis, which appeared to disprove the theory that the Viperidae may have been derived from opisthoglyphous colubrids.
Experiments made with the secretion of the parotid gland of Tropidonotus and Zamenis have shown that even aglyphous snakes are not entirely devoid of venom, and point to the conclusion that the physiological difference between so-called harmless and venomous snakes is only one of degree, just as there are various steps in the transformation of an ordinary parotid gland into a venom gland or of a solid tooth into a tubular or grooved fang. The question whether all snakes are immune to their own venom is not yet definitely settled. Most snakes certainly are, and it is a remarkable fact that certain harmless species, such as the North American Coronella getula and the Brazilian Rhacidelus brazili, are proof against the venom of the Crotalines which frequent the same districts, and which they are able to overpower and feed upon. The Cribo, Spilotes variabilis, is the enemy of the Fer-de-lance in St. Lucia, and it is said that in their encounters the Cribo is invariably the victor. Repeated experiments have shown our Common Snake, Tropidonotus natrix, not to be affected by the bite of Vipera berus and Vipera aspis, this being due to the presence, in the blood of the harmless snake, of toxic principles secreted by the parotid and labial glands, and analogous to those of the venom of these vipers.
The Hedgehog, the Mongoose, the Secretary Bird, and a few other birds feeding on snakes, are known to be immune to an ordinary dose of snake venom; whether the pig may be considered so is still uncertain, although it is well known that, owing to its subcutaneous layer of fat, it is often bitten with impunity. The garden dormouse (Eliomys quercinus) has recently been added to the list of animals refractory to viper venom.
Studies
The subject of snake venoms is one which has always attracted much attention and which has made great progress within the last quarter of a century. Especially noteworthy is progress regarding the defensive reaction by which the blood may be rendered proof against their effect, by processes similar to vaccination—antipoisonous serotherapy. The studies to which we allude have not only conduced to a method of treatment against snake-bites, but have thrown a new light on the great problem of immunity. They have shown that the antitoxic sera do not act as chemical antidotes in destroying the venom, but as physiological antidotes; that, in addition to the venom glands, snakes possess other glands supplying their blood with substances antagonistic to the venom, such as also exist in various animals refractory to snake venom, the hedgehog and the mongoose for instance. Unfortunately, the specificity of the different snake venoms is such that, even when the physiological action appears identical, serum injections or graduated direct inoculations confer immunity towards one species or a few allied species only. Thus, a European in Australia who had become immune to the poison of the deadly Australian Tiger Snake, Notechis scutatus, manipulating these snakes with impunity, and was under the impression that his immunity extended also to other species, when bitten by a Denisonia superba, an allied Elapine, died the following day. In India, the serum prepared with the venom of Naja tripudians has been found to be without effect on the venom of the two species of kraits of the genus Bungarus, and the Old World vipers Vipera russelli and Echis carinatus, and the pit viper Trimeresurus popeiorum. Vipera russelli serum is without effect on colubrine venoms, or those of Echis and Trimeresurus. In Brazil, serum prepared with the venom of the New World pit viper Lachesis lanceolatus is without action on Crotalus venom. Antivenom snakebite treatment must be matched as the type of envenomation that has occurred. In the Americas, polyvalent antivenoms are available that are effective against the bites of most pit vipers. These are not effective againts coral snake envenomation, which requires a specific antivenom to their neurotoxic venom. The situation is even more complex in countries like India, with its rich mix of vipers family Viperidae and highly the neurotoxic cobras and kraits of the family Elapidae.
This article is based on the 1913 book The Snakes of Europe, by G. A. Boulenger, which is now in the public domain in the United States (and possibly elsewhere) because of its age. Because of its age, the text in this article should not been viewed as reflecting the current knowledge of snake venom.
"Snake venom"