POISON APPARATUS--DIFFERENT KINDS OF POISONS
The gland which secretes the poison 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.
In the Vipers, which furnish examples of the most highly developed
poison apparatus, although inferior to some in its toxic effects, the
poison 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.
When the snake bites, the jaws close up, causing the gland to be
powerfully wrung, and the poison pressed out into the duct. 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
poison fang, described above (p. 55), which is ensheathed in a thick
fold of mucous membrane, the _vagina dentis_. By means of the movable
maxillary bone (_supra_, p. 49) 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 poison
discharged through the distal orifice in which it terminates.
[Illustration: FIG. 13--POISON APPARATUS OF RATTLESNAKE: VENOM GLAND
AND MUSCLES (LATERAL VIEW). (After Duvernoy)
_a_, Venom gland; _a´_, venom duct; _b_, anterior temporal muscle;
_b´_, mandibular portion of same; _c_, posterior temporal muscle;
_d_, digastricus muscle; _e_, posterior ligament of gland; _f_,
sheath of fang; _g_, middle temporal muscle; _h_, external pterygoid
muscle; _i_, maxillary salivary gland; _j_, mandibulary salivary
gland.]
In some of the Proteroglyphous Colubrids, as we have seen, the poison
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 poison 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 poison-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
connexion 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 poison-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 poison 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 poison 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 poison, which is mostly a clear limpid fluid of a pale straw or
amber colour, more 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 poison can be ejected otherwise than by a
bite, as in the so-called Spitting Snakes of the genera _Naia_ and
_Sepedon_. The fact that some of these deadly snakes when irritated are
in the habit of shooting poison 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 Malaya, 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
poison 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.
Snake poisons is a subject which has always attracted much attention,
and which has made great progress within the last quarter of a century,
especially as regards 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 serums do not act as chemical antidotes in
destroying the venom, but as physiological antidotes; that, in addition
to the poison glands, snakes possess other glands supplying their blood
with substances antagonistic to the poison, such as also exist in
various animals refractory to snake poison, the hedgehog and the
mungoose for instance. Unfortunately, the specificity of the different
snake poisons 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
_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 _Naia
tripudians_ has been found to be without effect on the poison of _Naia
bungarus_, the two species of _Bungarus_, and the Vipers _Vipera
russelli_, _Echis carinatus_, and _Lachesis gramineus_. _Vipera
russelli_ serum is without effect on Colubrine venoms, and on those of
_Echis_ and _Lachesis_. In Brazil, serum prepared with the venom of
_Lachesis lanceolatus_ has proved to be without action on _Crotalus_
poison. These examples, and others which could be given, show that the
hopes which were at first entertained as to the benefits to be conferred
on mankind by the serum treatment were somewhat over-sanguine--at least
as regards countries like India, where, different kinds of poisonous
snakes occurring together, it is sometimes impossible to know by which
the bite has been inflicted.
Chemistry teaches that snake venoms consist for the most part of
solutions of modified proteids, and all attempts to separate the toxic
principles from such proteids have hitherto been unsuccessful.
Accordingly, at the present time we must regard such toxic principles as
residing in some special grouping of a portion of the atoms in the
complex venom proteid molecule. The analysis of their physiological
actions has proved them to be made up of a great many more constituents
than would be imagined from their chemical composition.
The effect of the poison of Proteroglyphous Colubrids (Hydrophids,
Cobras, _Bungarus_, _Elaps_, _Pseudechis_, _Notechis_, _Acanthophis_) is
mainly on the nervous system, respiratory paralysis being quickly
produced by bringing the poison into contact with the central nervous
mechanism which controls respiration; the pain and local swelling which
follow a bite are not usually severe.
Viper poison (_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
poisoning. 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
poison (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 Colubrids, even of the smallest and
gentlest, such as the _Elaps_ or Coral-snakes, is, so far as known,
deadly to man. The Viperidæ differ much among themselves in the toxicity
of their venom. Some, such as the Indian _Vipera russelli_ and _Echis
carinatus_, the American _Ancistrodon_, _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 poison, 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 poison 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 poison 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 poison, 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 typus_, the symptoms, carefully recorded,
being those characteristic of Viperine poisoning, an important fact to
oppose to the conclusions, based on the physiological experiments on
_Coelopeltis_, which appeared to disprove the theory that the Viperidæ
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 poisonous snakes
is only one of degree, just as there are various steps in the
transformation of an ordinary parotid gland into a poison gland or of a
solid tooth into a tubular fang.
The question whether all snakes are immune to their own poison 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 _Rhachidelus brazili_, are
proof against the poison 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 _V. 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 Mungoose, the Secretary Bird, and a few other birds
feeding on snakes, are known to be immune to an ordinary dose of snake
poison; 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 (_Myoxus
quercinus_) has recently been added to the list of animals refractory to
Viper poison.
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