Anatomical Peculiarities of the Poison Apparatus in Snakes : an Aspect of Classical Zoology

Snakes are ectothermic reptiles belong to the suborder Serpentes and thought to have evolved from lizards developing their specialized features by the Cretaceous times. Though the different species of snakes exhibit diversities in structure and habit but are closely resemble with each other. Many of them bears well formed biting apparatus, that coupled with toxic venoms, products by the poison gland, made them a dreadful creature.

Snakes show different types of defense strategies to protect themselves:

1. Venomous snakes may contain distinctive markings on their hoods to warn the predators that they are poisonous.

2. Hognose snake creates the sound 'hiss' to generate fear in the mind of predator, and some snakes like Rattle snake make noise by twitching their tails in dry grass for the same purpose.

3. The Costa Rican parrot snake frightens its predator by opening its bright coloured buccal cavity.

4. The Arizona coral snake possesses vibrant yellow and orange stripes, for that, predators can be frightened to think that they are poisonous.

5. Some other non-poisonous snakes mimic the outlooking of poisonous snakes.

The skull of a snake possess several unique features as follows :

1. The quadrate is highly streptostylic and is freely articulated with posterior end of the elongated squamosal.

2. The facial part of the skull is fairly loose and both upper and lower jaws are exceedingingly mobile.

3. The rami of the mandible are connected by elastic ligamentous tissue.

4. Small curved teeth develop in maxilla, palatine and pterygoid.

5. The premaxillae are small, fused and toothless.

Poison apparatus in snakes:
# 1
Less than one-third of the 2,500 to 3,000 snake species are listed in poisonous category, and less than 300 species may be harmful to human. Snake venom contains different types of chemicals those damage the victims body in several ways. Some poisons attack the nervous system (neurotoxic), some attack blood-vascular system (haemotoxic), and some attack muscular system (myotoxic) etc. The neurotoxins damage the nervous system, bring paralysis, even cause respiratory failure as found in Cobras; the haemotoxins attack walls of the blood vessels and blood corpucles and cause internal hemorrhage as found in rattle snakes, and myotoxins cause the damage to the muscular tissues as found in sea snakes.

# 2
a) Poison glands are modified superior labial or parotid glands.

b) This glands are usually situated on the either side of the upper jaw.

c) These are sac like structure and are thickly encapsulated with fibrous tissue.

d) A narrow duct rises from the anterior end of the poison sacs and passes forward along the side of the upper jaw and loops over itself just in front of the fang.

e) The capsule supports vascular fibrous septa that separate the glandular substances into secretory pockets.

f) In few snakes the poison gland has migrated back into the neck extending as far as the heart region.

# 3
Snakes eject venoms by their two hollow maxillary teeth called fangs. These are long sharply pointed and hook like, being extremely hard and calcified with a superficial enamel layer.

According to the structural differences poison fangs are of two categories :

1. Open type :

These are furrowed by a groove for the conduction of the venom.

2. Closed type :

In this category the fang has becomes almost or totally tubular, hypodermic syringe like structure due to approximation of the groove tips. The canals open almost at the extremity of the fang by the discharge aperture and the ducts of the poison gland open into the fang by an intake aperture.

According to the position, the poison fang may be of the following categories ;

1. Opisthoglyphodont :

In this type the fangs lie at the back of the maxilla and are of open category.

2. Proteroglypodont :

In this type the fangs are situated at the front end of the maxilla and are at closed category.

It is additionally to be stated that in Viper snakes fangs can be rotated into different positions and in Cobra snakes the fangs are fixed in position.

# 4
Some muscles and ligaments are also related with the poison apparatus:

1. Adductor mandibulae superficialis muscle :

This muscle connects the side wall of the cranium with the lower jaw.

2. Protractor and Levator pterygoidas :

This arises from the sphenoid bone and is inserted to the pterygoid.

3. Cervico-mandibularis muscle :

This is inserted on the lower end of the quadrate and arises from the neural spines of the anterior neck vertebrae.

4. Retractor pterygoid :

This arises from the parasphenoid and is inserted onto the palatines and other bones of the upper jaw.

5. Quadrate-maxillary ligament :

This connects the back of the maxilla with the quadrate.