Poison Apparatus in Snake

A poisonous snake is a type of snake that produces venom, a toxic substance that can cause harm to other organisms, including humans. Venomous snakes have specialized structures, such as venom glands and fangs, which they use to deliver their venom. The venom is used primarily for immobilizing and digesting prey, but it can also serve as a defense mechanism.

Not all snakes are venomous, and the term "poisonous snake" is often used interchangeably with "venomous snake".

Snake Venom Glands

The venom apparatus in snakes is a highly specialized system composed of a pair of venom glands, their associated ducts, and a pair of fangs. In venomous snakes, these glands are situated on either side of the upper jaw, likely derived from superior labial glands or parotid glands. Each gland is sac-like and contains a narrow duct at its anterior end. The capsule surrounding the gland sends vascular fibrous septa, which divide the glandular tissue into secretory pockets. The duct extends forward along the upper jaw, looping over itself before terminating at the base of the fang or at the base of a tunnel on the fang. Ligaments securely hold the venom gland in place; an anterior ligament attaches the gland to the maxilla, while a posterior ligament connects it to the quadrate. Additionally, fan-shaped ligaments are positioned between the side walls and the squamoso-quadrate junction, providing further stabilization.

Fangs

Fangs, the specialized maxillary teeth in venomous snakes, are crucial for venom delivery. These teeth are long, sharply pointed, and hook-like, with a superficial enamel layer that makes them extremely hard and calcified. When a fang is lost, it regenerates. Fangs are categorized based on structural differences and their position within the mouth.

Structural Classification of Fangs

1. Open Type: In snakes like cobras, the venom groove in the fang is open.
2. Closed Type: In vipers, the venom groove forms a tunnel with openings at both the base and near the apex of the fang.

Positional Classification of Fangs

Snakes possess several tooth-bearing bones, including the premaxilla, maxilla, pterygoid, and palatine in the upper jaw, and the dentine in the lower jaw. The teeth of modern snakes are generally divided into four types, three of which are fangs, named with the Greek word "glyph," meaning grooves.

1. Solenoglyphous Fangs:
   • These long, tubular fangs are attached to the snake’s maxillary bone. The maxilla is hinged, allowing the fangs to be folded back parallel to the jaw when the mouth is closed or erected perpendicularly when striking. The pterygoid and dentine teeth manipulate food once inside the mouth.
   • Solenoglyphous fangs resemble hypodermic needles, with a hollow core that channels venom from the gland through an entrance orifice near the base and injects it through a slit-like exit orifice near the tip. If the opening were at the very tip, it would compromise the fang's strength and sharpness.
   • Example: Vipers and rattlesnakes.
2. Proteroglyphous Fangs:
   • Located at the front of the mouth, these fangs are about three times shorter than solenoglyphous fangs. Unlike solenoglyphs, some proteroglyphs have other teeth on the maxilla behind the fang, but the fang is separated by a gap called a diastema.
   • Some species have multiple functional fangs on each side, with at least two fangs present on each maxilla at any time—one in use and one reserve. Both fangs are covered by a fang sheath composed of connective tissue and skin.
   • Some proteroglyphs, including dangerous species like mambas and taipans, have partially movable fangs. Spitting cobras have modified fangs with smaller, rounder exit orifices, enhancing the velocity of venom ejection.
   • Example: Elapids and sea snakes.
3. Opisthoglyphous Fangs:
   • These grooved fangs are positioned near the back of the maxilla, behind the normal teeth. They are connected to Duvernoy's glands, which differ from true venom glands by lacking muscles to generate the pressure required for venom evacuation, as seen in solenoglyphous and proteroglyphous snakes.
   • The venom pressure in these glands is generally less than 5 psi, compared to over 30 psi in biting solenoglyphs and proteroglyphs. Due to the absence of a venom storage chamber, it is believed that opisthoglyphous snakes secrete venom primarily during chewing.
   • Example: Some colubrid snakes.
4. Aglyphous Teeth:
   • This term describes unmodified, non-fang teeth, used by all snakes, even those with fangs of the first three types, for gripping prey during swallowing. Some species have enlarged posterior maxillary teeth without grooves, classifying them as aglyphous.
   • Among colubrids, the distinction between opisthoglyphs and aglyphs is not entirely clear.
   • Example: Pythons and boas.

Muscles Involved in the Snake Biting Mechanism

The venom apparatus is associated with specialized muscle bands that facilitate the snake’s biting mechanism:

1. Digastric Muscle: Attached to the squamosal bone of the skull and the articular of the lower jaw, it aids in opening the jaws.
2. Sphenopterygoid Muscle: This muscle connects anteriorly to the spheroidal region and posteriorly to the dorsal surface of the pterygoid, helping to pull the pterygoid forward.
3. Anterior and Posterior Temporalis Muscles: These muscles attach to the side walls of the cranium and the lower jaw, playing a crucial role in closing the lower jaw.