Poison Apparatus And Biting Mechanism Of Snake, Lecture notes of Zoology

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Poison apparatus

and the biting

mechanism of

snakes

B.Sc. Part – I (Honours)

Prof. Jyoti Das

Contents

• Poisonous Apparatus Structure ..................................................................................
  • Poison Gland ...........................................................................................................
  • Fangs .......................................................................................................................
    • i. Open type and ii. Closed type...........................................................................
    • Solenoglyphous....................................................................................................
    • Proteroglyphous ...................................................................................................
    • Opisthoglyphous ..................................................................................................
    • Aglyphous ............................................................................................................
• Muscles associated with snake biting mechanism .....................................................
  • i. Digastric muscle ...............................................................................................
  • ii. Sphenopterygoid ..............................................................................................
  • iii. Anterior and posterior temporalis .................................................................
• Snake Biting Mechanism ...........................................................................................
  • I. The strike ..........................................................................................................
  • II. Opening of the mouth and elevation of the fangs ............................................
  • III. Closure of the mouth and the injection of venom .........................................
  • IV. Retraction of the fangs ................................................................................
• Difference between Non-poisonous Poisonous snakes: ..........................................
• Review Questions ....................................................................................................

i. Open type and ii. Closed type.

In open type as in cobras the poison groove is open and in closed type as in vipers, the poison groove forms a tunnel having two openings one at the base and one near the apex.

According to the position of poison fangs

Most snakes have several tooth bearing bones including four (the premaxilla, maxilla, pterygoid and palatine) in the upper jaw and the dentine in the lower jaw.

The teeth of modern snakes are classically divided into four types, three of which are classically called fangs. The name of all the teeth involves the Greek word glyph, which literally means grooves. They are as follows:

Solenoglyphous

Figure 2: Folding of Solenoglyphous fangs.  Fangs are long and tubular and attached to the snake’s maxillary bone.  Only single tooth, the fang is present on the maxilla, the maxilla is hinged so that the fangs can be folded back parallel to the jaws when the mouth is closed, or erected perpendicular to the jaw when the snake is striking.

 The teeth in pterygoid and dentine works together to manipulate food once inside the mouth.  Solenoglyphous fangs are strikingly similar to hypodermic needles. They have a hollow core that receives venom from the venom gland at the entrance orifice near the base and injects it from a slit-like exit orifice on the front of the fang near the tip. If the opening were at the very tip of the fang, its strength would be compromised and it would lack the sharp point needed to penetrate the target.  Example – vipers and rattle snakes.

Proteroglyphous  Proteroglyphous fangs are in the front of the mouth and are about three times shorter than solenoglyphous fangs.  Unlike solenoglyphs, some proteroglyphs have other teeth on the maxilla behind the fang. However, the fang is always separated from the other teeth by a gap, called a diastema. . Some elapids have more than one functional fang on each side.  In both vipers and elapids, there are usually at least two fangs on each maxilla at any one time, one that is in use and one that is a reserve fang.  Both fangs are draped in a layer of connective tissue and skin called the fang sheath.  Some proteroglyphs have partially movable fangs, including many of the most dangerous species such as mambas, taipans, and death adders.  A few, such as spitting cobras, have modified exit orifices to their fangs that are smaller and rounder than in other cobras, a modification that increases the velocity with which venom is ejected.  Example – Elapids and sea snakes.

Aglyphous  This word is used to describe unmodified teeth, essentially non-fangs. All snakes, even those that possess fangs of the first three types, have aglyphous teeth which they use for gripping their prey as they manipulate it during swallowing.  This species has enlarged posterior maxillary teeth that lack grooves, so they are by definition aglyphous.  Among colubrid, the distinction between opisthoglyphs and aglyphs has never been entirely clear.  Example – Python, Boa

Muscles associated with snake biting mechanism

The poison apparatus is associated with specialized bands of three types of muscles viz. i. digastrics ii. Sphenopterygoid iii, anterior and posterior temporalis

i. Digastric muscle – Attached to the squamosal of the skull at one end and articular of the lower jaw at the other end. It helps in opening jaws. ii. Sphenopterygoid – attached anteriorly to the spheroidal region and posteriorly to the dorsal surface of the pterygoid. It assists in pulling the pterygoid forward. iii. Anterior and posterior temporalis – attached to the side walls of the cranium and the lower jaw. They help in closing the lower jaw.

Figure 5: Muscles associated with poison apparatus

Snake Biting Mechanism

There are four distinct phases when a poisonous snake bites: (1) The strike; (2) opening of the mouth and elevation of the fangs; (3) closing of the jaws and the injection of venom; (4) retraction of the fangs.

I. The strike. – In this phase the snake throws itself forward with great rapidity and violence, the distance covered not generally exceeding one-third of its length. Vipers strike with greater velocity than the colubrids, some of which especially the hooded species raise the head from the ground thus compensating to some extent for the limited mobility of the fangs. II. Opening of the mouth and elevation of the fangs.- Most poisonous snakes commence the strike with closed jaws, but as the head approaches the victim the mandibles are depressed by a rapid contraction of the digastric, cervico-mandibular and vertebro-mandibular muscles and simultaneously the fangs are elevated or rotated forward by the forward swing of the pterygo-palatine-transverse arch produced by the contraction of the spheno- and parieto-pterygoid muscles. The fangs of the Colubrida are invariably grooved and are generally shorter than those of the viper, and their capacity for forward rotation is much more limited. The degree of elevation from extreme retraction to maximal protraction varies from 10°- 15 o^ to 45^0 - 500 in different species. Each pterygo-palatine-transverse arch acts as a single entity, and then the protractor muscles of the palate draw the endo-pterygoid forward, they invariably bring with it the palatine bone and the ecto-pterygoid which impinges on the posterior arm of the maxilla, driving the maxilla forwards and upwards on the articulating surface of the prefrontal. This produces a variable degree of elevation and forward rotation of the fangs, which are ankylosed to the inferior surface of the maxilla; its extent can be judged by the angle formed at the ecto- pterygoid maxillary junction which, in the resting position, forms a straight line. In these snakes, the smaller the maxilla the greater the forward movement of the pterygo-palatine-transverse arch and the greater the degree of forward projection of the fangs (i.e. the greater the maxillary index). This mechanism differs from that of the vipers in which the movement of the maxilla on the prefrontal is a

anatomical arrangement of muscles acting on the venom gland; expulsion of its contents is instantaneous and independent of fixation of the lower jaw.

Figure 8: Temporalis helps in closing the mouth.

IV. Retraction of the fangs.- Immediately following the insertion of the fangs, and actually accompanying the discharge of venom, contraction of the retractor muscles which operate on the pterygo-palatine-transverse arch occurs, dragging the elevated fangs downwards and backwards through the tissues.

Though the four stages including the inoculation of venom are described separately, they occur in nature as a series of rapidly co-ordinated movements, several lethal doses being injected in a fraction of a second in some instances- especially with the vipers.



Review Questions

1. State four differences between poisonous snake and non poisonous snakes citing at least three Indian examples of each. [4] 1997, 1999, 2006, 2008, 2010
2. Describe the structure of poison gland. [3] 1997, 2006, 2008, 2010
3. Give an account of the muscles involved in the biting mechanism of poisonous snakes, and also illustrate the biting mechanism. [3+3] 2005, 2006, 2008, 2010
4. Distinguish between proteroglyphous and solenoglyphous fangs. [4] 1999