The Buccal Buckle: The
Functional Morphology of
Venom Spitting In Cobras
Presented by: Qudirat Jamiu
Young, B., Dunlap, K., Koenig, K., and Singer, M. “The buccal buckle: the
functional morphology of venom spitting in cobras” The Journal
of Experimental Biology Volume 207 (2004) Pages 3483-3494.
Background

Asiatic and African cobras independently evolved
ability to expel venom as pressurized horizontal
stream



Some cobras can spit venom as far as 3 meters


Known as spitting
Form of active defensive behavior
Pressure required
Most venomous snakes have to make direct physical
contact with an object/organism to propel venom

Spitting cobras have the ability to expel venom without
making direct physical contact

Have specialized exit orifice of the
fang

Exit orifice is directed more craniad and
has a more circular opening than the exit
orifice of non-spitting cobras

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This explains why venom is propelled forward,
rather than downward.
Fangs surrounded by CT and epithelium

Attached to lateral and cranial surface of
the maxilla
No smooth or skeletal muscle
 Venom duct is also attached to lateral surface
of maxilla

Jaw Anatomy

The upper jaw/ palato-maxillary arch of
cobras consist of four bones


Pterygoid, ectopterygoid, palatine and
maxilla
Four skeletal muscles contact the
palato-maxillary

M. Protractor pterygoideus, M. levator
pterygoideus, M. Retractor pterygoideus,
and M. pterygoideus
Illustration of the lateral view of the skull, and ventral view of the palato-maxillary arch, of Naja nigricollis. Muscle
attachment sites are shown as solid colors (lateral surface) or broken colors (medial surface) for the following muscles:
M. adductor mandibular externus superficialis (yellow); M. levator pterygoideus (purple); M. protractor pterygoideus
(red); M. retractor pterygoideus (blue); M. pterygoideus (green). e, ectopterygoid; m, maxilla; pa, palatine; pf,
prefrontal; pt, pterygoid.
Objective

To determine the functional bases of
venom spitting


Muscle involvement
Bone Displacement
Materials and Methods


5 Black-necked sitting cobras Naja nigricollis,
1 Red spitting cobra Naja pallida, 1
Indochinese spitiing cobra Naja siamensis, 4
Egyptian cobras Naja haje, and 3 Forest
cobras Naja melanoleuca.
Venom expulsion measured using high speed
videography and photography


Multiple spitting episode recorded using
MotionScope 1000S at 500 frames s-1
Recorded venom expulsions of spitting and nonspitting cobras


Sagital bisections of the heads of
various species were examined to
understand anatomy of spitting
Analysis of mechanical role of the M.
protractor pterygoideus (PP) in spitting



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Exposure of muscle
Applied bipolar stimulating probe to
surface of mucsle
Electrical stimulations
Repeated stimulations with uniaxial strain
guage

Venom pressure measurements

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
Fang removed
Polyethylene (PE) tubing placed over fang
and attached to a pressure transducer
M. protractor pterygoideus (PP) and M.
adductor mandibulae externus superficialis
(AMES) exposed

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AMES directly contacts venom gland, PP does
not
Muscles were stimulated individually and
simultaneously

Electromyography



(Instrument that produces an visual record of
the electrical activity of a skeletal muscle by
electrode inserted into the muscle or placed
on the skin )
Inserted into PP and AMES through incisions
in head
Cobras were induced to spit by experimenter
Spit detector held in front of experimenters
face
 Striking sent signal to data system

VG =AMES
PG = PP
Results

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The high speed digital videography revealed that the
spitting behavior had consistent patterns of motion
Four displacements occurred immediately prior to
venom expulsion

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Snout complex rotated in sagital plane elevating snout
Lateral displacement of caudal maxilla causing
bulge/deformation of supralabial scales
Elevation of CT and epithelium surrounding fang to expose
fang tip
Spit released with mouth slightly open at ~ 25o

Deformations of the
palato-maxillary arch
during spitting. (A) Naja
nigricollis immediately
prior to spitting;
deformation of the
supralabial scales
(arrow) (B) a high-speed
digital videograph
recording of N.
nigricollis spitting

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The displacement of the palatomaxillary arch observed during spitting
resembled the unilateral motions of the
palato-maxillary arch associated with
prey ingestion known as the ‘pterygoid
walk’
Displacements observed during spitting
was never observed in non-spitting
cobras, nor was it observed from
spitting cobras engaged in any other
behavior, including other forms of
venom expulsion

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Gross histological morphology of palatomaxillary arch of spitting and nonspitting cobras revealed minor
differences except for the exit orifices
Contractions generated by electrical
stimulation produced
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displacements of palato maxillary arch
Protracted maxilla
Protracted palatine
Buckling of palato maxillary arch and
maxilloectopterygoid joint
Ventral view of the palate of an Indochinese spitting cobra Naja siamensis before (A) and after
(B) stimulation of the M. protractor pterygoideus. Both A and B are photos of the same side of
the same animal Note the protraction and rotations of the palato-maxillary arch. f, fang; pa,
palatine; pt, pterygoid.
12 electrical stimulations
measured
Displacement of
palatoperygoid and
maxilloectopterygoid can
be seen by the signals
obtained from the strain
guage
Clear pattern of
deformation observed
prior to spits recorded
Strain gauges placed on the palatal mucosa (A) or scales over the nasofrontal joint (B) of two
separate spitting cobras Naja nigricollis. In both experiments contraction of the M. protractor
pterygoideus, either artificially (A) or during spitting (B), resulted in deformations of the
palato-maxillary arch evident in the strain gauge tracings.

Venom Pressure

Individual stimulations of PP and AMES
produced clear patterns in pressure
changes
AMES produced little pressure
 PP produced displacement of palato-maxillary
arch and tissue surrounding fang and produced
pressure greater than that of the AMES


Two muscles stimulated together produced
pressure greater than the sum of the
individual muscle stimulations
Stimulation trials
exposed to single
twitch and train
stimuli
Venom pressure recorded from the fang tip of Naja nigricollis. Note that stimulation of the M.
adductor mandibulae externus superficialis (AMES) simultaneously with the M. protractor
pterygoideus (PP) produces greater venom pressure than when either is stimulated alone. This
pattern holds whether the muscles are given twitch (A) or train (B) stimuli.

EMG activity
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Electrical activity during spitting in the PP
lasted 37 ms and in AMES 96ms
Mean spit duration 66ms
Palatal projections appeared 3 ms prior to
onset of spitting
Conclusion

Analyses suggest that a two-component mechanism
form the functional basis of venom spitting in cobras.

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displacement of the palato-maxillary arch
increase in pressure caused by contraction of the M.
adductor mandibulae externus superficialis
Spitting cobras utilize a pressure-balance system for
venom expulsion; however, unlike all other venomous
snakes where displacement of the fang sheath is
passive, in spitting cobras the displacement is actively
produced by the contraction of the M. protractor
pterygoideus and the displacement of the palatomaxillary arch

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Phylogeny of snakes remains uncertain,
however functional convergence
between Elapidae (rattlesnakes) and
Viperidae (spitting cobras) has been
suggested.
Venom delivery system of Elapidae and
Viperidae

Nearly every aspect of the venom delivery
system of elapids and viperids is
morphologically distinct

It appears that rather than evolving a
suite of morphological specializations
for spitting, cobras have instead
modified a motor action pattern
employed for ingestion
Summary

Prior to spit discharge AMES involved in
increasing venom pressure

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AMES displaces the palato-maxillary arch
Contractions of the PP causes displacement of the
palato-maxillary arch and fang sheath which also
influences venom pressure
AMES and PP are activated simultaneously
This increases venom pressure within the venom
gland, propelling stream of venom through the
venom duct and out the fang