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procoelous

Snake skeleton

Skull

The typical Ophidian skull is characterized by a solidly ossified brain-case, with the distinct frontals and the united parietals extending downwards to the basisphenoid, which is large and produced forward into a rostrum extending to the ethmoidal region. The nasal region is less completely ossified, and the paired nasals are often attached only at their base. The occipital condyle is either trilobate and formed by the basioccipital and the exoccipitals, or a simple knob formed by the basioccipital; the supraoccipital is excluded from the foramen magnum. The basioccipital may bear a strong, curved ventral process or hypapophysis (in the Vipers).

The prefrontal bone is situated, on each side, between the frontal and the maxillary, and may or may not be in contact with the nasal.

The postfrontal, usually present, borders the orbit behind, rarely also above, and in the Pythons a supraorbital is intercalated between it and the prefrontal.

The premaxillary is single and small, and as a rule connected with the maxillary only by ligament.

The paired vomer is narrow.

The palatine and pterygoid are elongate and parallel to the axis of the skull, the latter diverging behind and extending to the quadrate or to the articular extremity of the mandible; the pterygoid is connected with the maxillary by the ectopterygoid or transverse bone, which may be very elongate, and the maxillary often emits a process towards the palatine, the latter bone being usually produced inwards and upwards towards the anterior extremity of the basisphenoid.

The quadrate is usually large and elongate, and attached to the cranium through the supratemporal (often regarded as the squamosal).

In rare cases, (Miodon, Polemon) the transverse bone is forked, and articulates with two branches of the maxilla.

The quadrate and the maxillary and palatopterygoid arches are more or less movable to allow for the distension required by the passage of prey, often much exceeding the calibre of the mouth. For the same reason, the rami of the lower jaw, which consist of dentary, splenial, angular, and articular elements, with the addition of a coronoid in the Boidae and a few other small families, are connected at the symphysis by a very extensible elastic ligament.

The hyoid apparatus is reduced to a pair of cartilaginous filaments situated below the trachea, and united in front.

There are various modifications according to the genera. A large vacuity may be present between the frontal bones and the basisphenoid (Psammophis, Coelopeltis); the maxillary may be much abbreviated and movable vertically, as in the Viperidae; the pterygoids may taper and converge posteriorly, without any connexion with the quadrate, as in the Amblycephalidae; the supratemporal may be much reduced, and wedged in between the adjacent bones of the cranium; the quadrate may be short or extremely large; the prefrontals may join in a median suture in front of the frontals; the dentary may be freely movable, and detached from the articular posteriorly.

The deviation from the normal type is much greater still when we consider the degraded wormlike members of the families Typhlopidae and Glauconiidae, in which the skull is very compact and the maxillary much reduced. In the former this bone is loosely attached to the lower aspect of the cranium; in the latter it borders the mouth, and is suturally joined to the premaxillary and the prefrontal. In both the transverse bone and the supratemporal are absent, but the coronoid element is present in the mandible.

The principal modifications of the skull in the European genera may be contrasted as in the following synopsis:

  • I. Quadrate articulating with the cranium, supratemporal absent; mandible much shorter than the skull, with coronoid bone; maxillary small, on lower aspect of cranium; pterygoids not extending to quadrate; nasals forming long sutures with the premaxillary, prefrontals, and frontal .......... Typhlops.
  • II. Quadrate suspended from the supratemporal; mandible at least as long as the skull; pterygoids extending to quadrate or mandible.

*A. Mandible with coronoid bone; nasals in sutural contact with frontals and prefrontals; transverse bone short, not projecting much beyond cranium; maxillary not half as long as mandible, which is not longer than skull (to occiput) .......... Eryx.
*B. No coronoid bone; nasals isolated.
*1. Maxillary elongate, not movable vertically.
*a. Maxillary half as long as mandible.
*Supratemporal half as long as skull, projecting far beyond cranium; mandible much longer than skull .......... Tropidonotus.
*Supratemporal not half as long as skull, projecting far beyond cranium; mandible much longer than skull .......... Zamenis.
*Supratemporal not half as long as skull, projecting but slightly beyond cranium; mandible much longer than skull .......... Coluber.
*Supratemporal not half as long as skull, not projecting beyond cranium; mandible not longer than skull .......... Coronella, Contia.
*b. Maxillary not half as long as mandible, which is longer than skull; supratemporal not half as long as skull, projecting beyond cranium.
*Quadrate longer than supratemporal; maxillary much longer than quadrate, nearly straight in front of prefrontal; a large vacuity between the frontal bones and the basisphenoid .......... ''Coelopeltis".
*Quadrate not longer than supratemporal; maxillary little longer than quadrate, strongly curved in front of prefrontal .......... Macroprotodon
*Quadrate longer than supratemporal; maxillary little longer than quadrate, nearly straight in front of prefrontal .......... Tarbophis
*2. Maxillary much abbreviated and erectile; supratemporal not half as long as skull; mandible much longer than skull; basioccipital with a strong process.
*Maxillary bone solid .......... Vipera.
*Maxillary bone hollowed out .......... Ancistrodon.
*The vertebrae number 130 to 500—in the European forms 147 (Vipera ursinii) to 330 (Coluber leopardinus).

Vertebrae

The vertebral column consists of an atlas (composed of two vertebrae) without ribs; numerous precaudal vertebrae, all of which, except the first or first three, bear long, movable, curved ribs with a small posterior tubercle at the base, the last of these ribs sometimes forked; two to ten so-called lumbar vertebrae without ribs, but with bifurcate transverse processes (lymphapophyses) enclosing the lymphatic vessels; and a number of ribless caudal vertebrae with simple transverse processes. When bifid, the ribs or transverse processes have the branches regularly superposed.

The centra have the usual cup-and-ball articulation, with the nearly hemispherical or transversely elliptic condyle at the back (procoelous vertebrae), whilst the neural arch is provided with additional articular surfaces in the form of pre- and post-zygapophyses, broad, flattened, and overlapping, and of a pair of anterior wedge-shaped processes called zygosphene, fitting into a pair of corresponding concavities, zygantrum, just below the base of the neural spine. Thus the vertebrae of snakes articulate with each other by eight joints in addition to the cup-and-ball on the centrum, and interlock by parts reciprocally receiving and entering one another, like the joints called "tenon-and-mortice" in carpentry. The precaudal vertebrae have a more or less high neural spine which, as a rare exception (Xenopholis), may be expanded and plate-like above, and short or moderately long transverse processes to which the ribs are attached by a single facet. The centra of the anterior vertebrae emit more or less developed descending processes, or haemapophyses, which are sometimes continued throughout (Fig. II, A), as in Tropidonotus, Vipera, and Ancistrodon, among European genera.

In the caudal region, elongate transverse processes take the place of ribs, and the haemapophyses are paired, one on each side of the haemal canal. In the Rattlesnakes the seven or eight last vertebrae are enlarged and fused into one.

Limbs

No living snake shows any remains of the pectoral arch, but remains of the pelvis are found in:-

  • Typhlopidae: a single bone (ilium ?) on each side.
  • Glauconiidae: ilium, pubis, and ischium, the ischium forming a ventral symphysis, and a rudimentary femur.
  • Boidae: a long ilium, attached to the lower branch of the first bifurcate transverse process of the lumbar vertebrae, bearing three short bones, the longest of which, regarded as the femur, terminates in a claw-like spur which, in males at least, usually appears externally on each side of the vent.
  • Ilysiidae

References

  • George Albert Boulenger. The snakes of Europe, 2nd edition. London: Methuen & Co., Ltd., 1913.
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