This order of Reptiles has been divided
- The palatine bones widely separated, and their long axes longitudinal; a transverse bone; the pterygoids united with the quadrate bones.
- None of the maxillary teeth grooved or canalieulated.
- Some of the posterior maxillary teeth grooved.
- Grooved anterior maxillary teeth succeeded by solid teeth.
- Maxillary teeth few, canaliculated, and fanglike.
- The palatine bones meet, or nearly meet, in the base of the skull, and
their long axes are transverse; no transverse bone; the pterygoids
are not connected with the quadrate bone.
All the Snakes possess a scaly epidermic investment, which
it. usually shed in one piece, and reproduced at definite intervals.
As a general rule these scales are flat, and overlap one
another; but sometimes, as in Acrochordus
, they become
more tubercle-like, and do not overlap. In the Rattlesnakes
) the body is terminated by several loosely-conjoined
rings of horny matter, which consist of the modified epidermis
of the end of the tail.
The derm does not become ossified in the Ophidia.
|Fig. 71. - Anterior and posterior views of the dorsal vertebra of a Python: z. s, zygosphene
z. a., zygantrum; p. z., prezzgapophysea; pt. z., postgygapophyses; t, p., transverse
The number of the vertebrae in the Snakes is always considerable,
and in some cases becomes very great, amounting
to more than four hundred in some of the large Pythons. The
spinal column is divisible only into caudal and precaudal regions,
as there is no sacrum, nor any distinction between cervical,
dorsal, and lumbar vertebrae. The atlas and the odontoid
vertebra are similar to those of the Lizards, and the atlas
is the only precaudal vertebra which is devoid of ribs. The
centra have nearly hemispherical articular surfaces, and thus
differ from those of ordinary Lacertilia
, while the superadded
articular processes found only in certain Lizards attain a great
development in the Snakes. The zygapophyses are broad
and flattened, and the outer surfaces of the anterior pair are
commonly prolonged into a process. The anterior surface of
the arch above the neural canal is produced into a strong
wedge-shaped zygosphene, which fits into a corresponding
zygantrum of the next preceding vertebra; and, on the posterior
surface of the arch, there is a zygantrum for the zygo-sphene of the next preceding vertebra. (Fig. 71.)
The transverse processes are short and tubercle-like, and
the heads of the ribs which articulate with them are simple.
Each rib usually gives off a short upward process at a little
distance from its head; it is curved, usually hollow, and terminates,
inferiorly, in a cartilage which is always free, no
trace of a sternum existing. Strong descending processes are
given off from the undersides of many of the presacral vertebrae. In the caudal region, elongated transverse processes
take the place of the ribs. Chevron-bones, like those of the Lacertilia
, do not exist, but the caudal vertebrae possess bifurcated
descending processes, which bear similar relations to
the caudal vessels.
The skull differs from the ordinary Lacertilian cranium in
the following points:
- That vertical elevation and lateral compression of the
presphenoidal region, which give rise to the interorbital septum,
are wanting; the floor of the cranium being nearly flat,
and the vertical height of its cavity diminishing gradually in
front, so that it remains spacious between the eyes, and in the
frontal region generally. The periotic region is not produced
into parotic processes.
- The boundary-walls of the front half of the cranial cavity
are as well ossified as those of its posterior moiety, and the
bones which constitute the brain-case are firmly united together.
- On the other hand, the nasal segment is less completely
ossified, and may be movable. The premaxillae are usually
represented by a single small bone, which very rarely bears
teeth. It is connected with the maxillae only by fibrous tissue.
- The palatine bones never unite directly with the vomer,
or with the base of the skull, but they are usually connected
with the maxillae by transverse bones; and, by the pterygoids,
with the mobile quadrate bones. Hence the connection of the
palato-maxillary apparatus with the other bones of the skull ia
always less close in Ophidia than in Lacertilia, and sometimes
it is exceedingly lax.
- The two rami of the mandible are united at the symphysis
only by ligamentous fibres, which are often extremely elastic.
- The hyoidean apparatus is very rudimentary, consisting
only of a pair of cartilaginous filaments, which are united together
in front, and lie parallel with one another beneath the
trachea. They have no connection with the skull.
|Fig. 72. - The skull of a Python, viewed from the left side, and in longitudinal section; Cm stapes; Tl, turbinal bone.
These are the most apparent differences between the
Ophidian and the Lacertilian skull. But there are others, of
a less obvious but more remarkable character, by which the
skulls of the Ophidian depart not only from that of the Lizard,
but from that of other Vertebrata. Thus the basi-sphenoid
passes in front of the sella turcica, into a great rostrum,
which extends forward to the ethmoidal region, and probably
results from a parasphenoidal ossification. In many adult Ophidia
two cartilaginous rods he in grooves on the upper
face of this rostrum, and pass behind into the basisphenoid,
while in front they are continued into the cartilaginous ethmoidal
septum. These rods are the trabeculae cranii
of the foetus,
which do not become united in Snakes, as they do in all the
other abranchiate Vertebrata
. The roof and side-walls of the
Ophidian skull are completed in front of the occipital segment,
by two pairs of bones, which appear to be parietals and frontals.
The "frontal" bones not only completely wall in the
sides of the frontal region, but extend inward below, and meet
in the middle line, above the basisphenoidal rostrum and the
persistent trabeculse. The "parietals" unite suturally with
the basisphenoid. These relations are not usual in true frontals
or parietals (though the latter unite with the basisphenoid
, and the frontals unite in the middle line of the
floor of the skull in some Mammals); and as there are only
two bones in the place of four in this region of the skull, it
becomes a matter for inquiry whether the two bones, on each
side, respectively represent orbitosphenoids + frontals, and
alisphenoids + parietals; or whether they represent overgrown
frontals and parietals only; or whether, lastly, they are
the result of an excessive development of the orbitosphenoids
and alisphenoids, true frontals and parietals being absent.
According to Rathke's elaborate investigation into the developement
of the skull in Coluber natrix
, the two bones on each
side are formed from single centres of ossification, which appear
in patches of "cartilage," which are situated, at first, in
the superolateral regions of the skull, in the place normally
occupied by orbitosphenoids and alisphenoids, and that these
grow up and meet in the middle line. In this case the bones
in question are orbitosphenoids and alisphenoids, and Ophidia
have no true frontals or parietals; but the existence of so
remarkable a deviation from the ordinary construction of the
vertebrate skull cannot be admitted until the development of
the Snake's skull has been carefully reexamined.
usually possess well-developed post-frontals,
and they have large membrane-bones in front of the orbit,
which lie upon the cartilaginous nasal chambers, and are ordinarily
regarded as lachrymals. Large nasals lie upon the
upper surface of the nasal capsule between the lachrymals;
and, forming the floor of the front part of the nasal chamber,
on each side, is a large concavo-convex bone (Tl
, Fig. 72),
which extends from the ethmoidal septum to the maxilla, protects
the nasal gland, and is commonly termed a turbinal,
though, if it be a membrane-bone, it does not truly correspond
with the turbinals of the higher Vertebrata
. The squamosals
are usually well developed. There is no jugal, or quadrato jugal.
Though the general conformation of the skull in the Opihidia
is that which has now been described, it presents remarkable
modifications in different members of the order, especially
in the form and disposition of the bones of the jaws. In the
great majority of the Ophidia
, the elongated palatine bones
have their long axes longitudinal, lie on the outer sides of the
internal nasal apertures, and do not enter into the formation
of the posterior boundaries of those apertures. Each is connected
by a transverse bone with the maxilla, which lies at the
side of the oral cavity; and the pterygoids diverge posteriorly
toward the quadrate bones, with which they are connected by
|Fig. 78. - Under-view of the
left half of the sknll and 4-
clal bones oi Python.
But, in the remarkable group of the Typhlopidae
, the slender
palatine bones meet upon the base of the skull in the middle
line, and are directed transversely, in such a manner as
to bound the posterior nasal apertures behind, as in the Satrachia.
There is no transverse bone. The pterygoids lie parallel
with one another under the base of the skull, and are not
connected with the quadrate bones. The maxillae are short
plates of bone which are connected with the outer extremities
of the palatine bones, and are directed obliquely toward the
middle line of the oral cavity, into which their free edges,
armed with teeth, depend.
Again, the first-mentioned, or typical, form of Ophidian
skull exhibits two extreme modifications, between which lie
all intermediate gradations. At the one end of the scale are
the non-venomous Snakes, and especially Python
(which belong to the division Aglyphodontia
); at the other
the poisonous Snakes, and especially Crotalus (Solenoglyphia)
(Figs. 73 and 73) has well-marked premaxillae,
large maxillary bones, palatine bones
which are firmly united with the pterygoids,
and transverse bones which bind
the maxillaries and palato-pterygoid
bars into one solid framework.
The maxillaries give attachment to
a long series of recurved teeth, which are not very unequal in size. And Python
, but unlike all other Ophidia
) possesses teeth in the premaxillae.
The squamosal bones are very long,
and adhere to the skull, upon which
they are slightly movable, only by their
anterior ends; and the quadrate bones
are borne upon the posterior ends of
the squamosals, and are thus, as it were,
thrust away from the walls of the skull,
The rami of the mandible are loosely
connected by an elastic symphysial ligament.
Thus, not only can these rami
be widely separated from one another,
but the squamosal and quadrate bones
constitute a kind of jointed lever, the
straightening of which permits of the
separation of the mandibles from the
base of the skull. And all these arrangements, taken together,
allow of that immense distention of the throat which is requisite
for the passage of the large and undivided prey of the
this mechanism does not exist, the short quadrate
bone being directly articulated with the skull, while the
squamosal, like the post-frontal, is rudimentary. The maxillary
bones are also almost fixed to the skull.
In the Rattlesnakes (Crotalus
, Fig. 74), the premaxillae
are very small and toothless. The maxillary bone has no
longer the form of an elongated bar, but is short, subcylindrical,
and hollow; its cavity lodges the fossa formed by the integument
in front of the eye, which is so conspicuous in these,
and sundry other, poisonous Snakes. The upper and inner
part of the maxilla articulates with a pulley-like surface furnished
to it by the lachrymal, so that the maxilla plays freely
backward and forward upon that bone. The lachrymal, again,
has a certain amount of motion upon the frontal. The upper
edge of the posterior wall of the maxilla is articulated by a
hinge-like joint with the anterior end of the transverse bone,
which has the form of an extremely elongated and flattened
bar connected posteriorly with the pterygoid.
|Fig. 74. - A, the skull of Crotalus, vlew from the left side; B, a transverse section taken at the point. B, in Fig. A, showing T, the persistent cartilaginous trabeculae. The maxilla
is supposed to be transparent, and the anterior half of the palatine bone is seen
The latter is long and stout, and, as usual, is united, benind,
with the distal end of the quadrate bone. In front of,
and internal to, its union with the transverse it is prolonged
forward, and becomes united, by a movable joint, with the
short palatine bone, which is flattened from side to side,
and lies on the outer side of the posterior nasal aperture. Its
anterior end is connected only by fibrous tissue with the base
of the skull. The inferior edge of the palatine bears a few
small teeth, and other sharp, recurved, solid teeth are attached
to the under-surface of the anterior moiety of the pterygoid.
When the mouth is shut, the axis of the quadrate bone
is inclined downward and backward. The pterygoid, thrown
as far back as it can go, straightens the pterygo-palatine joint,
and causes the axes of the palatine and pterygoid bones to
coincide. The transverse, also carried back by the pterygoid,
similarly pulls the posterior part of the maxilla, and causes
its proper palatine face, to which the great channelled poisonfangs
are attached, to look backward. Hence these fangs lie
along the roof of the mouth, concealed between folds of the
mucous membrane. But, when the animal opens its mouth
for the purpose of striking its prey, the digastric muscle, pulling
up the angle of the mandible, at the same time thrusts
the distal end of the quadrate bone forward. This necessitates
the pushing forward of the pterygoid, the result of which is
twofold; firstly, the bending of the pterygo-palatine joint;
secondly, the partial rotation of the maxillary upon its lachrymal
joint, the hinder edge of the maxillary being thrust downward
and forward. In virtue of this rotation of the maxillary,
through about a quarter of a circle, the dentigerous Tace of the
maxilla looks downward, and even a little forward, instead of
backward, and the fangs are erected into a vertical position.
The snake "strikes:" by the simultaneous contraction of the
crotaphite muscle, part of which extends over the poisongland,
the poison is injected into the wound through the canal
of the fang; and, this being withdrawn, the mouth is shut, all
the previous movements are reversed, and the parts return to
their first position.
No Ophidian possesses any trace of anterior extremities,
but the Typhlopidae
, the Pythons, Boas, and Tortrices
rudiments of a pelvis, and the latter Snakes even possess very
short representatives of hind-limbs terminated by claws.
The teeth of the Ophidia
are short and conical, and become
anchylosed to the bones by which they are supported.
They may be developed in the premaxillaries, maxillaries,
palatines, pterygoids, and the dentary piece of the mandible,
but their presence in the premaxillaries is exceptional. In Uropeltis
and some other genera, there are no palatine teeth;
and in the egg-eating African snake, Rachiodon
, the teeth
are small and rudimentary upon all the bones which usually
bear them. But the inferior spines of eight or nine of the
anterior vertebrae are long, and tipped, at their apices, with a
dense enamel-like substance. These project through the
dorsal wall of the oesophagus into its cavity, and the eggs,
which are swallowed whole, are thus broken in a position in
which all their contents must necessarily be saved.
In the majority of the non-venomous Snakes the teeth are
simply conical, but in the others, and in all the poisonous
Snakes, some of the maxillary teeth (which are usually longer
than the rest) become grooved in front. In the Solenoglyphia
or Vipers and Rattlesnakes, the maxillary teeth are reduced
to two or three long fangs, the groove in the front of which
is converted into a canal open at each end, by the meeting of
its edges. The teeth of the Snakes are replaced by others
which are developed close to the bases of the old ones.
are not known in the fossil state before the older