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  Section: Anatomy of Vertebrate Animals » The Classification and the Osteology of the Reptilia
 
 
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The Ophidia

 
     
 

This order of Reptiles has been divided as follows:
  1. The palatine bones widely separated, and their long axes longitudinal; a transverse bone; the pterygoids united with the quadrate bones.
    1. None of the maxillary teeth grooved or canalieulated.
    2. 1. Aglyphodontia.
    3. Some of the posterior maxillary teeth grooved.
    4. 2. Opisthoglyphia.
    5. Grooved anterior maxillary teeth succeeded by solid teeth.
    6. 3. Proteroglyphia,
    7. Maxillary teeth few, canaliculated, and fanglike.
    8. 4. Solenoglyphia.
  2. 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.
  3. 5. Typhlopidae.

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 (Crotalus) 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.

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
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
processes.
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:

  1. 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.

  2. 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.
  3.    
  4. 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.
  5.    
  6. 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.
  7.    
  8. The two rami of the mandible are united at the symphysis only by ligamentous fibres, which are often extremely elastic.
  9.    
  10. 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.

The skull of a Python, viewed from the left side, and in longitudinal section; Cm stapes; Tl, turbinal bone
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 in Cholonia, 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.

The Ophidia 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 ligaments.

Under-view of the left half of the sknll and 4- clal bones oi Python
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 and Tortrix (which belong to the division Aglyphodontia); at the other the poisonous Snakes, and especially Crotalus (Solenoglyphia).

Thus, Python (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 (like Tortrix, 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 serpent.

In Tortrix, 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.

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 through it.
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 through it.
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, have 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.

Ophidia are not known in the fossil state before the older tertiaries.

 
     
 
 
     



     
 
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