STRUCTURE OF THE SKULL OF THE PYTHON.

193

The basisphenoid, 5, and presphenoid, 9, form a single bone, and the chief keel of the cranial superstructure. The posterior articular surface looks obliquely upwards and backwards, and supports that of the vertebral centrum behind, as the posterior ball of the ordinary vertebræ supports the oblique cup of the succeeding vertebræ; here, however, all motion is abrogated between the two vertebræ, and the co-adapted surfaces are rough and sutural. The basisphenoid presents a smooth cerebral channel above for the mesencephalon, in front of which a deep depression (sella) sinks abruptly into the expanded part of the bone, and there bifurcates, each fork forming a short culde-sac in the substance of the bone.

The alisphenoids, 6, form the anterior half of the fenestra ovalis, which is completed by the exoccipitals; and in their two large perforations for the posterior divisions of the fifth pair of nerves, as well as in their relative size and position, the alisphenoids agree with those of the frog. Each alisphenoid is a thick suboval piece, with a tubercular process on its under and lateral part; it rests upon the basisphenoid and basioccipital, supports the posterior part of the parietal and a portion of the mastoid, 8, and unites anteriorly with the descending lateral plate of the parietal bone.

The parietal, 7, is a large and long, symmetrical, roof-shaped bone, with a median longitudinal crest along its upper surface, where the two originally distinct moieties have coalesced. It is narrowest posteriorly, where it overlaps the superoccipital, and is itself overlapped by the mastoid: it is convex at its middle part on each side of the sagittal spine, and is continued downwards and inwards, to rest immediately upon the basisphenoid. This part of the parietal seems to be formed by an extension of ossification along a membranous space, like that which permanently remains so in the frog, between the alisphenoid and orbitosphenoid: the mesencephalon and the chief part of the cerebral lobes are protected by this unusually developed spine of the mesencephalic vertebra. The optic foramina are conjugational ones, between the anterior border of the lateral plate of the parietal and the posterior border of the corresponding plate of the frontal.

The frontals, 11, rest by descending lateral plates, representing connate orbitosphenoids, 12, upon the attenuated, pointed prolongation of the basisphenoid: the upper surface of each frontal is flat, subquadrate, broader than long in the boa, and the reverse in the python, where the roof the orbit is continued outwards by a detached superorbital bone: there is a distinct, oval, articular surface near the anterior median angle of each frontal to which the prefrontal is attached: the angle itself is slightly produced, to form the articular process for the nasal bones. The smooth orbitosphenoid plate of the frontal joins the outer margin of the upper surface of the frontal at an acute angle; the inner side of each frontal is deeply excavated for the prolongation of the cerebral lobes, and the cavity is converted into a canal by a median vertical plate of bone at the inner and anterior end of the frontal. The frontals join the parietals and postfrontals behind, and, by the anchylosed orbital plates, the presphenoid below, the prefrontals and nasals before, and the superorbitals at their lateral margins. The orbitosphenoids have their bases extended inwards, and meet below the prosencephalon and above the presphenoid, as the neurapophyses of the atlas meet each other above the centrum. The anterior third part of such inwardly-produced base is met by a downward production of the mesial margin of the frontal, forming a septum between the olfactory prolongations of the brain, but is not confluent with the frontal bone: the outer portion of the orbitosphenoids ascends obliquely outwards, and is confluent with the under part of the frontal; it is smooth externally, and deeply notched posteriorly for the optic foramen.

ORGANIC NATURE.-No. VII.

N

194

SKULL OF THE BOA CONSTRICTOR.

The post-frontal, 12, is a moderately long trihedral bone, articulated by its expanded cranial end to the frontal and parietal, and bent down to rest upon the outer and fore angle of the ectopterygoid. It does not reach that bone in the boa, nor in poisonous serpents. In both the boa and python it receives the anterior sharp angle of the parietal in a notch.

The natural segment which terminates the cranium anteriorly, and is formed by the vomerine, prefrontal, and nasal bones, is very distinct in the ophidians.

The vomer, 13, is divided, as in salamandroid fishes and batrachians, but is edentulous: each half is a long, narrow plate, smooth and convex below, concave above, with the inner margin slightly raised; pointed anteriorly, and with two processes, and an intervening notch above the base of the pointed end. The prefrontals, 14, are connate with the lacrymals, 73. The two bones which intervene between the vomerine and nasal bones are the turbinals, 19; they are bent longitudinally outwards in the form of a semicylinder about the termination of the olfactory nerves.

The spine of the nasal vertebra is divided symmetrically, as in the frog, forming the nasal bones, 15; they are elongated, bent plates, with the shorter upper part arching outwards and downwards, completing the olfactory canal above, and with a longer median plate, forming a vertical wall, applied closely to its fellow, except in front, where the nasal process of the premaxillary is received in the interspace of the nasals.

The acoustic capsule remains in great part cartilaginous: there is no detached centre of ossification in it; to whatever extent this capsule is ossified, it is by a continuous extension from the alisphenoid. The sclerotic capsule of the eye is chiefly fibrous, with a thin inner layer of cartilage; the olfactory capsule is in a great measure ossified, as above described.

Maxillary Arch. The palatine, 20, or first piece of this arch, is a strong, oblong bone, having the inner side of its obtuse anterior end applied to the sides of the prefrontals and turbinals, and, near its posterior end, sending a short, thick process upwards and inwards for ligamentous attachment to the lacrymal, and a second similar process outwards as the point of suspension of the maxillary bone. Between these processes the palatine is perforated, and behind them it terminates in a point.

28

Fig. 14.

The chief part of the maxillary, 21 (Fig. 14), is continued forwards from its point of suspension, increasing in depth, and terminating obtusely; a shorter process is also, as usual, continued backwards, and terminates in a point. The point of suspension of the maxillary forms a short, narrow, palatine process. A space occupied by elastic ligament intervenes between the maxillary and the premaxillary, 22, which is single and symmetrical, and firmly wedged into the nasal interspace; the anterior expanded part of this small triangular bone supports two teeth. Thus the bony maxillary arch is interrupted by two ligamentous intervals at the sides of the premaxillary key-bone, in functional relation to the peculiar independent movements of the maxillary and palatine bones required by serpents during the act of engulfing their usually large prey. Two bones extend backwards as appendages to the maxillary arch: one is the "pterygoid," 24, from the palatine; the other the ectopterygoid, 25, from the maxillary. The pterygoid is continued from the posterior extremity of the palatine to abut against the

SKULL OF BOA CONSTRICTOR.

SKULL OF POISONOUS SERPENTS.

195

end of the tympanic pedicle; the under part of the anterior half of the pterygoid is beset with teeth. The ectopterygoid, 25, overlaps the posterior end of the maxillary, and is articulated by its posterior-obliquely cut end to the outer surface of the middle expanded part of the pterygoid.

Mandibular Arch.-The tympanic bone, 28 (Figs. 14 and 15), is a strong trihedral pedicle, articulated by an oblique upper surface to the end of the mastoid, and expanded transversely below to form the antero-posteriorly convex, transversely concave, condyle for the lower jaw. This consists chiefly of an articular and a dentary, with a small coronoid and splenial, piece. The articular piece ends obtusely, immediately behind the condyle; it is a little contracted in front of it, and gradually expands to its middle part, sends up two short processes, then suddenly contracts and terminates in a point wedged into the posterior and outer notch of the dentary piece. The articular is deeply grooved above, and produced into a ridge below. The coronoid is a short compressed plate; the splenial is a longer, slender plate, applied to the inner side of the articular and dentary, and closing the groove on the inner side of the latter. The outer side of the dentary offers a single perforation near its anterior end, which is united to that of the opposite ramus by elastic ligament.

By the above-described mode of union of the extremities of the maxillary and mandibular bones, those on the right side can be drawn apart from those on the left, and the mouth can be opened not only vertically, as in other vertebrate animals, but also transversely, as in insects. Viewing the bones of the mouth that support teeth in the great constricting serpents, they offer the appearance of six jaws-four above and two below; the inner pair of jaws above are formed by the palatine and pterygoid bones, the outer pair by the maxillaries, the under pair by the mandibles, or "rami," as they are termed, of the lower jaw.

Each of these six jaws, moreover, besides the movements vertically and laterally, can be protruded and retracted, independently of the other: by these movements the boa is enabled to retain and slowly engulf its prey, which may be much larger than its own body. At the first seizure the head of the prey is held firmly by the long and sharp recurved teeth of all the jaws, whilst the body is crushed by the overlapping coils of the serpent; the death-struggles having ceased, the constrictor slowly uncoils, and the head of the prey is bedewed with an abundant slimy mucus: one jaw is then unfixed, and its teeth withdrawn by being pushed forward, when they are again infixed, further back upon the prey; the next jaw is then unfixed, protruded, and reattached; and so with the rest in succession-this movement of protraction being almost the only one of which they are susceptible whilst stretched apart to the utmost by the bulk of the animal encompassed by them; thus, by their successive movements, it is slowly and spirally introduced into the wide gullet.

21

The bones of the mouth, in the poisonous serpents, have characters distinct from those of the constricting serpents. These characters consist chiefly in the modification of form and attachments of the superior maxillary bone (Fig. 15), 21, which is moveably articulated to the palatine, ectopterygoid, and lacrymal bones; but chiefly supported by the latter, which presents the form of a short, strong, three-sided pedicle, extending from the anterior external angle of the frontal to the anterior and upper part of the maxillary. The

Fig. 15.-SKULL OF A POISONOUS SNAKE.

196

VERTEBRE OF THE RATTLESNAKE.

articular surface of the maxillary is slightly concave, of an oval shape; the surface articulating with the ectopterygoid on the posterior and upper part of the maxillary is smaller and convex. The maxillary bone is pushed forward and rotated upon the lacrymal joint by the advance of the ectopterygoids, which are associated with the movements of the tympanic pedicle of the lower jaw by means of the true pterygoid bones. The premaxillary bone (Fig. 13), 22, is edentulous. A single, long, perforated poison-fang is anchylosed to the right maxillary, and sometimes two similar fangs, as in the cobra figured in Cut 13. The palatine bones have four or five, and the pterygoids from eight to ten small, imperforate, pointed, and recurved teeth. The frontal bones are broader than they are long; there are no superorbitals. A strong ridge is developed from the under surface of the basisphenoid, and a long and strong recurved hypapophysis from that of the basioccipital; these give insertion to the powerful “longi-colli” muscles by which the downward stroke of the head is performed in the infliction of the wound by the poison-fangs.

Fig. 16.

ns

VERTEBRA OF THE

The characteristics of the trunk-vertebræ of the ophidian reptiles are as follow:The autogenous elements, except the pleurapophyses (Fig. 16), pl, coalesce with one another in the vertebræ of the trunk; and the pleurapophyses also become anchylosed to the diapophyses in those of the tail. There is no trace of suture between the neural arch (ib.), n, and centrum, c. The outer substance of the vertebra is compact, with a smooth or polished surface. The vertebræ are "procoelian;" that is, they are articulated together by ball-and-socket joints, the socket being on the fore part of the centrum, where it forms a deep cup with its rim sharply defined; the cavity looking not directly forwards, but a little downwards, from the greater prominence of the upper border; the well-turned prominent ball terminates the back part of the centrum rather more obliquely, its aspect being backwards and upwards. The hypapophysis, hy, is developed in different proportions from differby ent vertebræ, but throughout the greater part of the trunk presents a considerable size in the cobra and crotalus (Figs. 13 RATTLESNAKE (Crotalus). and 16), hy; it is shorter in the python and boa. A vascular canal perforates the under surface of the centrum, and there are sometimes two or even three smaller foramina. In the python a large, vertically oblong, but short diapophysis extends from the fore part of the side of the centrum obliquely backwards: it is covered by the articular surface for the rib, convex lengthwise, and convex vertically at its upper half, but slightly concave at its lower half. In the rattlesnake the diapophysis developes a small, circumscribed, articular tubercle, d, for the free vertebral rib or pleurapophysis, pl; a parapophysis, p, extends downwards and forwards below the level of the centrum; the anterior zygapophysis, z, seems to be supported by a similar process from the upper end of the diapophysis. The base of the neural arch swells outward from its confluence with the centrum, and developes from each angle a transversely-elongated zygapophysis; that from the anterior angle looking upwards, that from the posterior angle downwards, both surfaces being flat, and almost horizontal, as in the batrachians. The neural canal is narrow; the neural spine, ns, is of moderate height, about equal to its antero-posterior extent; it is compressed and truncate. A wedge-shaped process (the "zygosphene"), zs, is developed from the fore part of the base of the spine; the lower apex of the wedge being, as it were, cut off, and its sloping sides presenting two smooth, flat, articular surfaces.

This wedge is received into a cavity (the "zygantrum") excavated in the posterior expansion of the neural arch, and having two smooth articular surfaces to which the zygosphenal surfaces are adapted.

Thus the vertebræ of serpents articulate with each other by eight joints in addition to those of 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. In the caudal vertebræ, the hypapophysis is double, the transition being effected by its progressive bifurcation in the posterior abdominal vertebræ. The diapophyses become much longer in the caudal vertebræ, and support in the anterior ones short ribs which usually become anchylosed to their extremities.

The pleurapophyses or vertebral ribs in serpents have an oblong articular surface, concave above and almost flat below in the python, with a tubercle developed from the upper part, and a rough surface excavated on the fore part of the expanded head for the insertion of the precostal ligament. They have a large medullary cavity, with dense but thin walls, and a fine cancellous structure at their articular ends. Their lower end supports a short cartilaginous hæmapophysis, which is attached to the broad and stiff abdominal scute. These scutes, alternately raised and depressed by muscles attached to the ribs and integument, aid in the gliding movements of serpents; and the ribs, like the legs in the centipede, subserve locomotion; but they have also accessory functions in relation to breathing and constriction. The anterior ribs in the cobra (Fig. 13), pl, are unusually long, and are slightly bent; they can be folded back one upon another, and can be drawn forward, or erected, when they sustain a fold of integument, peculiarly coloured in some species—e.g., the spectacled cobra—and which has the effect of making this venomous snake more conspicuous at the moment when it is about to inflict its deadly bite. The ribs commence in the cobra, as in other serpents, at the third vertebra from the head.

The centrum of the first vertebra coalesces with that of the second, and its place is taken by an autogenous hypapophysis: this, in the python, is articulated by suture to the neurapophyses; it also presents a concave articular surface anteriorly for the lower part of the basioccipital tubercle, and a similar surface behind for the detached central part of the body of the atlas, or "odontoid process of the axis." The base of each neurapophysis has an antero-internal articular surface for the exoccipital tubercle, the middle one for the hypapophysis, and a postero-internal surface for the upper and lateral parts of the odontoid; they thus rest on both the separated parts of their proper centrum. The neurapophyses expand and arch over the neural canal, but meet without coalescing. There is no neural spine. Each neurapophysis developes from its upper and hinder border a short zygapophysis, and from its side a still shorter diapophysis. In the second vertebra, the odontoid presents a convex tubercle anteriorly, which fills up the articular cavity in the atlas for the occipital tuberole; below this is the surface for the hypapophysial part of the atlas, and above and behind it are the two surfaces for the atlantal neurapophyses. The whole posterior surface of the odontoid is anchylosed to the proper centrum of the axis, and in part to its hypapophysis. The neural arch of the axis developes a short ribless diapophysis from each side of its base; a thick sub-bifid zygapophysis from each side of the posterior margin; and a moderately long bent-back spine from its upper part. The centrum terminates in a ball behind, and below this sends downwards and backwards a long hypapophysis.

At the opposite extreme of the elongated body, two or three much simplified