Chapter X The Skeleton of the Newt (Molge Cristata)
I. EXOSKELETON.
The skin of the Newt is quite devoid of any exoskeletal structures. The only exoskeletal structures that the animal possesses are the teeth, and these are most conveniently described with the endoskeleton.
II. ENDOSKELETON.
The endoskeleton of the Newt, though ossified to a considerable extent, is more cartilaginous than is that of the frog. It is divisible into an axial portion including the vertebral column, skull, ribs, and sternum, and an appendicular portion including the skeleton of the limbs and their girdles.
1. The Axial Skeleton.
A. The Vertebral column.
This consists of about fifty vertebrae arranged in a regular continuous series. The first vertebra differs a good deal from any of the others; the seventeenth or sacral vertebra and the eighteenth or first caudal also present peculiarities of their own. The remaining vertebrae are divided by the sacrum into an anterior series of trunk vertebrae which bear fairly large ribs, and a posterior series of caudal vertebrae, all of which except the first few are ribless.
The trunk vertebrae.
Any vertebra from the second to the sixteenth may be taken as a type of the trunk vertebrae.
The general form is elongated and somewhat hour-glass shaped, and the centra are convex in front and concave behind; an opisthocoelous condition such as this is quite exceptional in Anura. The notochord may persist intervertebrally54, but in the centre of each vertebra it becomes greatly constricted or altogether obliterated, and replaced by marrow. The superficial portion of the centrum is ossified, while the articular surfaces are cartilaginous. The neural arches are low and articulate together by means of zygapophyses borne on short diverging processes. The anterior zygapophyses look upwards, the posterior downwards. Each neural arch is drawn out dorsally into a very slight cartilaginous neural spine.
On each centrum, at a little behind the middle line, there arise a pair of short backwardly-directed transverse processes; each of which becomes divided into two slightly divergent portions, a dorsal portion which meets the tubercular process of the rib and is derived from the neural arch, and a ventral portion which meets the capitular process of the rib and is derived from the ventral or haemal arch. The division between these two parts of the transverse processes can be traced back as far as the sacrum.
The first vertebra as already mentioned differs much from all the others. It has no ribs, and presents anteriorly two slightly divergent concave surfaces which articulate with the occipital condyles of the skull. Between these surfaces the dorsal portion of the anterior face of the centrum is drawn out into a prominent odontoid process, the occurrence of which renders it probable that the first vertebra of the newt is really the axis, and that the atlas with the exception of the odontoid process has become fused with the skull. The sacral vertebra or sacrum differs from the vertebrae immediately in front of it only in the fact that its transverse processes are stouter and more obviously divided into dorsal and ventral portions.
The caudal vertebrae.
The caudal vertebrae are about twenty-four in number. The anterior ones have hour-glass shaped centra, and short backwardly-directed transverse processes. The middle and posterior ones have rather shorter centra, and are without transverse processes. The neural arches resemble those of the trunk vertebrae, but each is drawn out into a rather high cartilaginous neural spine abruptly truncated anteriorly. All the caudal vertebrae except the first have also a haemal arch, which is very similar to the neural arch, and is drawn out into a haemal spine quite similar to the neural spine. Both neural and haemal arches are ossified continuously with the centra.
B. The Skull.
The skull of the newt is divisible into three principal parts:—
(1) an axial part, the cranium proper, which encloses the brain and to which
(2) the capsules of the auditory and olfactory sense organs are fused;
(3) the skeleton of the jaws and hyoid apparatus. The skull is much flattened and expanded, though not so much as in the frog.
(1) The cranium proper.
The cranium proper or brain case is an unsegmented tube which remains partly cartilaginous, and is partly converted into cartilage bone, partly sheathed by membrane bone. The roof and floor of the cartilaginous cranium are, as is the case also in the frog, pierced by holes or fontanelles, and these are so large that the main part of the roof and floor comes to be formed by membrane bone.
Two pairs of large ossifications take place in the cranial walls. Of these the more posterior on each side represents the exoccipital and all three periotic bones. It bears a small convex patch of cartilage for articulation with the atlas, and with its fellow forms the boundary of the foramen magnum.
Two foramina pierce the exoccipital just in front of the occipital condyle and transmit respectively the glossopharyngeal and pneumogastric (fig. 21, X) nerves. Lying laterally to these nerve openings is seen a patch of cartilage, the stapes, which is homologous with the stapes or proximal element of the columellar chain in the frog. Further forward in front of the stapes is the small opening for the exit of the facial nerve, and seen in a lateral view close to the orbitosphenoid, that for the trigeminal (fig. 21, C, 5).
In front of these large bones the lateral parts of the cranial walls remain cartilaginous for a short distance, and then there follow two elongated bones, the orbitosphenoids (fig. 21, B and C, 11), pierced by the foramina for the exit of the optic nerves. These bones partly correspond to the sphenethmoid of the frog.
The membrane bones connected with the cranium are the parietals, frontals and prefronto-lachrymals on the dorsal surface, and the parasphenoid on the ventral surface.
The parietals (fig. 21, A and C, 6) roof over the posterior part of the great dorsal fontanelle and overlap the exoccipito-periotics. They meet one another along a sinuous suture in the middle line, as do also the frontals which overlap them in front. The frontals and parietals both extend for a short distance down the sides of the cranium and meet the orbitosphenoids. The prefronto-lachrymals (fig. 21, A and C, 7) connect the frontals with the maxillae.
On the ventral surface is the large parasphenoid (fig. 21, B, 10), which is widest behind and overlapped anteriorly by the vomero-palatines.
(2) The sense capsules.
The auditory capsules become almost completely ossified continuously with the exoccipitals; they have been already described.
The nasal capsules are large and quite unossified though they are overlain by membrane bone. They appear on the dorsal surface between the anterior nares and the nasal process of the premaxillae. They enclose the nasal organs, bound the inner side of the anterior narial opening, and are connected with one another posteriorly by a cartilaginous area.
Developed in connection with the nasal capsules are a pair of rather large nasals (fig. 21, A and C, 4), which lie on the dorsal surface immediately in front of the frontals. Each forms part of the posterior boundary of one of the anterior nares, and the two are separated from one another in the middle line by the nasal process of the premaxillae (fig. 21, A, 1), and the opening of the middle narial passage (fig. 21, A and B, 22), which passes right through the skull.
On the ventral surface of the skull and forming the greater part of the boundary of the posterior nares are two large bones, the vomero-palatines (fig. 21, B and C, 9). Each consists of a wide anterior portion, partly separated from its fellow in the middle line by the ventral opening of the middle narial passage, and of a long pointed posterior portion which is separated from its fellow by the parasphenoid, and bears a row of small pointed teeth formed of dentine capped with enamel.
(3) The jaws.
The upper jaw of the newt is a discontinuous structure divided into two parts, an anterior part which consists of membrane bones, the maxillae and premaxillae, and a posterior part which remains mainly cartilaginous.
The premaxillae are united, forming a single bone, which in a ventral view is seen to meet the maxillae and vomero-palatines, and in a dorsal view to send back a nasal process (fig. 21, A, 1) between the nasals.
The maxillae are large bones, each terminating in a point posteriorly. A single row of teeth similar to those on the vomero-palatines runs along the outer margin of the maxillae and premaxillae.
The posterior part of the upper jaw forms a mass of cartilage which extends forwards towards the maxillae as a long pointed process whose ventral surface and sides are overlapped by a membrane bone, the pterygoid (fig. 21, 12).
The suspensorial bones include the quadrate and squamosal. The quadrate (fig. 21, 15) which forms the true suspensorium is directed forwards and outwards, and is terminated by a patch of cartilage with which the mandible articulates.
The lower jaw or mandible remains partly cartilaginous, while its ossifications include two membrane bones and one cartilage bone. The cartilage bone is the articular (fig. 21, C, 18), it forms the posterior part of the ramus, extends forwards for some distance along its inner side, and is terminated posteriorly by a patch of cartilage which articulates with the quadrate. The dentary (fig. 21, C, 20) is a large bone which forms the anterior part and nearly all the outer half of each ramus, and bears teeth similar to those of the upper jaw. Attached to its inner face is a long slender splenial (fig. 21, C, 21).
The Hyoid apparatus.
This consists of the hyoid arch and part of the first two branchial arches.
The hyoid arch (fig. 29, A, 2) consists of a pair of cornua, each of which is divided into two halves. The dorsal half forming the cerato-hyal is mainly ossified though tipped with cartilage, and is connected by ligament with the suspensorium. The ventral half (hypo-hyal) is cartilaginous, and is connected with the basibranchial.
The branchial arches consist of a median piece, the basibranchial, which is ossified in the centre and cartilaginous at either end, and of two pairs of cerato-branchials which are attached to the cartilaginous part (fig. 29, A, 8) of the basibranchial. The first cerato-branchial is chiefly ossified, the second (fig. 29, A, 4) is a good deal smaller and is cartilaginous. Both are united dorsally to a single epi-branchial, which is terminated by a small cartilaginous area at the free end but is elsewhere well ossified.
C. The Ribs.
The ribs are short imperfectly ossified structures, bifid at their proximal end where they articulate with the transverse processes, and tipped both proximally and distally with cartilage. The dorsal portion of the proximal end corresponds to the tuberculum of the ribs of higher animals, and the ventral portion to the capitulum. Some of the anterior ribs have a step-like notch on their dorsal surfaces.
The second to twelfth ribs are fairly equal in size, but further back they decrease slightly. The ribs which connect the sacral vertebrae with the ilia are however large. The short ribs borne on the anterior caudal vertebrae are cartilaginous.
D. The Sternum.
The sternum (fig. 22, A, 6) is a rather broad plate of cartilage, drawn out posteriorly into a median process marked by a prominent ridge. On its antero-lateral margins it bears surfaces for articulation with the pectoral girdle.
2. The Appendicular Skeleton.
A. The Pectoral girdle.
This is of a very simple character, and remains throughout life in an imperfectly ossified condition. It consists of a dorsal scapular portion, and a ventral coracoid portion partially divided into an anterior part, the precoracoid, and a posterior part, the coracoid.
The scapular portion is a slightly curved oblong plate; its proximal third the scapula (fig. 22, 1) is ossified and bounds part of the well-marked glenoid cavity (fig. 22, 4); its distal portion forms a large oblong cartilaginous plate, the suprascapula (fig. 22, 2).
The precoracoid (fig. 22, 5) forms a small forwardly-directed cartilaginous plate. The coracoid (fig. 22, 3) forms a much larger plate, the greater part of which is unossified and overlaps its fellow in the middle line, the two being overlapped by the sternum. Around the glenoid cavity is an area which is mainly ossified and is continuous with the scapula.
B. The Anterior limb.
This is divisible into three parts, the upper arm or brachium, the fore-arm or antibrachium, and the manus.
The upper arm includes a single bone, the humerus.
The humerus is a slender bone cylindrical in the middle and expanded at either end, the proximal part forms a rounded head which articulates with the glenoid cavity. Along the proximal part of the anterior or pre-axial surface runs a strong deltoid ridge. The proximal part of the postaxial surface also bears a small outgrowth.
The fore-arm contains two bones, the radius and ulna, both of which are small and imperfectly ossified at their terminations.
The radius (fig. 23, B, 11) or pre-axial bone is rather the larger of the two, and is considerably expanded at its proximal end. The ulna or postaxial bone is somewhat expanded distally, but is not drawn out proximally into an olecranon process.
The manus consists of two parts, a group of small bones forming the carpus or wrist, and the hand.
The carpus is in a very simple unmodified condition as compared with that of the Frog. It consists of a proximal row of two bones and a distal row of four, with one, the centrale, interposed between. All these bones are small and polygonal and are imbedded in a plate of cartilage.
The bones of the proximal row are a smaller pre-axial bone, the radiale (fig. 23, B, 13), and a larger postaxial bone, which represents the fused ulnare and intermedium of the very simple carpus described on pp. 26 and 27.
The four bones of the distal row are respectively carpalia 2, 3, 4 and 5.
The hand consists of four digits, that corresponding to the thumb of the human hand, judging from the analogy of the frog probably being the one that is absent.
Each digit consists of a somewhat elongated metacarpal and of two or three phalanges. The metacarpals are contracted in the middle and expanded at either end. They are connected with the carpus by cartilage, and the articulations between the several phalanges, and between the metacarpals and phalanges are also cartilaginous. The second, third, and fifth digits have two phalanges apiece, the fourth, which is the longest, has three. The second metacarpal in the specimen examined and figured articulates partly with carpale 2, partly with carpale 3.
C. The Pelvic girdle.
The pelvic girdle of the Newt is in a much less modified condition than is that of the Frog (see p. 165). It consists of a dorsal element, the ilium, a posterior ventral element, the ischium, and an anterior ventral element, the pubis, to which is attached an epipubis.
The ilium is a somewhat cylindrical bone which at its ventral end meets the ischium, and forms part of the acetabulum. It is then directed upwards and slightly backwards, and is attached to the ribs of the sacral vertebra.
The ischia are a pair of somewhat square bones which meet one another in the middle line; they form part of the acetabulum, and are united to the ilia above.
In front of the ischia is a narrow cartilaginous area which represents the pubes. Projecting forwards from it is a bifid cartilaginous epipubis.
D. The Posterior limb.
This is divisible into a proximal portion, the thigh, a middle portion, the crus or shin, and a distal portion, the pes.
The thigh consists of a single bone, the femur (fig. 23, A, 1), which has a thin shaft and expanded ends. The anterior part of the pre-axial border and posterior part of the postaxial border bear slight outgrowths.
The crus or shin includes two short bones, the tibia and fibula, which are nearly equal in length. The pre-axial bone or tibia is a straight bone thickest at its proximal end, the postaxial bone or fibula (fig. 23, A, 3) is a rather stouter curved bone of nearly equal diameter throughout.
The pes includes the tarsus or ankle, and the foot.
The tarsus consists of eight small bones arranged in a proximal row of three, the tibiale, intermedium and fibulare, and a distal row of four tarsalia, with one bone, the centrale (fig. 23, A, 7), interposed between the two rows. In the specimen examined, the tibiale, is a small bone articulating with the tibia, the intermedium (fig. 23, A, 5) is larger and articulates with both tibia and fibula, the fibulare is the largest of the three and articulates with the fibula.
The bones of the distal row are tarsalia 1, 2, 3, and a bone representing 4 and 5 fused. In the specimen examined tarsale 1 is pushed away dorsally (fig. 23, A, 8), so as to lie between the tibiale and tarsale 2. All the tarsal bones are small and somewhat polygonal, and are connected with one another, and with the tibia and fibula on the one hand, and with the metatarsals on the other by a thin layer of cartilage.
The five digits of the foot each consist of a metatarsal and of a certain number of phalanges. In the specimen examined, owing to the shifting of tarsale 1, the first metatarsal as well as the second articulates with tarsale 2, while the fifth metatarsal articulates partially with the bone representing the fused tarsalia 4 and 5, partially with the fibulare. All the bones of the digits except the distal phalanges are terminated at each end by cartilaginous epiphyses, the distal phalanx of each digit has a cartilaginous epiphysis only on its proximal end.
The first, second, and fifth digits have two phalanges apiece, the third and fourth have three.
Figure 31 B, showing a Newt’s tarsus copied from Gegenbaur, has precisely the arrangement generally regarded as primitive for the higher vertebrates, except that tarsalia 4 and 5 are fused.