188

METAMORPHOSES OF THE FROG'S SKELETON.

didactyla (Fig. 33) the ray is ossified: its first joint (ib.), 53, is long, its second (ib.), 54, 55, is bifid, and a cartilage at the end of this supports two short terminal rays. This is the pattern of the subdivision of the appendage both of the scapular and pelvic arches, in all the higher vertebrates: hence, in consequence of the vast modifications of the several segments, the necessity for their special names. In the fore-limb the first segment (Fig. 33), 53, is the "arm," and its bone, the "humerus," No 53; the second segment is the fore-arm-its two bones are the "radius," No. 55, and "ulna," No. 54; the third segment is the "hand"-its rays are the "fingers ;" and its bones are subdivided into "carpals," No. 56, "metacarpals," and "phalanges," No. 57. In the hind-limb (Fig. 34) the first segment is the "thigh," and its bone, the "femur," No. 65; the second segment is the "leg,” and its two bones are the “tibia” No. 66, and "fibula," No. 67; the third segment is the "foot"-its rays are the "toes;" its bones are subdivided into "tarsals," "metatarsals," and "phalanges."

In the siren the pelvic arch and limbs are not developed; but they coexist with the scapular arch and limbs in all other batrachia. In the proteus the last segment of the forelimb divides into three rays, that of the hind-limb into two rays; in other words, it has three fingers and two toes. The menobranchus has four fingers and four toes. The axolotl has four fingers and five toes. The menopome has five fingers and five toes.

The ultimate subdivisions of the radiated or diverging appendages of the scapular and pelvic arches do not exceed five in any existing air-breathing animal, and their further complexity is due to the specialization of each digit, so as to combine in associated action, instead of their indefinite multiplication, which causes the seeming complexity of the same appendages in fishes.

In all the fish-like batrachia, called, from a retention of more or less of the branchial apparatus, "perennibranchia," the limbs are short, and the rays of the terminal segments of each limb are, more or less, united by a web: the body is long, and the tail long and compressed. But a great ascent in the scale of life is made in the batrachian order: all the species when hatched have the fish-like form, and gills for breathing water; most of them exist for some time, under this form, in water; and these undergo so strange a modification of form and structure before arriving at maturity, that it has been called a" metamorphosis." They change their aquatic for a terrestrial life; they breathe air instead of water; and from being omnivorous become carnivorous. The tadpoles of our common toad and frog afford ready and abundant instances for tracing these stages. The following is an outline of the main phenomena of the change observable in regard to the osseous system :

In the development of the skeleton of the common frog, a fibrous and cartilaginous framework is originally laid down conformably with the aquatic habits and life of the larva. A large cartilaginous cranium with four hæmal arches, and one of these supporting the framework of the branchial apparatus,—a short series of fibro-cartilaginous vertebræ, minus the hæmal arches, in the trunk, and a series of fibrous septa diverging from the fibrous capsule of the notochord, and defining and giving attachment to the mus cular segments along the tail,-constitute the skeleton of the newly hatched tadpole. As it grows, ossification begins; but only in those part of the skeleton which are to be retained in the future frog. Thus, the centrums and neurapophyses of the head and trunk are ossified, but not those of the tail. In the trunk, ossification of the vertebral body proceeds centripetally by layers, successively diminishing in extent, and conical interspaces are left, consisting of the changed fibrous capsule of the notochord with the inclosed gelatinous cells, their liquefied contents forming the balls of fluid, between the

METAMORPHOSES OF THE FROG'S SKULL.

189

biconcave vertebræ, as in fishes. But ossification proceeds to fill up the hinder cavity of the centrum, and to project into the front cavity of the succeeding vertebra, with which it is finally connected by a synovial ball-and-socket joint. Thus, the firmer intervertebral articulations are established, which adapt the vertebral column to the support of a body which is to be suspended upon limbs, and transported by them along the surface of the dry ground. Whilst this change is proceeding, the tail is undergoing rapid absorption, the retained fibro-cartilaginous condition of its vertebræ rendering them more ready for removal. In the last fused rudiments of the caudal vertebræ, ossification extends continuously, and the peculiar style (Fig. 12), c, at the end of the vertebral series in the frog and other tail-less batrachians, is thus established.

14

22

21

Fig. 12.

In the conversion of the biconcave into cup-and-ball vertebræ in batrachian larvæ, ossification commonly, but not always, proceeds to obliterate the hinder cavity. In the land salamanders, however, it extends from the front cavity; so that in the adult vertebræ the ball is anterior, and the cup posterior, as in certain salamandroid fishes-e. g., lepidosteus. In those batrachians that retain more or less of the branchial apparatus, with the outward form and natatory tail adapted to aquatic life, the vertebræ of the tail are ossified like those of the trunk, but the biconcave structure and intervening gelatinous joints are retained throughout life.

The chief changes which take place in the conversion of the cartilaginous skull of the larva to the ossified one of the imago, or perfect frog, are seen in the shape and relative position of the hæmal arches and their appendages-i. e., of the maxillary, mandibular, hyoid, and scapular arches. The maxillary arch expands in breadth, the mouth widens, and the horny mandibles are shed. As the mouth advances forwards, the tympanic pedicles are elongated, and are placed more obliquely; their proximal end retrograding from the post-frontal to the mastoid region of the skull, and their distal end inclining forwards with the attached lower jaw, Nos. 29, 33, on which the denticles now begin to be developed. For the still more extraordinary changes of the hyoid arch, No. 41, and its branchial appendages, No. 46, the student is referred to Dugè's "Recherches sur l'Ostéologie des Batraciens," 4to, 1835; and to the writer's "Archetype of the Vertebrate Skeleton," pp. 70, 71.

29

50

33

...28

65

56

54.

55

lr m

26

SKELETON OF THE FROG (Rana esculenta).

The scapular arch, which was close to the occiput, whilst protecting and supporting

the branchial heart-its primary function-begins, as the rudiments of the forelimbs bud out, to recede backwards, like the mandibular and branchial arches, but to a greater extent, the attachment to the occipital segment being wholly lost. The scapular and coracoid portions of the arch become first ossified; the suprascapular plate remains long cartilaginous, and always partly so; the sternum is developed in proportion as the hyoid arch is reduced, and the branchial arches are removed; thus a strong fulcrum is completed for the articulation of the shoulder-joints. The pelvic arch had previously been completed, and the iliac bones and sides of the sacrum become co-elongated: then the ilia continue to extend backwards as the tail is being absorbed, and the hind-limbs are lengthened out and finished.

Thus metamorphosed, the skeleton of the frog presents the following structure (Fig. 12):-The number of vertebræ of the trunk, exclusive of the coxygeal style, c, is nine; the first, or atlas, has no diapophyses, but these are present and long on the rest, especially on the third, d, and ninth, s, vertebræ; in the latter they are thick, stand outwards, and support two other long, curved, rib-like bones, 62, which expand at their distal ends, and unite to two bony plates, 63, completing the hæmal arch of the ninth segment of the trunk. The bones of the hinder extremities are attached to the point of union of the above costal and hæmal pieces, one of which answers to the ilium, 62, and the other to the ischium, 63. The superior development of this arch relates to the great size and strength of the hinder extremities in the tail-less tribe. The bodies of the vertebræ are articulated by ball-and-socket joints, the cup being anterior, the ball posterior, a modification which relates to the more terrestrial habits and locomotion of these higherorganized batrachia. The caudal vertebræ are represented by a single, elongated, cylindrical style, c, having an anchylosed neural canal. In the seven vertebræ, between the atlas and the sacrum, two zygapopophyses, looking upwards, two zygapophyses, z, looking downwards, and a short spine, are developed from each neural arch.

The suprascapula, 50, is very broad, and in great part ossified; the scapula, 51, divides at its humeral end into an acromial and coracoid process; the latter articulates with the true coracoid bone, 52, the acromion with the expanded extremity of the clavicle, 58: the glenoid cavity is formed by both the scapula and the coracoid. An episternal bone, 59, supporting a broad cartilage, is articulated to the mesial union of the clavicles, from which a bony bar is continued backwards between the expanded and partially conjoined ends of the coracoids. The sternum, 60, is articulated to the posterior part of the same extremities of the coracoids, and supports a broad "xiphoid” cartilage. The proximal end of the humerus, 53, is an epiphysis; the distal end presents a hemispherical ball between a small external ridge, and a large internal condyloid process. The antibrachial bones have coalesced, but an anterior and posterior indentation at the distal half indicates the radius, 55, ahd ulna, 54; their distal articular extremities are represented by a single epiphysis. The ulnar portion of the bone developes a short and broad olecranon, o. The bones of the carpal series now receive definite names, and are as follows:-(Fig. 12), s, scaphoid; 7, lunare; c and p, cuneopisiforme; t, trapezium; tr, trapezoides; m, magnum; u, unciforme-here two distinct bones. The first digit, I, has one bone, a metacarpal; the second digit, II, has a metacarpal and two phalanges; the third, III, the same; the fourth, IV, has a metacarpal and three phalanges; and the fifth, V, the same.

Both the proximal and the distal extremities of the femur, 65, are in the condition of epiphyses. The tibia and fibula are connate, 66: a longitudinal impression on the front and back part of the expanded distal end indicates their division, but a single

SKELETON OF THE SERPENT.

191

epiphysis, partially anchylosed, forms the proximal extremity, and a similar one the

Fig. 13.-SKELETON OF THE COBRA (Najá tripudians).

distal extremity, of the connate bones; they are perforated near their middle, from before backwards, by a vascular canal. The tarsal bones are now distinguished by names.

The astragalus, a, and calcaneum, cl, are much elongated; the former is slightly bent, the latter straight; they have coalesced at their proximal and also at their distal extremities with each other, and with the scaphoid, s, and cuboid, b, bones. Three cuneiform bones, c, ci, remain detached, and immediately support the three inner toes and a cartilaginous appendage. The first toe, i, and second toe, ii, have each a metatarsal and two phalanges; the third toe, iii, has a metatarsal and three phalanges; the fourth toe, iv, has a metatarsal and four phalanges; the fifth toe, v, a metatarsal and three phalanges. The great length and strength of the pelvic arch, and its appendages, the hind-limbs, give the frog the power of executing the long leaps for which it is proverbial.

All the batrachia present this structure in common with fishes, viz., that the ribs of the trunk, when present, are free, consist only of "pleurapophyses," and do not encompass the thoracic-abdominal cavity. The absence of unyielding osseous girdles at this part seems to relate to a peculiarity of their generation, viz., the almost simultaneous ripening of the spermcells and ova, causing a great and sudden distension of the abdomen at the breeding period.

Osteology of the Ophidia, or Serpent Tribe.-There are certain tropical land batrachia-the Cecilia, e. g.-in which the body is

192

STRUCTURE OF THE SKULL IN SERPENTS.

as long and slender as in serpents, includes almost as numerous vertebræ, and is devoid of all trace of limbs. But the osteology of the typical Ophidian reptiles differs from that of the batrachians in the more elongated ribs; in the distinct basi- and superoccipitals; in the superoccipital forming part of the ear-chamber; in the basioccipital combining with the exoccipital to form a single articular condyle for the atlas; in the ossification of the membranous space between the elongated parietals and the sphenoid; in the constant coalescence of the parietals with one another; in the constant confluence of the orbitosphenoids with the frontals, and in the meeting of the orbitosphenoids below the prosencephalon, upon the upper surface of the presphenoid; in the presence of distinct postfrontals, and the attachment thereto of the ectopterygoids, whereby they form an anterior point of suspension of the lower jaw, through the medium of the pterygoid and tympanic bones; lastly, in the connation of the prefrontals and lacrymals.

In studying the osteology of the head of the python, as the type of the Ophidian Order, by the aid of the following description, the student may compare the disarticulated skull, No. 628, with that of the large skeleton, No. 602, in the Museum, Royal College of Surgeons: the bones are numbered as here referred to.

The basioccipital, 1, is subdepressed, broadest anteriorly, subhexagonal; smooth and concave at the middle above, with a rough sutural tract on each side, and a hypapophysis below, produced into a recurved point. The hinder facet of the basioccipital is convex, forming the lower half of the occipital condyle, which is supported on a short peduncular prolongation. The basioccipital unites above and laterally with the exoccipitals and alisphenoids, and in front with the basisphenoid, upon which it rests obliquely, and it supports the medulla oblongata on its upper smooth surface.

The exoccipitals, 2, 2, are very irregular subtriangular bones; each is produced backwards into a peduncular process, supporting a moiety of the upper half of the occipital condyle. The outer and fore part of the exoccipital expands into the irregular base of the triangle: it is perforated by a slit for the eighth pair of nerves; it articulates below with the basioccipital; it is excavated in front to lodge the petrosal cartilage, where it articulates with the alisphenoid; it unites above with the superoccipital. The superoccipital, 3, is of a subrhomboidal form, sends a spine from its upper and hinder surface, expands laterally into oblong processes, is notched anteriorly, and sends down two thin plates from its under surface, bounding on the mesial side the surface for the cerebellum, and by the outer side forming the inner and upper parts of the acoustic cavities. The superoccipital articulates below with the exoccipitals and alisphenoids, and in front with the parietal, by which it is overlapped in its whole extent. The occipital vertebra is as if it were sheathed in the expanded posterior outlet of the parietal one (Fig. 17), the centrum resting on the oblique surface of that in front, and the anterior base of the neural spine entering a cavity in and being overlapped by that of the preceding neural spine: the analogy of this kind of "emboitement" of the occipital in the parietal vertebra with the firm interlocking of the ordinary vertebræ of the trunk is very interesting the end gained seems to be, chiefly, an extra protection of the epencephalon -the most important segment to life of all the primary divisions of the cerebro-spinal axis. The thickness of its immediately protecting walls (formed by the basi-, ex-, and super-occipitals) is equal to that of the same vertebral elements in the human skull, but they are moreover composed of very firm and dense tissue throughout, having no diploë: the epencephalon also derives a further and equally thick bony covering from the basisphenoid and the parietals, the latter being overlapped by the mastoids, which form a third covering to the cerebellum.