Читать книгу The Vertebrate Skeleton - Sidney H. Reynolds - Страница 12

Table of Contents

I. EXOSKELETON.

The exoskeleton includes

(1) Scales. These are of the type known as cycloid and consist of flat rounded plates composed of concentrically arranged laminae of calcified matter, with the posterior margin entire. The anterior end of each scale is imbedded in the skin and is overlapped by the preceding scales.

(2) The teeth. These are small, pointed, calcified structures arranged in large groups on the premaxillae, mandible, vomer, and superior and inferior pharyngeal bones.

(3) The fin-rays. These are delicate, nearly straight bony rods which support the fins.

II. ENDOSKELETON.

The endoskeleton of the Codfish, though partially cartilaginous, is mainly ossified.

It is divisible into an axial portion, including the skull, vertebral column, ribs, and skeleton of the median fins, and an appendicular portion, including the skeleton of the paired fins and their girdles.

1. The Axial Skeleton.

A. The Vertebral Column.

This consists of a series of some fifty-two vertebrae, all completely ossified.

It is divisible into two regions only, viz. the trunk region, the vertebrae of which bear movable ribs, and the caudal or tail region, the vertebrae of which do not bear movable ribs.

Trunk vertebrae.

These are seventeen in number; the ninth may be described as typical of them all. It consists of a short deeply biconcave centrum whose two cavities communicate by a narrow central canal. From the dorsal surface of the anterior half of the centrum arise two strong plates, the dorsal or neural processes, which are directed obliquely backwards and meet forming the dorsal or neural arch. This is produced into a long backwardly-directed dorsal or neural spine.

From the lower part of the anterior edge of each neural arch arise a pair of blunt triangular projections which overhang the posterior half of the preceding centrum, and bear a pair of flattened surfaces which correspond to the anterior or prezygapophyses of most vertebrae, they differ however from ordinary prezygapophyses in the fact that they look downwards and outwards. From the posterior end of the centrum arise a pair of short blunt processes each of which bears an upwardly- and inwardly-directed articulating surface corresponding to a postzygapophysis.

The two halves of the ventral arch form a pair of large ventri-lateral processes which arise from the anterior half of the centrum and pass outwards and slightly backwards and downwards.

Behind these there arises on each vertebra a second outgrowth which is small and flattened, and like the ventri-lateral process serves to protect the air-bladder. The surface of the centrum is marked by more or less wedge-shaped depressions, one in the mid-dorsal line, and two on the ventral surface immediately mesiad to the bases of the ventri-lateral process. There are also a number of smaller depressions.

The space between one centrum and the next is in the fresh skeleton filled up by the gelatinous remains of the notochord.

The first few vertebrae differ from the others in having very short centra and no ventri-lateral processes.

The first vertebra comes into very close relation to the posterior part of the skull, articulating with the exoccipitals. In the next few vertebrae the centra gradually lengthen, and at the fourth or fifth vertebra the ventri-lateral processes appear and gradually increase in size as followed back. They likewise gradually come to arise at a lower level on the centrum, and also become more and more downwardly directed, till at the last trunk vertebra they nearly meet.

The neural spines of the anterior trunk vertebrae are much longer than those of the posterior ones, that of the first vertebra being the largest and longest of all, and articulating with the skull. The spinal nerves pass out through wide notches or spaces between the successive neural arches.

Caudal vertebrae.

The caudal vertebrae are about thirty-five in number, each consists of a centrum with a slender backwardly-directed dorsal or neural arch, similar to those of the posterior trunk vertebrae. The two halves of the ventral or haemal arch however do not form outwardly-directed ventri-lateral processes, but arise on the ventral surface of the centrum, and passing downwards meet and enclose a space; they thus form a complete canal, and are prolonged into a backwardly-directed ventral or haemal spine. The anterior haemal arches are much larger than the corresponding neural arches, but when followed back they gradually decrease in size, till at about the twenty-fourth caudal vertebra they are nearly as small as the neural arches. The last caudal vertebra is succeeded by a much flattened hypural bone or urostyle, which together with the posterior neural and haemal spines supports the tail-fin.

B. The Ribs.

The ribs are slender, more or less cylindrical bones attached to the poster-dorsal faces of the ventri-lateral processes of all the trunk vertebrae except the first and second. The earlier ones are thicker and more curved; the later ones thinner and more nearly straight. The ribs are homologous with the distal parts of the haemal arches of the caudal vertebrae.

Associated with the ribs are a second series of rib-like bones, the intermuscular bones. These are slender, curved bones which arise from the ribs or from the ventri-lateral processes at a distance of about an inch from the centra, and curve upwards, outwards and backwards. In the anterior region where the ventri-lateral processes are short they arise from the ribs, further back they arise from the ventri-lateral processes.

C. The Unpaired or Median Fins.

These are six in number, three being dorsal, one caudal and two anal.

The dorsal and anal fins each consist of two sets of structures, the fin-rays and the interspinous bones. Each fin-ray forms a delicate, nearly straight, bony rod which becomes thickened and bifurcated at its proximal or vertebral end, while distally it is transversely jointed and flexible, frequently also becoming more or less flattened.

The first dorsal fin has thirteen rays, the second, sixteen to nineteen, the third, seventeen to nineteen. The first anal fin has about twenty-two, the second anal fourteen. In each fin the posterior rays rapidly decrease in size when followed back.

The interspinous bones of the dorsal and anal fins alternate with the neural and haemal spines respectively, and form short, forwardly-projecting bones, each attached proximally to the base of the corresponding fin-ray.

The caudal fin consists of a series of about forty-three rays which radiate from the posterior end of the vertebral column, being connected with the urostyle or hypural bone, and with the posterior neural and haemal spines without the intervention of interspinous bones. Like the other fin-rays those forming the caudal fin are transversely jointed, and are widened and frayed out distally.

The tail-fin in the Cod is homocercal, i.e. it appears to be symmetrically developed round the posterior end of the vertebral column, though in reality a much greater proportion is attached below the end of the vertebral column than above it. It is a masked heterocercal tail.

The Skull.

Owing to the fact that very little cartilage remains in the skull of the adult Codfish, its relation to the completely cartilaginous skull of the Dogfish is not easily seen. Before describing it therefore, the skull of the Salmon will be described, as it forms an intermediate type.

THE SKULL OF THE SALMON[36].

The Salmon's skull consists of (1) the chondrocranium, which remains partly cartilaginous and is partly converted into cartilage bone, especially in the occipital region, (2) a large series of plate-like membrane bones.

The Chondrocranium.

This is an elongated structure, wide behind owing to the fusion of the large auditory capsules with the cranium, and elongated and tapering considerably in front; in the middle it is much contracted by the large orbital cavities.

Dorsal surface of the Cranium.

In the centre of the posterior end of the dorsal surface is the supra-occipital (fig. 9, A, 1) with a prominent posterior ridge. It is separated by two tracts of unossified cartilage from the large series of bones connected with the auditory organ. The first of these is the epi-otic (fig. 9, 2), which is separated by only a narrow tract of cartilage from the supra-occipital, and is continuous laterally with the large pterotic (fig. 9, A, 3) which overlaps in front a smaller bone, the sphenotic (fig. 9, 4). Both epi-otic and pterotic are drawn out into rather prominent backwardly-projecting processes.

Fig. 9. A. dorsal and B. ventral view of the cranium of a Salmon (Salmo salar) from which most of the membrane bones have been removed (after Parker). Cartilage is dotted.

1. supra-occipital.	12. opisthotic.
2. epi-otic.	13. alisphenoid.
3. pterotic.	14. orbitosphenoid.
4. sphenotic.	16. foramen for passage of an
5. frontal.	artery.
6. median ethmoid.	17. pro-otic.
7. parietal.	18. articular surface for
8. lateral ethmoid.	hyomandibular.
9. parasphenoid.	II. VII. IX. X. foramina for the
10. vomer.	passage of cranial nerves.
11. exoccipital.

The greater part of the remainder of the dorsal surface is formed of unossified cartilage which is pierced by three large vacuities or fontanelles. The anterior fontanelle is unpaired, and lies far forward near the anterior end of the long cartilaginous snout, the two larger posterior ones lie just in front of the supra-occipital and lead into the cranial cavity. In front of the orbit the skull widens again, and is marked by two considerable lateral ethmoid (fig. 9, 8) ossifications. In front of these are a pair of deep pits, the nasal fossae, at the base of which are a pair of foramina through which the olfactory nerves pass out; they communicate with a space, the middle narial cavity, seen in a longitudinal section of the skull.

The long cartilaginous snout is more or less bifid in front, especially in the male (fig. 9).

Posterior end of the Cranium.

The foramen magnum forms a large round hole leading into the cranial cavity, and is bounded laterally by the two exoccipitals and below by them, and to a very slight extent by the basi-occipital, the three bones together forming a concave occipital condyle by which the vertebral column articulates with the skull.

The exoccipitals are connected laterally with a fourth pair of auditory bones, the opisthotics, and just meet the epi-otics dorsolaterally, while dorsally they are separated by a wide tract of unossified cartilage from the supra-occipital.

The opisthotics are connected laterally with the pterotics.

Side of the Cranium.

At the posterior end is seen the basi-occipital in contact above with the exoccipital, which is pierced by a prominent foramen for the exit of the tenth nerve. In front of this lies a small foramen, sometimes double, for the ninth nerve.