Читать книгу The Vertebrate Skeleton - Sidney H. Reynolds - Страница 13

Fig. 10. Lateral view of the chondrocranium of a Salmon (Salmo salar) (after Parker). A few membrane bones are also shown. Cartilage is dotted.

1. supra-occipital.	12. olfactory pit; the vomerine
2. epi-otic.	teeth are seen just below.
3. pterotic.	14. pro-otic.
4. opisthotic.	15. basisphenoid.
5. exoccipital.	16. foramen for the passage of
6. basi-occipital.	an artery.
7. parasphenoid.	17. anterior fontanelle.
8. sphenotic.	18. posterior fontanelle.
9. alisphenoid.	I. II. V. VII. IX. X. foramina
10. orbitosphenoid.	for the passage of cranial nerves.
11. lateral- or ectethmoid.

In front of the exoccipital is the large pro-otic pierced by two prominent foramina. Through the more dorsal of these (fig. 10, VII.) the facial nerve passes out, while the more ventral (fig. 10, 16) is for the passage of an artery. Dorsal to the exoccipital are the opisthotic and pterotic, and dorsal to the pro-otic is the sphenotic. The pterotic is marked by a prominent groove often lined by cartilage, which is continued forwards along a tract of cartilage between the pro-otic and sphenotic. With this groove the hyomandibular articulates.

There are considerable ossifications in the sphenoidal region of the side of the cranium. The anterior boundary of the posterior fontanelle is formed by the large alisphenoid, which is continuous behind with the pro-otic and sphenotic, and below with a slender basisphenoid. Both in front of and behind the basisphenoid there are considerable vacuities in the walls of the cranium; through the posterior of these openings (fig. 10, V.) the main part of the trigeminal nerve passes out, and through the anterior one, the optic (fig. 10, II.). The alisphenoid is continuous in front with the orbitosphenoid (fig. 10, 10), which is pierced by the foramen for the exit of the first nerve (fig. 10, I.), and in front of the orbitosphenoid there is a large vacuity. The lateral ethmoid is seen in the side view as well as in the dorsal view. Further forwards are seen the olfactory pits, and the long cartilaginous snout.

A ventral view of the cartilaginous cranium shows much the same points as the side view. The basisphenoid appears on the surface immediately in front of the basi-occipital.

The Skull with membrane bones.

The dorsal surface. The greater part of the dorsal surface in front of the supra-occipital is overlaid by a pair of large rough frontals (figs. 9, A, 5, and 10, 5). They cover the posterior fontanelles and stretch over from the sphenotic to the lateral ethmoid, forming a roof for the orbit. They meet in the middle line behind, but in front are separated by a narrow tract of unossified cartilage, and are overlapped by the median ethmoid (figs. 9, A, 6, and 11, 6). At the sides of the supra-occipital behind the frontals are a pair of small parietals (figs. 9, A, 7, and 11, 7).

Fig. 11. Lateral view of the skull of a Salmon (Salmo salar) (after Parker). Cartilage is dotted.

1. supra-occipital.	19. maxillae
2. epi-otic.	20. premaxillae.
3. pterotic.	21. articular.
4. sphenotic.	22. angular.
5. frontal.	23. dentary.
6. median ethmoid.	24. hyomandibular.
7. parietal.	25. symplectic.
8. nasal.	26. epi-hyal.
9. lachrymal.	27. cerato-hyal.
10. suborbital.	28. hypo-hyal.
11. supra-orbital.	29. glosso-hyal.
12. cartilaginous sclerotic.	30. opercular.
13. ossification in sclerotic.	31. sub-opercular.
14. meso-pterygoid.	32. infra-opercular.
15. meta-pterygoid.	33. pre-opercular.
16. palatine.	34. supratemporal
17. jugal.	35. branchiostegal rays.
18. quadrate.	36. basi-branchiostegal.

In a ventral view the cranium is seen to be chiefly covered by two large membrane bones, the parasphenoid (fig. 9, B, 9) behind, the vomer in front. A view of the posterior end differs from that of the cartilaginous cranium only in the fact that the end of the parasphenoid appears lying ventral to the basi-occipital.

The lateral view differs very markedly from that of the cartilaginous cranium, there being a great development of membrane bone in connection with the jaws and branchial apparatus. Lying dorsally are seen the median ethmoid, frontal, parietal, and supra-occipital as before. Lying external to the middle of the median ethmoid is seen the small nasal (fig. 11, 8), and below the hinder part is the lachrymal. The lachrymal (fig. 11, 9) forms the first of a series of seven small bones which surround the orbit forming the orbital ring. Of these the one lying immediately in the mid-ventral line of the orbit is the suborbital, while the one lying in the mid-dorsal line and attached to the frontal is the supra-orbital (fig. 11, 11). The orbit has a cartilaginous sclerotic in which are two small ossifications (fig. 11, 13) laterally placed.

Bones of the upper jaw.

The palato-pterygo-quadrate bar is in a very different condition from that of the dogfish, it is partially cartilaginous, partially converted into cartilage bone, partially overlapped by membrane bone. It is narrow in front but becomes much broader and deeper when followed back. Its anterior end forms the palatine which bears teeth, and in front is completely ossified, while behind the cartilage is only sheathed by bone.

Just behind the palatine the outer part of the cartilage is ossified, forming two small bones, the pterygoid and meso-pterygoid, while behind them is a larger, somewhat square bone, the meta-pterygoid (fig. 11, 15).

Below the meta-pterygoid is a tract of unossified cartilage, and then comes the quadrate (fig. 11, 18).

The lower angle of the quadrate bears a cartilaginous condyle with which the mandible articulates. In front of the palatine the cartilaginous snout is overlapped by three membrane bones, the jugal, maxilla and premaxillae.

The premaxillae (fig. 11, 20), the largest of these, overlaps the maxilla behind; both bones bear teeth. The jugal (fig. 11, 17) lies above the maxilla and overlaps it in front.

The lower jaw.

The lower jaw is a strong bar and is like the upper jaw, partly cartilaginous, forming Meckel's cartilage, partly ossified, and sheathed to a considerable extent in membrane bone.

The outer side and posterior end is ossified, forming the large articular (fig. 11, 21), but the condyle is cartilaginous and the anterior part of the articular forms merely a splint on the outer side of Meckel's cartilage, which extends beyond it for a considerable distance. The angle of the jaw just below the condyle is formed by a small angular (fig. 11, 22), and the anterior two-thirds of the jaw is sheathed in the large tooth-bearing dentary (fig. 11, 23).

The Hyoid arch.

The hyoid arch has a number of ossifications in it and is closely connected with the mandibular arch.

The hyomandibular (fig. 11, 24) is a large bone which articulates with a shallow groove lined by cartilage and formed partly in the pterotic, partly in front of it. The hyomandibular is overlapped in front by the meta-pterygoid, while below it tapers and is succeeded by a small area of unossified cartilage followed by the forwardly-directed symplectic which fits into a groove in the quadrate.

The unossified tract between the hyomandibular and symplectic is continuous in front with a strong bar, which remains partly cartilaginous and is partly converted into cartilage bone. The proximal part is ossified, forming the epi-hyal, the middle part forms the cerato-hyal (fig. 11, 27), in front of which is the small hypo-hyal. The hyoid arches of the two sides are united by the large tooth-bearing glosso-hyal (fig. 11, 29). Attached to the lower surface of the hyoid arch are a series of twelve flat branchiostegal rays (fig. 11, 35). Each overlaps the one in front of it, the posterior one being the largest. The branchiostegal rays of the two sides are united in front by an unpaired membrane bone, the basi-branchiostegal (fig. 11, 36).

Opercular bones. Behind the hyomandibular there is a large bony plate, the operculum, formed of four large membrane bones. The anterior of these, the pre-opercular (fig. 11, 33), is crescentic in shape, and with its upper end a small supratemporal (fig. 11, 34) is connected.

Behind the upper part of the pre-opercular is the largest of the opercular bones, the opercular proper. Its lower edge overlaps the sub-opercular, and both opercular and sub-opercular are overlapped by the infra-opercular (fig. 11, 32) in front. The infra-opercular is in its turn overlapped by the pre-opercular.

Branchial arches.

There are five branchial arches, the first four of which bear gill rays. Each of the first three consists of a shorter upper portion directed obliquely backwards and outwards, and a longer lower portion forming a right angle with the upper and directed obliquely forwards and inwards. The greater part of each arch is ossified.

The upper part of either of the first two consists of a short tapering pharyngo-branchial directed inwards, and of a long epi-branchial tipped with cartilage at both ends. The junction of the upper and lower parts is formed by a cartilaginous hinge-joint between the epi-branchial and cerato-branchial. The cerato-branchial is a long bony rod separated by a short area of cartilage from the hypo-branchial, which is succeeded by the basibranchial meeting its fellow in the middle line. The fourth arch has a short epi-branchial and no ossified pharyngo-branchial, while the fifth is reduced to little more than the cerato-branchial, which bears a few teeth on its inner edge. All the branchial arches have projecting from their surfaces a number of little processes which act as strainers. The first and fourth arches have one series of these, the second and third have two.

THE SKULL OF THE CODFISH[37].

A full description having been already given of the Salmon's skull, that of the Codfish will be described in a briefer manner. The skull is very fully ossified, and the great number of plate-like bones render it a very complicated structure.

The Cranium.

At the posterior end of the dorsal surface is the large supra-occipital, which is drawn out behind into the large blade-like occipital spine. On each side of the supra-occipital are the small irregular parietals, while in front of it the roof of the skull is mainly formed by the very large unpaired frontal.

A complicated series of bones are developed in connection with the auditory capsule, which forms a large projecting mass united with the side of the cranium and drawn out behind into a pair of strong processes, the epi-otic and parotic processes. Both these processes are connected behind with a large V-shaped bone, the post-temporal (fig. 13, 1), which will be described when dealing with the pectoral girdle. The epi-otic process is formed by the epi-otic, which is continuous in front with the parietal. The parotic process is formed by two larger bones, a more dorsal one, the pterotic, and a more ventral and internal one, the opisthotic, which is continuous in front with the large pro-otic. Intervening between the pterotic and frontal is another rather large bone, the sphenotic, this articulates below with the pro-otic. The pterotic and sphenotic together give rise to a large concave surface by which the hyomandibular articulates with the cranium. Several of the cranial nerves pass out through the bones of the auditory capsule. The ninth leaves by a foramen near the posterior border of the opisthotic, the fifth and seventh by a notch in the anterior border of the pro-otic.

A number of bones are likewise developed in connection with the orbit forming the orbital ring. Of these the most anterior, the lachrymal, is much the largest, the others are five to seven in number, the most ventral being the suborbital. The sclerotic coat of the eye is cartilaginous.

Two pairs of bones and one unpaired bone are developed in connection with the olfactory capsules, of these, the nasals are narrow bones lying next the lachrymals, but nearer the middle line; they overlap the second pair of bones, the irregular lateral ethmoids. These meet one another in the middle line, and are overlapped behind by the frontal. They articulate laterally with the lachrymal and palatine, and ventrally with the parasphenoid.

In a posterior view the foramen magnum and the four bones which surround it and together form the occipital segment are well seen. On the ventral side is the basi-occipital, terminated posteriorly by a slightly concave surface which articulates with the centrum of the first vertebra. The sides of the foramen magnum are formed by the exoccipitals, a pair of very irregular bones, pierced by a pair of prominent foramina for the exit of the tenth nerves. The exoccipitals also bear a pair of surfaces for articulation with corresponding ones on the neural arch of the first vertebra. The most dorsal of the four bones is the supra-occipital.

On the ventral surface of the cranium in front of the basi-occipital is seen the parasphenoid, a very long narrow bone which underlies the greater part of the cranium. Behind, it articulates dorsally with the basi-occipital and dorsolaterally with the pro-otics and opisthotics, in front it articulates dorsally with the lateral ethmoid and ventrally with the vomer. At the sides of the parasphenoid are the small alisphenoids articulating above with the postfrontals, in front with the frontals, and behind with the pro-otics.

The vomer is an unpaired bone lying immediately in front of the parasphenoid. In front it terminates with a thickened curved margin bearing several rows of small teeth; behind it tapers out into a long process which underlies the anterior part of the parasphenoid. Immediately dorsal to the vomer is another median bone, the median ethmoid; this is truncated in front and tapers out behind into a process which fits into a groove on the ventral side of the frontal.

Bones in connection with the upper jaw.

These bear a close resemblance to those of the Salmon. The most anterior bone is the premaxillae, a thick curved bone meeting its fellow in the middle line. The point of junction of the two is drawn out into a short process, and the oral surface is thickly covered with small teeth. The dorsal ends of the premaxillae are seen in the fresh skull to meet a large patch of cartilage. Behind the premaxillae is the maxilla, a long rod-like toothless bone, somewhat expanded at the upper end where it articulates with the premaxillae and vomer.

Articulating in front with the anterior end of the maxilla and with the lateral ethmoid is a very irregular bone, the palatine (fig. 12, 1); it articulates behind with two flat bones, the pterygoid and meso-pterygoid. The pterygoid is united behind with two more bones, the quadrate (fig. 12, 4) and meta-pterygoid. The quadrate is a rather stout irregular bone, bearing on its lower surface a prominent saddle-shaped articulating surface for the mandible. The palatine, pterygoid and quadrate bones are the ossified representatives of the palato-pterygo-quadrate bar of the Dogfish.

Fig. 12. Mandibular and hyoid arches of a Cod (Gadus morrhua) × ½ (Brit. Mus.).

1. palatine.	9. articular.
2. meso-pterygoid.	10. dentary.
3. pterygoid.	11. inter-hyal.
4. quadrate.	12. epi-hyal.
5. symplectic.	13. cerato-hyal.
6. meta-pterygoid.	14. hypo-hyal.
7. hyomandibular.	15. uro-hyal.
8. angular.	16. branchiostegal rays.

The quadrate is united behind with the symplectic (fig. 12, 5), and the meta-pterygoid with the symplectic and hyomandibular, both of which bones will be described immediately in connection with the hyoid arch.

The Lower jaw.

The lower jaw or mandible like that of the Salmon is partly cartilaginous, forming Meckel's cartilage, partly formed of cartilage bone, partly of membrane bone. Meckel's cartilage is of course not seen in the dried skull.

The lower jaw includes one cartilage bone, the articular (fig. 12, 9), this is a large bone connected by a saddle-shaped surface with the quadrate. Meckel's cartilage lies in a groove on its under surface, and projects beyond it in front. The angular is a small thick bone united to the lower surface of the articular at its posterior end. The dentary (fig. 12, 10) is a large tooth-bearing bone meeting its fellow in the middle line in front, while the articular fits into a deep notch at its posterior end.

The hyoid arch.

The hyomandibular (fig. 12, 7) is a large irregular bone, articulating by a prominent rounded head with the sphenotic and pterotic. It is united in front with the meta-pterygoid and symplectic, and sends off behind a strong process which articulates with the opercular. The symplectic is a long somewhat triangular bone drawn out in front into a process which fits into a groove on the inner surface of the quadrate. The distal portion of the hyoid arch is strongly developed and consists of first the inter-hyal (fig. 12, 11), a short bony rod, which articulates dorsally with a patch of cartilage intervening between the posterior part of the hyomandibular and the symplectic. Below it is united with the apex of the triangular epi-hyal, a bone suturally connected with the large cerato-hyal (fig. 12, 13) which unites distally with two small hypo-hyals. To the cerato-hyal are attached a series of seven strong curved cylindrical rods, the branchiostegal rays. The first of these is the smallest and they increase in size up to the last. The four dorsal ones are attached to the outer surface of the cerato-hyal, the three ventral ones to its inner surface. Interposed between the hypo-hyals of the two sides is an unpaired somewhat triangular plate, the uro-hyal or basi-branchiostegal (fig. 12, 15).

The branchial arches.

The branchial arches are five in number and consist of the following parts on each side. The dorsal end is formed of the supra-pharyngeal bone, a large irregular bone covered ventrally with teeth of a fair size, and representing the fused pharyngo-branchials of the four anterior arches. Its external surface is continuous with four small epi-branchials which pass horizontally backwards and outwards. Their distal ends meet four long cerato-branchials which are directed forwards and inwards and form the principal part of the arches.

Each of the first three cerato-branchials articulates ventrally with a hypo-branchial, and the hypo-branchials of the two sides are united in the middle line by an unpaired basibranchial. The third hypo-branchial is much flattened. The fourth cerato-branchial is united by cartilage with the posterior surface of the third hypo-branchial, which it meets near the middle line.

The fifth arch consists only of the cerato-branchial, a wide structure covered with teeth and generally called the inferior pharyngeal bone.

The skeleton of the operculum consists of the same four bones as in the Salmon, namely the opercular, the infra-opercular, the pre-opercular and the sub-opercular. Of these the anterior bone, the pre-opercular, is the largest, while the infra-opercular is the smallest. The opercular has a facet for articulation with the hyomandibular.

2. The Appendicular Skeleton.

The Pectoral girdle.

This is of a highly specialised type. Membrane bones are greatly developed, and the cartilage bones, the scapula and coracoid, are much reduced in size and importance.

Fig. 13. The right half of the pectoral girdle and right pectoral fin of a Cod (Gadus morrhua) × ½ (Brit. Mus.).

1. post-temporal.	5. scapula.
2. supra-clavicle.	6. post-clavicle.
3. clavicle.	7. brachial ossicles.
4. coracoid.	8. dermal fin-rays.

The largest bone in the shoulder girdle is the clavicle (fig. 13, 3), which is irregularly crescent shaped, thick in front and tapering off behind. To the outer side of its upper part is attached a thick cylindrical bone, the supra-clavicle, which passes upwards and is connected with a strong V shaped bone, the post-temporal. The apex of the V meets the supra-clavicle, the inner limb articulates with the epi-otic process, the outer with the parotic process. Projecting downwards from the upper part of the clavicle is a long bony rod, flattened proximally and cylindrical and pointed distally, this is the post-clavicle (fig. 13, 6).

The scapula (fig. 13, 5) is a small irregular plate of bone attached to the inner side of the middle of the clavicle. The coracoid[38] is a larger plate of similar character, irregularly triangular in shape, attached to the inner side of the clavicle immediately below the scapula. The scapula and coracoid bear the pectoral fin.

The Pectoral fins.

Each of these consists of four small irregular bones, the brachial ossicles (fig. 13, 7), bearing a series of about nineteen dermal fin-rays. The brachial ossicles represent the reduced and modified radiale and basalia of cartilaginous fish such as the dogfish. The fin-rays (fig. 13, 8) which form the whole external portion of the fin are long slender rods having essentially the same character as those of the unpaired fins.

The Pelvic girdle.

The pelvic girdle in the Cod as in other Teleosteans is entirely absent, its place being taken by the enlarged basi-pterygia of the fins.

The Pelvic fins.

These have a very anomalous position in the Cod, being attached to the throat in front of the pectoral girdle. Each consists of a basal portion, the basi-pterygium, and of a number of dermal rays. The basi-pterygium consists of an expanded ventral portion which meets its fellow below in the middle line, and to which the rays are attached, and of an inwardly-directed dorsal portion which also meets its fellow and is imbedded in the flesh. The rays are six in number and are long slender structures similar to those of the other fins.