Fig. 22. A ventral, and B lateral view of the shoulder girdle and sternum of an old male Crested Newt(Molge cristata) × 3 (afterParker).1. scapula.4. glenoid cavity.2. suprascapula.5. precoracoid.3. coracoid.6. sternum.
Fig. 22. A ventral, and B lateral view of the shoulder girdle and sternum of an old male Crested Newt(Molge cristata) × 3 (afterParker).1. scapula.4. glenoid cavity.2. suprascapula.5. precoracoid.3. coracoid.6. sternum.
Thescapular portionis a slightly curved oblong plate; its proximal third thescapula(fig. 22, 1) is ossified and bounds part of the well-markedglenoid cavity(fig. 22, 4); its distal portion forms a large oblong cartilaginous plate, thesuprascapula(fig. 22, 2).
Theprecoracoid(fig. 22, 5) forms a small forwardly-directed cartilaginous plate. Thecoracoid(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, theupper armorbrachium, thefore-armorantibrachium, and themanus.
Theupper armincludes a single bone, thehumerus.
Thehumerusis a slender bone cylindrical in the middle and expanded at either end, the proximal part forms a roundedheadwhich articulates with the glenoid cavity. Along the proximal part of the anterior or pre-axial surface runs a strongdeltoid ridge. The proximal part of the postaxial surface also bears a small outgrowth.
Thefore-armcontains two bones, theradiusandulna, both of which are small and imperfectly ossified at their terminations.
Theradius(fig. 23, B, 11) or pre-axial bone is rather the larger of the two, and is considerably expanded at its proximal end. Theulnaor postaxial bone is somewhat expanded distally, but is not drawn out proximally into an olecranon process.
Themanusconsists of two parts, a group of small bones forming thecarpusorwrist, and thehand.
Thecarpusis 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, thecentrale, 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, theradiale(fig. 23, B, 13), and a larger postaxial bone, which represents the fusedulnareandintermediumof the very simple carpus described on pp. 26 and 27.
The four bones of the distal row are respectivelycarpalia2, 3, 4 and 5.
Thehandconsists 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.
Fig. 23. A right posterior, and B right anterior limb of a Newt× 1½ (Molge cristata).1. femur.10. humerus.2. tibia.11. radius.3. fibula.12. ulna.4. tibiale.13. radiale.5. intermedium.14. intermedium and ulnare fused.6. fibulare.15. centrale of carpus, the pointing7. centrale of tarsus.line passes across carpale 2.8. tarsale 1.16. carpale 3.9. tarsalia 4 and 5 fused.17. carpale 5.I. II. III. IV. V. digits.
Fig. 23. A right posterior, and B right anterior limb of a Newt× 1½ (Molge cristata).1. femur.10. humerus.2. tibia.11. radius.3. fibula.12. ulna.4. tibiale.13. radiale.5. intermedium.14. intermedium and ulnare fused.6. fibulare.15. centrale of carpus, the pointing7. centrale of tarsus.line passes across carpale 2.8. tarsale 1.16. carpale 3.9. tarsalia 4 and 5 fused.17. carpale 5.I. II. III. IV. V. digits.
Each digit consists of a somewhat elongatedmetacarpaland 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, theilium, a posterior ventral element, theischium, and an anterior ventral element, thepubis, to which is attached anepipubis.
Theiliumis a somewhat cylindrical bone which at its ventral end meets the ischium, and forms part of theacetabulum. It is then directed upwards and slightly backwards, and is attached to the ribs of the sacral vertebra.
Theischiaare 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 thepubes. Projecting forwards from it is a bifid cartilaginousepipubis.
D.The Posterior limb.
This is divisible into a proximal portion, thethigh, a middle portion, thecrusorshin, and a distal portion, thepes.
Thethighconsists of a single bone, thefemur(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.
Thecrusorshinincludes two short bones, thetibiaandfibula, which are nearly equal in length. The pre-axial bone or tibia is a straight bone thickest at its proximal end, the postaxial bone orfibula(fig. 23, A, 3) is a rather stouter curved bone of nearly equal diameter throughout.
Thepesincludes thetarsusorankle, and thefoot.
Thetarsusconsists of eight small bones arranged in a proximal row of three, thetibiale,intermediumandfibulare, and a distal row of fourtarsalia, with one bone, thecentrale(fig. 23, A, 7), interposed between the two rows. In the specimen examined, thetibiale, is a small bone articulating with the tibia, theintermedium(fig. 23, A, 5) is larger and articulates with both tibia and fibula, thefibulareis the largest of the three and articulates with the fibula.
The bones of the distal row aretarsalia 1,2,3, and a bone representing4and5fused. 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 fivedigitsof the foot each consist of ametatarsaland of a certain number ofphalanges. 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.
I. EXOSKELETON.
The skin of the frog is smooth and quite devoid of scales or other exoskeletal structures. The only exoskeletal structures met with in the frog are:—
1. Theteeth, which are most conveniently described with the endoskeleton.
2. The horny covering of the calcar or prehallux (see p. 167).
II. ENDOSKELETON.
The endoskeleton of the adult frog consists partly of cartilage, partly of bone and each of these types of tissue occurs in two forms. The cartilage may be hyaline, as in the omosternum and xiphisternum, or may be more or less calcified as in part of the suprascapula and the epiphyses of the limb bones. The bone may be cartilage bone, or membrane bone.
The skeleton is divisible into anaxial portionconsisting of the skull, vertebral column, and sternum, and anappendicular portionconsisting of the skeleton of the limbs and their girdles.
1.The Axial Skeleton.
A.The Vertebral column.
The vertebral column is a tube, formed of a series of ten bones which surround and protect the spinal cord. Of these ten bones nine are vertebrae, while the tenth is a straight rod, theurostyle, and is almost as long as all the vertebrae put together. The second to eighth vertebrae inclusive have a very similar structure, but the first and ninth differ from the others.
Any one of the second to eighth vertebrae forms a bony ring with a somewhat thickened floor, thecentrumor body, which articulates with the centra of the immediately preceding and succeeding vertebrae. The articulating surfaces are covered with cartilage and are procoelous, or convex in front and concave behind. The eighth vertebra is however amphicoelous or biconcave. The centrum of each vertebra encloses an isolated vestige of the notochord. Theneural archforms the roof and sides of the neural canal, which is very spacious in the anterior vertebrae, but becomes more depressed in the posterior ones. The arch bears theneural spine, a low median ridge of variable character, and is drawn out in front and behind, forming the two pairs of articulating surfaces orzygapophysesby means of which the vertebrae are attached together. Of these the anterior articulating surfaces orprezygapophyseslook upwards and slightly inwards, while the posterior articulating surfaces orpostzygapophyseslook downwards and slightly outwards. The sides of the neural arches are drawn out into a pair of prominenttransverse processes. Those of the second vertebra look somewhat forwards, those of the third look directly outwards or somewhat forwards, while those of the fourth, fifth, and sixth are directed slightly backwards, and those of the seventh and eighth nearly straight outwards.All the transverse processes are terminated by very small cartilaginousribs.
Special vertebrae.
Thefirst vertebrais a ring-like structure with a much depressed centrum. It bears in front two oval concave surfaces for articulation with the condyles of the skull, while the centrum is terminated behind by a prominent convex surface. There are as a rule no transverse processes, and the postzygapophyses look downwards and outwards. Occasionally however transverse processes do occur. Projecting forwards from the centrum is a minute process better developed in the Newt. This resembles an odontoid process, and it has hence been supposed that the first vertebra is homologous with the axis of mammalia, and that the atlas of the frog is fused with the skull.
Theninth vertebrahas very stout transverse processes directed backwards and somewhat upwards. They articulate with the pelvic girdle and hence this vertebra is regarded as thesacrum. The neural arch is much depressed, the centrum is convex in front and bears on its posterior surface two short rounded processes for articulation with the urostyle.
Theurostyleis a long rod-like bone forming the posterior unsegmented continuation of the vertebral column. It is probably equivalent to three vertebrae, the tenth, eleventh, and twelfth fused together, and to an unsegmented rod of cartilage which lies ventral to the notochord. The anterior end is expanded and bears two concave articular surfaces by means of which it articulates with the sacrum. A prominent ridge runs along the dorsal surface, but gradually diminishes when traced back. The anterior portion contains a canal which is a continuation of the neural canal. At a point not far from the anterior end, this canal communicates with the exterior by a pair of minute holes which correspond with the intervertebral foramina.
B.The Skull[56].
The skull of the Frog consists of three principal parts:—
(1) an axial part, thecranium proper, which encloses the brain. To it are firmly fused
(2) thecapsules of the olfactory and auditory sense organs,
(3) lastly there is thehyoid apparatusand theskeleton of the jaws.
The skull is by no means so completely ossified as is the vertebral column, but in addition to the cartilage bone, there is a great development of membrane bone in connection with it.
The skull has a peculiarly flattened and expanded form depending on the wide lateral separation of the jaws from the cranium.
(1)The Cranium properor Brain case.
This is an unsegmented tube, which is widest behind. It remains to a considerable extent cartilaginous, but is partly converted into cartilage bone, partly sheathed in membrane bone. Its roof is imperfect, being pierced by three holes orfontanelles, one large anterior fontanelle (fig. 25, A, 9), and two smaller posterior fontanelles (fig. 25, A, 10).
The cartilage bones of the cranium proper are the twoexoccipitalsand thesphenethmoid.
Theexoccipitals(figs. 24, 25, and 26, 6) are a pair of irregular bones bounding the foramen magnum at the posterior end of the skull. They almost completely surround the foramen magnum, and bear a pair of oval convex surfaces, theoccipital condyles, with which the first vertebra articulates. The bones generally called the exoccipitals of the frog include theepi-oticandopisthoticelements of many skulls, in addition to the exoccipitals.
Fig. 24. A dorsal, and B ventral views of the cranium of a Common Frog(Rana temporaria) × 2 (afterParker). In this and the next two figs. cartilage is dotted, cartilage bones are marked with dots and dashes, membrane bones are left white.1. sphenethmoid.14. vomer.2. fronto-parietal.15. posterior nares.3. pterygoid.16. palatine.4. squamosal.18. columella.6. exoccipital.19. quadrate.7. parasphenoid.20. occipital condyle.8. pro-otic.II. optic foramen.9. quadratojugal.V. VII. foramen for exit of10. maxillae.trigeminal and facial nerves.11. nasal.IX. X. foramina for exit of12. premaxillae.glossopharyngeal and13. anterior nares.pneumogastric nerves.
Fig. 24. A dorsal, and B ventral views of the cranium of a Common Frog(Rana temporaria) × 2 (afterParker). In this and the next two figs. cartilage is dotted, cartilage bones are marked with dots and dashes, membrane bones are left white.1. sphenethmoid.14. vomer.2. fronto-parietal.15. posterior nares.3. pterygoid.16. palatine.4. squamosal.18. columella.6. exoccipital.19. quadrate.7. parasphenoid.20. occipital condyle.8. pro-otic.II. optic foramen.9. quadratojugal.V. VII. foramen for exit of10. maxillae.trigeminal and facial nerves.11. nasal.IX. X. foramina for exit of12. premaxillae.glossopharyngeal and13. anterior nares.pneumogastric nerves.
The patch of unossified cartilage immediately external to the occipital condyle is pierced by two small foramina, through which the ninth and tenth nerves leave the cranial cavity. The ninth nerve passes through the more external of these foramina, the tenth through the one nearer the condyle.The foramina lie however very close together and are sometimes confluent. The cranial walls for a considerable distance in front of the occipitals are unossified, but the anterior end of the cranial cavity is encircled by another cartilage bone, thesphenethmoid(figs. 24 and 25, 1) or girdle bone. This partly corresponds to the orbitosphenoids of the Newt's skull. Anteriorly it is pierced by a pair of small foramina through which the ophthalmic branches of the trigeminal nerve pass out.
The anterior part of the cranial cavity is divided into two halves by a vertical plate, themesethmoid. Some little distance behind the sphenethmoid the ventro-lateral walls of the cartilaginous cranium are pierced by a pair of rather prominent holes, theoptic foramina(figs. 24 and 25, B, II), and at a similar distance further back, occupying a kind of notch in the pro-otic are the largetrigeminal foramina, through which the fifth and seventh nerves leave the cranium. Between the trigeminal and optic foramina are the very small foramina for the sixth nerves (fig. 25, B,VI).
Themembrane bonesof the cranium proper include thefronto-parietalsand theparasphenoid.
Thefronto-parietals(figs. 24 and 26, A, 2) form a pair of long flat bones closely applied to one another in the middle line, the line of junction being thesagittal suture. They cover over the fontanelles and overlap the sphenethmoid in front.
Theparasphenoid(figs. 24 and 26, B, 7) is a bone shaped like a dagger with a very short handle. It lies on the ventral surface of the cranium, the blade being directed forwards and underlying the sphenethmoid; its lateral processes underlie the auditory capsules.
(2)The sense capsules.
The sense capsules are cartilaginous or bony structures which surround the olfactory and auditory organs and are closely united to the cranium.
Fig. 25. A dorsal and B ventral view of the cranium of a common Frog(Rana temporaria) from which the membrane bones have mostly been removed. × 2 (afterParker).1. sphenethmoid.8. pro-otic.2. palatine.9. anterior fontanelle.3. pterygoid.10. right posterior fontanelle.4. quadrate.11. quadratojugal.5. columella.12. nasal capsule.6. exoccipital.II. V. VI. IX. X. foramina for7. ventral cartilaginous wall ofexit of cranial nerves.cranium.
Fig. 25. A dorsal and B ventral view of the cranium of a common Frog(Rana temporaria) from which the membrane bones have mostly been removed. × 2 (afterParker).1. sphenethmoid.8. pro-otic.2. palatine.9. anterior fontanelle.3. pterygoid.10. right posterior fontanelle.4. quadrate.11. quadratojugal.5. columella.12. nasal capsule.6. exoccipital.II. V. VI. IX. X. foramina for7. ventral cartilaginous wall ofexit of cranial nerves.cranium.
Theauditory capsulesare fused with the sides of the posterior end of the cranium just in front of the exoccipitals. They are largely cartilaginous, but include in their anterior walls a pair of irregular cartilage bones, thepro-otics(figs. 24 and 25, 8). The cartilaginous area lying ventral to the pro-otic and external to the exoccipital is pierced by a rather prominent hole, thefenestra ovalis, which forms a communication between the internal ear cavity, and a space the tympanic cavity, which lies at the side of the head, and is bounded externally by the tympanic membrane. The fenestra ovalis is occupied by a minute cartilaginous structure, thestapes, and articulated partly to this and partly to a slight recess in the pro-otic is thecolumella(fig. 25, B, 5), a rod in part bony and in part cartilaginous, whose outer end isattached to the tympanic membrane. The columella and stapes are together homologous with the mammalian auditory ossicles and with the hyomandibular of Elasmobranchs. Sometimes the term columella is used to include the whole ossicular chain,—the columella together with the stapes.
Theolfactoryornasal capsules(fig. 25, B, 12) are fused with the anterior end of the cranium and differ from the auditory capsules in being to a great extent unossified. There are however two pairs of membrane bones developed in connection with them, thevomersand thenasals. They are drawn out into three pairs of cartilaginous processes, on the dorsal surface into theprenasalandalinasalprocesses which bound the external nares, and on the ventral surface towards the middle line into the forwardly-projectingrhinalprocesses.
Thenasals(figs. 24 and 26, 11) form a pair of triangular bones lying dorsolaterally in front of the fronto-parietals. Their bases are turned towards one another and their apices are directed outwards and backwards. They correspond in position with the prefrontals of the reptilian skull as well as with the nasals.
Thevomersare a pair of irregular bones lying on the ventral surface of the olfactory capsules. Each bears on its inner and posterior angle a group of minute pointed teeth, while its outer border is drawn out into three or four small slightly diverging processes, the two posterior of which form the inner boundary of theposterior nares(fig. 24, B, 15).
(3)The jaws.
Theupper jawconsists of a rod of cartilage connected with the cranium near its two ends, but widely separated from it in the middle. It is almost completely overlain by membrane bone. With its posterior end the lower jaw articulates.
The membrane bones of the upper jaw include first thepremaxillae, a small bone meeting its fellow in the middle line, and forming the extreme anterior end of the upper jaw. It gives off on its dorsal surface a backwardly-projectingprocess. It is connected behind with themaxillae(figs. 24 and 26, 10), a long flattened bone which forms the greater part of the margin of the upper jaw, and gives off near its anterior end a short process which projects upwards and meets the nasal.
Fig. 26.A, Lateral view of the skull,B, Posterior view of the cranium of a Common Frog(Rana temporaria) × 2 (afterParker).1. sphenethmoid.14. mento-meckelian.2. fronto-parietal.15. dentary.3. pterygoid.16. angulo-splenial.4. squamosal.17. basilingual plate.5. tympanic membrane.19. quadrate.6. exoccipital.20. columella.7. parasphenoid.21. occipital condyle.8. pro-otic.22. anterior cornu of the hyoid9. quadratojugal.(cerato-hyal).10. maxillae.23. foramen magnum.11. nasal.II. IX. X. foramina for the exit12. premaxillae.of cranial nerves.13. anterior nares.
Fig. 26.A, Lateral view of the skull,B, Posterior view of the cranium of a Common Frog(Rana temporaria) × 2 (afterParker).1. sphenethmoid.14. mento-meckelian.2. fronto-parietal.15. dentary.3. pterygoid.16. angulo-splenial.4. squamosal.17. basilingual plate.5. tympanic membrane.19. quadrate.6. exoccipital.20. columella.7. parasphenoid.21. occipital condyle.8. pro-otic.22. anterior cornu of the hyoid9. quadratojugal.(cerato-hyal).10. maxillae.23. foramen magnum.11. nasal.II. IX. X. foramina for the exit12. premaxillae.of cranial nerves.13. anterior nares.
Both maxillae and premaxillae are grooved ventrally, and bear, attached to the outer more prominent margin of the groove, a row of minute conical teeth. These teeth arepleurodont, that is, are ankylosed by their bases and outer sides to the margin of the jaw. Each tooth is a hollow cone, the basalpart of which is formed of bone, the apical part of dentine, capped by a very weak development of enamel.
The posterior end of the maxillae is overlapped by a small bone, thequadratojugal(figs. 24 and 26, 9), whose posterior end forms part of the articular surface for the lower jaw. Just behind the quadratojugal there is a small unossified area which lies at the angle of the mouth, and is connected by a narrow bar of cartilage with the cranium; this forms thequadrate(figs. 24 and 26, 19). A backwardly-directed outgrowth from the cartilaginous bar more or less completely surrounds the tympanic membrane, forming the tympanic ring. When followed back the maxillae and quadratojugal diverge further and further from the cranium, till the angle of the mouth comes to be separated from the foramen magnum by a space nearly double the width of the cranium. This space is bridged over to a considerable extent by two triradiate bones, thepterygoidandsquamosal.
Thepterygoid(figs. 24 and 26, 3) is a large bone, whose anterior limb runs forwards meeting the maxillae and palatine; while its inner limb meets the auditory capsule and parasphenoid, and its outer limb runs backwards and outwards to the angle of the mouth. Thepalatineis a small transversely-placed bone, which connects the pterygoid with the anterior part of the sphenethmoid. Thesquamosal(figs. 24 and 26, 4) is a T-shaped bone whose anterior arm is pointed and passes forwards to meet the pterygoid. The posterior upper arm is closely applied to the pro-otic, while the posterior lower arm meets the pterygoid and quadratojugal at the angle of the jaw, and surrounds the narrow cartilaginous bar of the quadrate which goes to join the cranium. The squamosal is probably homologous with the squamosal together with the pre-opercular of Bony Ganoids.
The quadrate and squamosal form thesuspensoriumby which the lower jaw is connected with the cranium.
Thelower jawormandibleconsists of a pair of cartilaginous rods (Meckel's cartilages) in connection with eachof which there are developed two membrane bones and one cartilage bone. The cartilage bone is themento-meckelian(fig. 26, A, 14), a very small ossification at the extreme anterior end. The membrane bones are theangulo-splenialand thedentary. Theangulo-splenialis a strong flat bone which forms the inner and lower part of the mandible for the greater part of its length. Its dorsal surface is produced into a slightcoronoid process. Thedentary(fig. 26, A, 15) is a flat plate which covers the outer surface of the anterior half of the mandible, as far forwards as the mento-meckelian. The lower jaw is devoid of teeth. The part of Meckel's cartilage which in most vertebrates ossifies, forming the articular bone, remains unossified in the Frog.
The Hyoid apparatus.
Thehyoidof the adult Frog is formed of the modified hyoid and branchial arches of the tadpole. It consists of a broad thin plate of cartilage, thebasilingual plate(fig. 29, B, 1), drawn out into two pairs of long processes, thecornua. The basilingual plate is broader in front than behind, and is formed from the fused ventral ends of the hyoid and branchial arches of the tadpole.
Theanterior cornua(fig. 29, B, 2) form a pair of long slender cartilaginous rods which project from the body of the hyoid at first forwards, then backwards, and finally upwards and somewhat forwards again, to be united to the auditory capsules just below the fenestrae ovales. They are formed from the dorsal portion of the hyoid arch of the tadpole and are homologous with the cerato-hyals of the Dogfish.
Theposterior cornuaform a pair of straight bony rods diverging outwards from the posterior end of the basilingual plate. They are formed from the fourth branchial arches of the tadpole, and differ from the rest of the hyoid apparatus in being well ossified.
Thecolumellar chain, which has been already described (p. 157), should be mentioned with the hyoid as it is homologous to the hyomandibular of fishes.
Thesternumof theFrog, though regarded as part of the axial skeleton, is so intimately connected with the pectoral girdle, that it will be described with the appendicular skeleton.
2.The Appendicular Skeleton.
This consists of the skeleton of the two pairs of limbs and their respective girdles. It is at first entirely cartilaginous but the cartilage becomes later on mainly replaced by bone. The only bone developed in connection with the appendicular skeleton, which has no cartilaginous predecessor, is theclavicle.
A.The Pectoral girdle.
This consists originally of two half rings of cartilage encircling the sides of the body a short way behind the head. These two halves meet one another in the ventral middle line, and separate the anterior elements of the sternum from the posterior ones.
Each half-ring bears on the middle of its outer and posterior surface a prominent cup, theglenoid cavity, with which the proximal arm-bone articulates. This cup divides the half-arch into a dorsalscapularand a ventralcoracoidportion.
Thescapular portionconsists of two parts, thesuprascapulaand thescapula.
Thesuprascapula(fig. 30, A, 2) is a wide, thin plate attached by its ventral and narrowest border to the scapula. Its proximal and anterior half is imperfectly ossified, its whole border or sometimes only its dorsal and posterior borders consist of unaltered hyaline cartilage, while the rest of it is composed of calcified cartilage. Thescapula(fig. 30, A, 3) is a fairlystout rod of bone constricted in the middle, and forming the dorsal half of the glenoid cavity.
Thecoracoid portionconsists of three parts, thecoracoid,precoracoidandclavicle.
The largest and most posterior of these is thecoracoid(fig. 30, A, 4) which like the scapula, is contracted in the middle and expanded at the ends, especially at the ventral end. It forms a large part of the glenoid cavity. The ventral ends of the coracoids which meet one another in the middle line are unossified, and form narrow strips of calcified cartilage, theepicoracoids(fig. 30, A, 5); these are often regarded as sternal elements.
Theprecoracoidforms a narrow strip of cartilage lying in front of the coracoid, from which it is separated by the widecoracoid foramen(fig. 30, A, 9). The dorsal end is continuous with an area of unossified cartilage which separates the coracoid and scapula and forms part of the glenoid cavity.
Theclavicleis a narrow membrane bone closely attached to the anterior surface of the precoracoid, its dorsal end is expanded.
The Sternum.
The sternum consists of four parts arranged in two groups; two parts to each group. The anterior members are the episternum and omosternum.
Theepisternum(fig. 30, A, 10) is a thin almost circular plate of cartilage much of which remains hyaline.
Theomosternum(fig. 30, A, 11) is a slender bony rod widest at its posterior end; it connects the episternum with the ventral ends of the precoracoids.
Thesternum properis a short rod of cartilage sheathed in bone; it is contracted in the middle and expanded at each end. It bears attached to its posterior end a broad somewhat bilobed plate of partially calcified cartilage, thexiphisternum(fig. 30, A, 13).
B.The Anterior limb.
This is divisible into three parts, theupper armorbrachium, thefore-armorantibrachium, and themanus.
All the larger bones have their ends formed by prominent epiphyses which do not unite with the shaft till late in life. Their articulating surfaces are covered by hyaline cartilage.
In theupper armthere is a single bone, thehumerus.
This has a more or less cylindrical shaft and articulates by a prominent roundedheadwith the glenoid cavity. The distal end shows a large rounded swelling on either side of which is acondylar ridge, the inner or postaxial one being the larger. A prominentdeltoid ridgeruns along the proximal half of the anterior surface, and in the male frog a second equally prominent ridge runs along the distal half of the posterior surface.
Thefore-armconsists of two bones, theradiusandulna, united together and forming theradio-ulna. The two bones are quite fused at their proximal ends where they form a deep cup which articulates with the distal end of the humerus, and is drawn out into a rather prominent backwardly-projectingolecranon process, which ossifies from a centre distinct from that of the shaft. The distal end is distinctly divided by a groove into an anterior radial and a posterior ulnar portion.
Themanusconsists of two parts, thewristorcarpusand thehand.
Thecarpus[57]consists of six small bones arranged in two rows. The three bones of the proximal row are theulnare,radialeandcentrale. Theulnareandradialeare about equal in size and articulate regularly with the radio-ulna. Thecentraleis pushed out of its normal position and lies partly on the pre-axial side, partly in front of the radiale. Of the three bones of the distal row the two pre-axial ones,carpalia 1and2, are small; carpale 2 articulates with the second metacarpal,carpale 1 with both the first and second. The third bone is large and articulates with the third, fourth and fifth metacarpals, it representscarpalia 3-5, with probably in addition the representative of a second centrale.
Thehandconsists of four complete digits, and a vestigialpollexreduced to a short metacarpal.
Each of the four complete digits consists of ametacarpaland a variable number ofphalanges. The first digit, as just mentioned, has no phalanges, the second and third have two, and the fourth and fifth have three.
C.The Pelvic girdle.
The pelvic girdle of the Frog is much modified from the simple or general type found in the Newt (p. 149).
It is a V-shaped structure consisting of two halves which are fused together in the middle line posteriorly, while in front they are attached to the ends of the transverse processes of the sacral vertebra. Each half bears at its posterior end a deep cup, theacetabulum, with which the head of the femur articulates.
Each half of the pelvis ossifies from two centres. The anterior and upper half of the acetabulum, and the long laterally compressed bar extending forwards to meet the sacral vertebra ossify from a single centre and are generally called theilium; it is probable however that they represent both theiliumandpubisof mammals[58]. The posterior part of this bone meets its fellow in a median symphysis.
The posterior third of the acetabulum is formed by a small bone, theischium, which likewise meets its fellow in a median symphysis.
The ventral portion of the pelvic girdle never ossifies, even in old animals being formed only of calcified cartilage. This is generally regarded as the pubis, but it perhaps corresponds to theacetabular boneof mammals.
D.The Posterior limb.
This corresponds closely to the anterior limb and, like it, is divisible into three parts, thethigh, theshinorcrusand thepes.
As was the case with the anterior limb, all the long bones have their ends formed by prominent epiphyses which do not unite with the shaft till late in life.
In thethighthere is only a single bone, thefemur.
Thefemuris a moderately long, slender bone with a well-ossified hollow shaft slightly curved in a sigmoid manner. Both ends are expanded, the proximal end is hemispherical and articulates with the acetabulum, the distal end is larger and more laterally expanded.
Theshinlikewise includes a single bone, thetibio-fibula, but this, as can be readily seen by the grooves at the proximal and distal ends of the shaft, is formed by the fusion of two distinct bones, thetibiaandfibula. The tibio-fibula is longer and straighter than the femur.
Thepesconsists of two parts, theankleortarsusand thefoot.
Thetarsusconsists of two rows of structures, very different in size. The proximal row consists of two long bones, thetibialeandfibulare, which are united by common epiphyses at the two ends, while in the middle they are widely separated. The tibiale lies on the tibial or pre-axial side, and the fibulare which is the larger of the two bones on the fibular or postaxial side. The distal row of tarsals consists of three very small pieces of calcified cartilage. The postaxial of these is the largest, it articulates with the second and third metatarsals and is probably homologous with tarsalia 2 and 3 fused. The middle one is very small, it articulates with the first metatarsal and is probably tarsale 1. The pre-axial one articulates with the metatarsal of the calcar, a structure to be described immediately, and has been regarded as acentrale.
Thefootincludes five complete digits and a supplemental toe as well. Each of the five digits consists of a longmetatarsalwith epiphyses at both ends, and of a variable number of phalanges. The first digit orhalluxand the second have two phalanges, the third three, the fourth, which is the largest, four, and the fifth, three. The distal phalanges have epiphyses only at their proximal ends, the others at both ends.
On the pre-axial side of the hallux is the supplemental digit, theprehalluxorcalcar. It consists of a short metatarsal and one or two phalanges, and is terminated distally by a horny covering of epidermal origin.
EXOSKELETON.
The exoskeleton, at any rate in most living forms, is very slightly developed in Amphibia. The only representatives of the epidermal exoskeleton are (1) the minute horny beaks found coating the premaxillae and dentaries inSirenand the tadpoles of most Anura, (2) the nails borne by the first three digits of the pes inXenopusand by the Japanese SalamanderOnychodactylus, (3) the horny covering of the calcar or prehallux of frogs. The Urodela and nearly all the Anura, which form the vast majority of living Amphibia, have naked skins. A few Anura belonging to the generaCeratophrysandBrachycephalushave bony dermal plates developed in the skin of the back, and these plates become united with some of the underlying vertebrae.
In the Gymnophiona the integument bears small cycloid scales arranged in rings which are equal in number to the vertebrae. These scales contain calcareous concretions. Scales also occur between the successive rings.
In the Labyrinthodontia the dermal exoskeleton is in many genera greatly developed. It is generally limited to the ventral surface and consists principally of a buckler formed of three bony plates, one median and two lateral. Theseplates protect the anterior part of the thorax, and are closely connected with the adjacent endoskeleton. They probably represent the interclavicle and clavicles. Behind this buckler numerous scutes are generally developed, which often cover the whole ventral surface, and may cover the whole body.
Teeth[59].
In Amphibia teeth are generally present on the maxillae, premaxillae and vomers, and except in Anura on the dentaries; sometimes they occur on the palatines as in many Urodela, most Labyrinthodontia, and the Gymnophiona; less commonly on the pterygoids as inMenobranchus,Siredon, some Labyrinthodontia, andPelobates cultripes[60], or on the splenials as inSirenandMenobranchus, or parasphenoid as inPelobates cultripes,Spelerpes belliandBatrachoseps. In some Anura such asBufoandPipathe jaws are toothless.
In Gymnophiona,Menobranchus, andSiredon, the teeth are arranged in two concentric curved rows. The teeth of the outer row are borne on the premaxillae and maxillae if present, (the maxillae are absent inMenobranchus), the teeth of the second row on the vomers and pterygoids inMenobranchusandSiredon, and on the vomers and palatines in Gymnophiona. In some Gymnophiona there is a double row of mandibular teeth. The vomerine, palatine and parasphenoid teeth of all forms are numerous and are not arranged in rows.
The teeth of all living Amphibia are simple conical structures ankylosed to the bone, and consisting of dentine, coated or capped with a thin layer of enamel. In the Labyrinthodontia teeth of more than one size are sometimes present. The dentine of the basal part of the larger teeth is in some genera very greatly folded, causing the structure to be highly complicated. These folds, the intervals between which are filled with cement,radiate inwards from the exterior and outwards from the large pulp cavity. The basal part of the teeth ofCeratophrys(Anura) has a similar structure.
ENDOSKELETON.
Vertebral column.
Four regions of the vertebral column can generally be recognised in Amphibia, viz. the cervical, the trunk or thoraco-lumbar, the sacral and the caudal regions. In the limbless Gymnophiona, however, only three regions, the cervical, thoracic, and post-thoracic can be made out. The cervical region is limited to a single vertebra which generally differs from the others in having no transverse processes or indication of ribs. It is generally called the atlas, but it commonly bears a small process arising from the anterior face of the centrum which resembles the odontoid process of higher animals, and renders it probable that the first vertebra of Amphibia corresponds to the axis, not to the atlas. Amphibia generally have a single sacral vertebra.
Three elements go to make up the vertebral column in Amphibia, viz.
1. the notochord,
2. the long vertebral centra,
3. intervertebral cartilage which forms the joints between successive centra.
The relations which these three elements bear to one another are subject to much variation. The successive stages can be well traced in the Urodela.
1. The first stage is found in larval Urodeles in general and in adult Ichthyoidea, and some Salamandrina. In these forms the notochord persists and retains approximately the same diameter throughout the whole length of the vertebral column. Bony biconcave centra are present and constrict itto a certain extent vertebrally, while intervertebrally there is a development of cartilage. The connection between the bony vertebrae is effected mainly by the expanded notochord.
2. In the next stage, as seen inGyrinophilus porphyriticus, the growth of intervertebral cartilage has caused the almost complete obliteration of the notochord intervertebrally, and its entire disappearance vertebrally, i.e. in the centre of each vertebra. The intervertebral cartilage now forms the main connection between successive vertebrae, and sometimes cases are found whose condition approaches that of definite articulations. Readily recognisable remains of the notochord are still found at each end of the intervertebral constriction.
3. In the third stage differentiation and absorption of the intervertebral cartilage has given rise to definitely articulating opisthocoelous vertebrae. These are found in most adult Salamandrina.
The transverse processes of the earlier trunk vertebrae are divided into two parts, a dorsal part which meets the tubercular process of the rib and is derived from the neural arch, and a ventral part which meets the capitular process of the rib, and is derived from the ventral or haemal arch. In the caudal vertebrae and often also in the posterior trunk vertebrae the two processes are fused.
SirenandProteus, although they possess minute posterior limbs, have no sacral vertebrae, whileCryptobranchus lateralishas two. The caudal vertebrae, except the first, have haemal arches very similar to the neural arches.
In Labyrinthodontia the centra of the vertebrae are generally well ossified biconcave discs. In some forms however, likeEuchirosaurus, the centra are imperfectly ossified, and consist of bony rings traversed by a wide notochordal canal. Each ring is formed of four pieces, a large well-ossified neural arch, a basal piece, and a pair of lateral pieces. Vertebrae of this type are calledrachitomous.
In the tail region of other forms each vertebra consists of an anterior centrum bearing the neural arch, and a posterior intercentrum[61]bearing chevron bones. Vertebrae of this type are calledembolomerous. Haemal arches similar to the neural arches are often found as in Urodela. The transverse processes are sometimes well developed and are divided into tubercular and capitular portions.
In Gymnophiona the vertebrae are biconcave and are very numerous, they sometimes number about two hundred and thirty. Only quite the last few are ribless and so can be regarded as post-thoracic vertebrae. The first vertebra has nothing of the nature of an odontoid process.
In Anura the number of vertebrae is very greatly reduced, only nine and the urostyle being present. Of these, eight are presacral and one sacral. The urostyle is post-sacral and corresponds to three or more modified vertebrae. The first vertebra is without transverse processes, the remaining presacral vertebrae have the transverse processes fairly large, while the sacral vertebra has them very large, forming in some genera widely expanded plates. The urostyle is a long cylindrical rod which articulates with the sacrum generally by two facets. Ankylosed to its anterior end are the remains of two neural arches.
In Anura remains of the notochord are found in the centre of each vertebra, i.e. vertebrally, while in the Urodela they only occur intervertebrally.
The vertebrae in Anura are, as a rule, procoelous. The eighth vertebra is however generally amphicoelous, while the ninth commonly has one convexity in front, and two behind.
In some forms such asBombinator,Pipa,DiscoglossusandAlytesthey are opisthocoelous; in others likePelobatesthey are variable.
The Skull[62].
Cranium and mandible.
In the Amphibian skull there are as a rule far fewer bones than in the skull of bony fish. The primordial cartilaginous cranium often persists to a great extent. Only quite a few ossifications take place in it; namely in the occipital region—the exoccipitals, further forwards—the pro-otics, and at the boundary of the orbital and ethmoidal regions—the sphenethmoid. The basi-occipital and basisphenoid are never ossified. As in Mammalia there are two occipital condyles formed by the exoccipitals.
Large vacuities commonly occur in the cartilage of both floor and roof of the primordial cranium. These are roofed over to a greater or less extent by the development of membrane bone. Thus on the roof of the cranium there are paired parietals, frontals, and nasals, and on its floor are paired vomers, and a median unpaired parasphenoid.
In all living forms the parietals meet and there is no interparietal foramen, though this exists in Labyrinthodonts.
The palato-pterygo-quadrate bar is united at each end with the cranium, but elsewhere in most cases forms a wide arch standing away from it. The suspensorium is, as in Dipnoi and Holocephali, autostylic. The palato-pterygo-quadrate bar sometimes remains entirely cartilaginous, sometimes its posterior half is ossified forming the quadrate. In connection with it a number of membrane bones are generally developed, viz. the maxillae, premaxillae, palatines, pterygoids, quadratojugals, and squamosals. The pterygoids are, however, sometimes partially formed by the ossification of cartilage. Thecartilage of the lower jaw and its investing membrane bones generally have much the same relations as in bony fishes.
Urodela.The skulls of the various Urodeles show an interesting series of modifications and differ much from one another, but all agree in the absence of the quadratojugals, in the fact that the palatines lie parallel to the axis of the cranium, and in the large size of the parasphenoid.
The lower typesMenobranchus,Siren,Proteus, andAmphiumahave longer and narrower skulls than do the higher types.
Menobranchushas a very low type of skull which remains throughout life in much the same condition as that of a young tadpole or larval salamander. The roof and floor of the cranium internal to the membrane bones are formed of fibrous tissue, not of well-developed cartilage. The epi-otic regions of the skull are ossified, forming a pair of large bones which lie external to, and distinct from, the exoccipitals.Proteusand the Labyrinthodonts are the only other Amphibia which have these elements separately ossified. The parietals send a pair of long processes forwards along the sides of the frontals. Nasals and maxillae are absent, as is likewise the case inProteus. Teeth are borne on the vomers, premaxillae, pterygoids, dentaries and angulo-splenials. The suspensorium is forwardly directed.
The skull ofSirenresembles that ofMenobranchusin several respects, as in the forward direction of the suspensorium and in the absence of maxillae, but differs in the possession of nasals, in the toothless condition of the premaxillae and dentaries, and in the fusion and dentigerous condition of the vomers and palatines.
Amphiumahas a skull which, though narrow and elongated, differs from those ofMenobranchus,Proteus, andSiren, and resembles those of higher types in the following respects:—
(1) the suspensorium projects nearly at right angles tothe cranium instead of being directed forwards, (2) the maxillae are well developed, and the premaxillae are completely ankylosed together, (3) there are no palatines.
The skulls ofMegalobatrachus,CryptobranchusandSiredonresemble those of the highest Urodeles the Salamanders in their wide form, in having the pro-otics distinct from the exoccipitals which are ossified continuously with the epi-otics and opisthotics, and in having no palatines, but differ in having the two premaxillae separate, and in the arrangement of the vomerine teeth which inMegalobatrachusandCryptobranchusare placed along the anterior boundaries of the bones, these meeting in the middle line. InSiredonthe vomers are separated by the very large parasphenoid.
The suspensorium inMegalobatrachusandCryptobranchusprojects at right angles to the cranium; inSiredonit projects somewhat downwards and forwards as in the Salamandrina.
Modifications of the vomers, pterygoids and palatines accompany the changes of the larval ichthyoidSiredoninto the adult salamandroidAmblystoma, the vomers especially come to resemble to a much greater extent those of the Salamandrina.
The ossification of the skull in the Salamandrina is carried further than in the Ichthyoidea, though the supra-occipital and basi-occipital are not ossified. The skull differs from that in the Ichthyoidea in the size of the part of the vomero-palatines which lies in front of the teeth, in the frequent union of the two premaxillae and in the ossification of all the periotic bones continuously with the exoccipital.
The skull differs from that of Anura in the following respects:—
(1) the bones of the upper jaw do not form a complete arch standing away from the cranium, and the maxillae are not united to the quadrates by quadratojugals, (2) the long axis of the suspensorium passes obliquely downwards and forwards instead of downwards and backwards, (3) there is nosphenethmoid encircling the anterior end of the brain, its place being partly taken by a pair of orbitosphenoids, (4) there is no definite tympanic cavity.