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6. CHAPTER VI.
THE SKELETON OF THE FROG

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It may cause surprise to speak of the skin of the common Frog as part of its skeleton, consisting as the skin does of small membranous structures only.

The term "skeleton," however, should properly include all the membranous and gristly, as well as the bony structures. [14]

Moreover, more or less of the skin may attain to so solid a condition as to justify its comprehension under the name "skeleton," even in the popular signification of that term.

The skin of Vertebrate animals consists of two Layers: an outer layer (the epidermis or ecteron), and an inner layer (the dermis or enderon). The epidermis, and any projections or processes developed from it when they take on a dense or hardened structure, become horny. Of such horny nature are hairs, feathers, nails, and scales; they are more or less epidermal appendages. The dermis when hardened becomes bony, and of such nature are the bony skin-plates or "scutes" and teeth. They are dermal

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appendages. Now both layers of the skin of the common Frog are entirely soft and utterly destitute
Fig. 26.—Dorsal surface of the Carapace of a Freshwater Tortoise (Emys). 1-8, expanded neutral spines; r¹-r8, expanded ribs; nu, first median (or nuchal) plate; py, last median (or pygal) plate; m, marginal scutes. The dark lines indicate the limits of the plates of the horny epidermal tortoise-shell; the thin sutures indicate the lines at the junction of the bony scutes.
Fig. 27.— Dactylethra capensis.
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of any of these appendages. Allied forms, however, present us with examples of some interesting epidermal conditions. Thus in old male Toads, in Dactylethra and in one of the Japanese efts, the Epidermis of some of the finger-tips becomes hardened and horny, in other words we begin to meet with incipient "nails". "Incipient" because, in ascending from the lowest vertebrates, "nails" are first met with in the Frog's class, and these only very rarely and in an imperfectly developed condition.

As has been mentioned, in two kinds of Frog (Ceratophrys and Ephippifer) the skin of the back is furnished with bony plates. These are found in the deeper layer or dermis, and are therefore "scutes."

Fig. 28.—Diagram of a vertical section of both Carapace and Plastron of a Tortoise, made transversely to the long axis of the skeleton. c, vertebral centrum; ns, neural spine which expands above into a median dorsal scute; r, rib which forms one mass with a lateral scute and terminates at a marginal plate, ic, interclavicular scute; hp, hyo-sternal scute.

The remarkable circumstance, however, is that we have here a lower stage (as if it were an incipient condition) of that more developed dermal skeleton which exists in tortoises and turtles. In most of these reptiles both the back and the belly are protected by bony plates which adjoin one another, and together form a solid box in which the body is enclosed.

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Moreover the bony plates of tortoises and turtles are invested by large horny epidermal scales ("tortoise-shell") which scales do not agree in either size or number with the bony plates on which they are superimposed (fig. 26).

Again, the middle series of bony plates of the back are continuous with the subjacent joints of the back-bones, and the lateral series of dorsal plates are continuous with the ribs beneath them (fig. 28 ).

Fig. 29.—A Mud-tortoise (Trionyx), showing the dorsal plates.

There are certain Chelonians, however—"mud-tortoises"—(of the genus Trionyx), which have the dorsal plates much less developed and not connected

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with the ventral plates save by means of soft structures. In these latter Chelonians then we have in reptiles an interesting approximation to the condition we have seen to exist in those exceptional Anourans, Ceratophrys and Ephippifer. Moreover this resemblance is still further increased by the fact that in Trionyx the bony plates are not covered with any tortoise-shell, but are merely invested by soft skin as in the genera of dorsally-shielded Batrachians.

Have we then here a true sign of genetic affinity? Are these tortoises to be deemed the more specially modified, descendants of shielded frogs or of some, as yet unknown, slightly-shielded animals which were the common ancestors both of frogs and tortoises?

Certainly tortoises cannot be the direct descendants of frogs, they agree with all reptiles in characters which are both too numerous and too important to allow such an opinion to be entertained for a moment.

The other opinion is hardly less untenable; for if all the multitudinous species of frogs (together with a number of reptilian forms more closely allied to the tortoise than any frogs are) descended from slightly shielded animals, how comes it that all frogs and toads, save one or two species in no other way peculiar, have every one of them lost every trace of such shielded structure, which nevertheless cannot easily be conceived to have been in any way prejudicial to their existence and survival?

On the other band, it cannot but strike us with surprise that structures so similar—extending even to

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the continuity of the dorsal plates with the subjacent joints of the backbone—should have arisen twice in nature spontaneously. Here we seem to have a remarkable example of the independent origin of closely similar structures; and if so, what caution is not necessary before concluding that any given similarity of structure are undoubted marks of genetic affinity!

The skin of the frog is also interesting from a physiological point of view. Our own skin is by no means popularly credited with the great importance really due to it. "Only the skin!" is an exclamation not unfrequently heard, and wonder is very often felt when death supervenes after a burn which has injured but a comparatively small surface of the body. Yet our skin is really one of our most important organs, and is able to supplement, and to a very slight extent even to replace, the respective actions of the kidneys, the liver, and the lungs. [15]

In the frog we have this cutaneous activity developed in a much higher degree. Not only does its perspiratory action take place to such an extreme degree that a frog tied where it cannot escape the rays of a summer's sun speedily dies—nay, more, is soon perfectly dried up—but, its respiratory action is both constant and important. This has been experimentally demonstrated by the detection of the carbonic acid given out in water by a frog over the head of which a bladder had been so tightly tied as to prevent the possibility of the escape of any exhalation from the lungs. The fact of cutaneous respiration

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has also been proved by the experiment of confining frogs in cages under water for more than two months and a half, and by the cutting out of the lungs, the creature continuing to live without them for forty days. Indeed it is now certain that the skin is so important an agent in the frog's breathing that the lungs do not suffice for the maintenance of life without its aid.

It is no less true that in Batrachians which breathe by means of permanent gills—as, e.g. the Axolotl—

Fig. 30.—Backbone of the Frog (dorsal aspect)
Fig. 31—Backbone of the Frog (ventral aspect).
such gills are not necessary to life, as the late M. Aug. Duméril and Dr. Günther have established by cutting
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them away without inducing any apparent injurious effects. In the whole class of Batrachians skin respiration seems, then, to be of very great importance.

The internal skeleton (or the skeleton commonly so called) of the frog presents some points of considerable interest, especially as exhibiting its intermediate position between fishes on the one hand, and higher vertebrates on the other. First, as regards the backbone, it may be remembered that it is made up of distinct bony joints (or vertebræ), in which it agrees with all animals above fishes and with bony fishes; its hinder termination, however, is essentially fish-like.

Fig. 32.—Coccyx of Frog, lateral view, a black line indicates the course of the sciatic nerve.

It is fish-like because the terminal piece, as it is called, or "coccyx" (unlike the coccyx in man or in birds) is not composed of rudimentary vertebræ which subsequently blend and anchylose together, but is formed by the ossification continuously of the membrane investing (or sheath of) the hindermost part of that primitive continuous rod, or notochord, [16] which, as has been said, precedes, in all vertebrate animals, the development of the backbone, making its appearance beneath the primitive groove.

The vertebræ are shaped like rings, and enclose within their circuit the spinal marrow upon which, as it were, these rings are strung. From the side of each

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ring (except at the two ends of the backbone) there juts out a bony prominence called a "transverse process," and to a certain number of these a bony "rib" is in most vertebrate animals attached (though there are none in the frog), often extending round to join the breast-bone in front, and being capable of more or less motion, so as (by their simultaneous movement) to be able to enlarge or to contract the cavity of the chest, which they thus enclose and protect.

That part of each vertebra which is placed next the body cavity is generally the thickest part, and is called the "body," or "centrum." The series of

Fig. 33—The Axis Vertebra of Man. c, centrum; s, neural spine; d, tubercular process; p, capitular process; a, anterior articular surface for atlas; z», post-zygapophysis; o, odontoid process; hy, median vertical ridge beneath centrum.
bodies (or centra) occupy the position which was at first filled by the primitive notochord, the rest of the vertebral rings having been formed in the sides and roof of the canal formed by the upgrowth and union of the two sides of the primitive groove of the embryo.

The Frog order is distinguished amongst vertebrates as that which has the absolutely smallest number of joints in the backbone. In the frog there are but nine in the front of the coccyx. In the Pipa toad

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there are but seven, the eighth vertebra (to the transverse processes of which the haunch bones are attached) having become solidly joined in one bone with the coccyx.

In all higher vertebrates, i.e. in all beasts, birds, and reptiles, the head is supported on an especially ring-like vertebra which—because it so supports—is called the atlas, and this (in almost all) can turn upon a peculiar vertebra termed (from this circumstance)

Fig. 34.—The: Atlas Vertebra of Man. s, rudiment of neural spine; d. tubercular process; p, capitular process; a, articular surface for skull; hy, plate of bone holding the place of a cranium, and articulating with the odontoid process of the axis vertebra.
the axis, and provided with a toothlike (odontoid) [17] process, round which, as round a pivot, the "atlas" works. Nothing of the kind exists in any fish.

Fig. 35.—Lateral, Dorsal, and Ventral view of first Vertebra of Amphiuma.

In the frog (and in all its class) we find but a single vertebra representing these two, but in some allied forms, e.g. in Amphiuma, this vertebra develops a

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median process, reminding us of the odontoid process of the axis.

The frog, as has been said, has no ribs, in spite of the long "transverse processes" which project out on each side of the backbone. Ribs are not necessary to it, for it could apply them to none of the purposes to which ribs are ever applied.

In all beasts ribs aid importantly in respiration, serving by their motions alternately to inflate or empty the lungs by enlarging or contracting the cavity of the chest in the way before mentioned. The frog, however, breathes exclusively, as regards the lungs, by swallowing air by a mechanism which will be described shortly.

In serpents the ribs are the organs of locomotion, as also in the Flying Dragon before referred to; but in frogs locomotion is effected exclusively by the limbs. In the very aberrant species, Pipa and Dactylethra, there are on each side of the anterior parts of the body two enormously long transverse processes, each process bearing at its extremity a short flattened, straight osseous or cartilaginous rib. These little ribs can, however, take no part in such functions as those just referred to.

Ribs, moreover, are found in the other existing orders of the frog's class, i.e. both in the Urodela and Ophiomorpha. In none, however, do they join a breast-bone, or sternum, another character in which Batrachians agree with fishes, though they differ from fishes in that they have a sternum at all. In ascending from fishes through the Vertebrate sub-kingdom, a sternum first appears in the class Batrachia.

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In a certain North African Salamander named Pleurodeles the ribs are not only elongated, but their apices, if they do not actually perforate the skin, are so prominent as to seem to do so, when the finger is drawn from behind forwards along the side of the animal's body.

The several joints of the backbone are connected together by surfaces which are not the same on both the anterior and posterior sides of the centrum, or body, of the same vertebra. Each of the first seven vertebræ is furnished with a round prominence, or head, on the hinder side of its centrum, and each of the precoccygeal vertebra, except the first and last, has the anterior surface of its centrum excavated as a cup for the reception of the ball of the hinder surface of the vertebra next in front. The first vertebra has in front two concavities, side by side, to articulate with the skull. The eighth vertebra has a concavity at each end of its "body." The ninth vertebra has a body provided with a single convexity in front and a double convexity behind, to articulate with the concavities placed side by side on the front end of the coccyx. These arrangements are not constant in the frog's order, still less in its class. In Bombinater and Pipa the vertebra are concave behind each centrum, instead of in front: and the same is the case in Salamandra. In many tailed Batrachians the vertebræ are biconcave, as e.g. in Spelerpes, Amphiuma, Proteus, and Siren.

The biconcave shape is an approximation towards the condition which is almost universal in bony fishes,

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though not quite universal, since the bony pike (Lepidosteus) has a ball at one end of each vertebra and a cup at the other. Moreover, even in some reptiles (e.g. the lizards called Geekoes) the vertebræ are biconcave, and the same was the case with the majority of those species of crocodiles the remains of which are found in strata older than the chalk, and even in existing crocodiles the first vertebra of the tail is biconcave.

Vertebræ with a cavity in front of the centrum and a ball behind it are found in the crocodiles now living as well as in the frog, while vertebræ with a ball in front and a concavity behind are found even amongst beasts as in the joints of the neck of Ruminants, e.g. the sheep. Thus though the vertebræ of the frog's class exhibit no very decided signs of affinity, they show more resemblance to those of fishes than to those of any other non-batrachian class.

The transverse processes of the ninth or last vertebra in front of the coccyx, articulate with the haunch bones, but are not very remarkable in shape. In some frogs and toads the transverse processes of this vertebra become enormously expanded, and the expanded or non-expanded condition of this part is

Fig. 36.—Anterior aspect of Coccyx, showing the double articular concavities placed side by side beneath the neural arch.
a character made use of in zoological classification. The coccyx is made up mainly, as has been said, of
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a continuous ossification of the sheath of the notochord, and never consists of distinct vertebra. Nevertheless, the small bony arches which are at first distinct coalesce with it. These arches are called "neural" because they arch over the hinder part of the spinal marrow. The great nerve of the leg (the sciatic nerve) proceeds outwards on each side through a foramen situated at the anterior end of the coccyx from the spinal marrow—the spinal marrow being that structure which gives origin to the great mass of the nerves pervading the entire frame (fig. 32).

The skull of the frog presents numerous points of interest, but only four of these can be here referred to, as other matters demand our attention.

The first of these four relates to its mode of articulation with the vertebral column. As has been said, the first vertebra presents a pair of concavities to the skull. The skull develops from its hinder (or occipital) region a corresponding pair of articular convexities or "condyles." Now in this matter the frog differs from both birds and reptiles, every member of those classes possessing a single median (occipital) condyle for articulation with the vertebral column.

Yet every member of the frog class, not only every toad and newt, but also every species of the Ophiomorpha, and even every one of the long extinct Labyrinthodons (with the doubtful exception of the probably immature and larval Archegosaurus) has a similar pair of occipital condyles. The interesting matter is that man and all beasts have also two occipital condyles. Is this then a mark of affinity,

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and can we, as it were, reach beasts by a short cut through Batrachians, leaving all the reptiles and birds on one side, as a special outstanding and diverging developement?

We shall presently see that other yet more striking facts may be brought forward in support of the latter view. Nevertheless it must be remembered that there are fishes, though very few and exceptional, which also possess a pair of occipital condyles, and that in one respect most fishes are more like mammals than are any Batrachians, since they, like mammals, have a well-ossified median bone at the base of the skull in the occipital region, a structure which all Batrachians, without a single exception, are destitute of.

The second point of interest concerns the lower part, or base, of the skull, which exhibits a striking agreement with the same part as developed in bony fishes.

This agreement consists in the fact that the middle of the floor of the skull is not formed as in all beasts, birds, and reptiles, by a deposition of bony substance in pre-existing gristle (ossification of cartilage), to which the name Basi-sphenoid is applied, but, as in bony fishes, by a great bone called Parasphenoid, which shoots forwards and also extends backwards to the hinder end of the skull floor, but is formed by the decomposition of bony substance in pre-existing membrane.

Although this great membrane bone is constant in Batrachians and bony fishes, and is represented, if at all, only by minute rudiments in higher vertebrates; nevertheless in serpents we once more meet with a far-reaching and well-developed parasphenoid.

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Yet it can hardly be conceived that serpents have carried off from Piscine ancestors and carefully

Fig. 37—Upper Surface of the Skull of a Frog (after Parker). e, os en ceinture, or girdle-bone; eo, exoccipital; f, frontal part of frontoparietal bone; mx, maxillary bone; n, nasal; op, opisthotic; p, parietal part of fronto-parietal bone; pm, pre-maxilla; po, pro-otic; pt, pterygoid; q, quadrato-jugal; sq, squamosal; sus, suspensorium of lower jaw.
Fig. 38—Under surface of the Skull of a Frog (after Parker). e, girdle-bone; eo, exoccipital; mx, maxilla; par, parasphenoid; pm, pre-maxilla; po, pro-otic; pt, pterygoid; q, quadrato-jugal; sus, suspensorium of lower jaw, the lower end of which represents the quadrate bone; v, vomer; 1, optic foramen; 2, foramen ovale; 3, condyloid foramen.
preserved this peculiarity of structure which all their other class fellows have lost. It seems much more
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probable that this structure has independently appeared through the action of peculiar conditions, and hence that we have here again a remarkable instance of the independent origin of similar structures.

The third peculiarity of the frog's skull consists in the form and conditions of the bony supports of the tongue.

It would not be easy to find a better example of the need of widely extended observations in order duly to understand structures apparently very simple indeed. The bone of the tongue in man—the os hyoides [18]—is a small structure, and one to all appearance of little significance It is placed at the root of the tongue and above the larynx, and consists of a body with a pair of processes on each side, one large (the posterior or great cornu), and one small. (the anterior or lesser cornu, or corniculum).

Even in man's own class (mammalia) the relative development of the parts may vary greatly. Thus the cornicula may be large and may each be represented by two or three distinct applications as in the dog and horse.

The cornua also may take on. a development very much greater than that existing in man as is the case in some other Mammals. These facts may prepare us to expect much greater divergences in lower forms; and yet throughout the two great classes of birds and reptiles (as well as beasts)—though varying conditions as to the proportions of the parts present themselves

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—the os hyoides continues essentially the same in structure, and even in the adult frog this bone exhibits nothing but a rather wide "body" with two long and slender "cornicula" and a pair of shorter "cornua."

Fig. 39.—Of the Larynx of Man, the thyroid cartilage being supposed to be transparent, and allowing the right arytenoid cartilage (Ar), vocal ligament (V), and thyro-arytenoid muscle (Th A), the upper part of the cricoid cartilage (Cr), and the attachment of the epiglottis (Ep), to be seen. C th, the right cricothyroid muscle; Tr, the trachea; Hy, the body of the hyoid bone. The right lesser cornu appears as a very small process, extending upwards and backwards from the body of the hyoid behind the letters Hy, and in front of the Epiglottis. The right, great cornu is shown extending backwards from the body of the Hyoid and terminating beneath the letters Ep.
Fig. 40—Extracranial portion of hyoidean apparatus of Dog, front views sh, stylohyal; eh, epihyal; ch, ceratohyal (these three constitute the "anterior cornu"); bh, basihyal, or "body" of hyoid; th, thyrohyal, or "posterior cornu." (From Flower's "Osteology.")

Let us now pass for a moment to the other end of the Vertebrate sub-kingdom. We find in fishes a complex framework for the support of the gills, or structures, by which they effect their aquatic respiration. This framework consists of a number of arches (placed in series one behind another) extending on

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each side of the throat upwards towards the back-bone, and supporting on their outer sides the gills or branchia, on which account they are called the branchial arches. In front of these arches and forming as it were the first of the series, is an arch which ascends and becomes connected with the skull.

Fig. 41.—Skeleton of left series of Branchial Arches of a Perch, seen from above. 1, glosso-hyal; 2, 3, and 4, basi-branchials; 5, hypo-branchials; 6, cerato-branchials; 7. epi-branchials; 8, styliform pharyngo-branchials; 9, pharyngo-branchials; 6'''' inferior pharyngeal bone; 9' and 9'' superior pharyngeal bones; 5, 6, 7, and 8, first branchial arch; 5', 6', 7', and 9, second branchial arch; 5'', 6'', 7'' and 9', third branchial arch; 5'', 6''', and 7''', fourth branchial arch; 6'''', fifth branchial arch.
Fig. 42.—First three branchial Arches from the left side of a Perch. On the outer (convex) side of each branchial arch the series of closely-set gill filaments (or leaflets or lamellæ) are seen to be attached. On the inner (concave) side of the first branchial arch are the series of elongated processes (supporting minute denticles) which help to prevent particles of food, or other foreign bodies, passing from the mouth to the gill chamber.

Turning now to those Batrachians which breathe throughout their lives in the manner of fishes, we find a corresponding system of branchial arches. Thus in the Siren we find a series of gill-supporting branchial arches, placed behind another arch which is connected with the skull.

But the frog passes the first part of its life in a fishlike

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manner, and in the tadpole accordingly we find an apparatus similar to that of the Siren. There are, in fact, on each side of the throat, four branchial arches, placed behind another arch, which is connected with the skull. As development proceeds these branchial arches become gradually absorbed and all but disappear. Relics of them, however, exist even in the adult condition, and thus serve to indicate the true nature of parts which otherwise would be little understood. The central portion of the structure—that from which arches diverge on each side—increases in relative as well as absolute size, and becomes the
Fig. 43.—Diagram of the changes undergone by the hyoid in a Frog in passing from the Tadpole stage to the adult condition (constructed from Parker's Memoir). Uppermost left-hand figure, the youngest condition; lowest right-hand figure, the adult. h, the hyoidean arch, ultimately the corniculum; b¹-b4, the four branchial arches which become gradually atrophied, the cornua (or thyro-hyal) th being their representative in the adult; b', another branchial rudiment; bh, the body of the hyoid.
"body" of the os hyoides. That arch on each side which is connected with the skull and is placed
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immediately in front of the branchial arches, continues to be so connected and becomes one of the two "cornicula," while the rudimentary relics of the branchial arches which persist become what we have seen in the adult as the cornua of the os hyoides.

Thus the anatomy of the tongue-bone of the frog, studied in its progressive changes, reveals to us that otherwise unsuspected relations exist in certain parts of the tongue-bone of man. It exhibits to us the cornua of his os hyoides as related to those large and complex branchial arches which play so important a part in the fish and form so relatively large a portion of its skeleton.

The fourth circumstance (the last here to be noticed) connected with the frog's skull concerns the relative position and size of certain of its enveloping bones.

When the skull of the frog is viewed from above, a large vacuity is seen to exist on each side, between the brain-case and the great arch of the upper jaw. In the hinder part of this space is situate the temporal muscle, which by its contraction pulls up the lower

Fig. 44—Dorsal view of skull of Pelobates, showing bony lamellæ behind the orbits.
jaw and closes the mouth; and the hollow in which this muscle lies is called the temporal fossa.
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In a certain frog before noticed, called Pelobates, as also in Calyptocephalus, a similar view of the skull exhibits no such great vacuity. The reason of such absence is that the temporal fossa in these animals is roofed over and enclosed by the meeting together of bony lamellæ developed from the bones surrounding it, which thus bound the orbit posteriorly, and give to the cranium an altogether false appearance of great capacity.

Fig. 45.—Lateral view of skull of Turtle (Chelonia), showing bony lamellæ behind the orbit. a, naso-præfontal; b, maxillary; c, palatine, d, basi-sphenoid; e, præmaxillary; f, frontal; g, post-orbital; h, parietal; i, jugal: k, quadrato-jugal; l, quadratum; m, squamosal; p, super-occipital; 1, dentary; 3, angular; 4, surangular; 5, articular; 6, coronoid.

This very singular structure is found to exist also in the marine turtles, amongst the Chelonians, and here we have another striking resemblance between the Chelonia and the Anoura, apparently reinforcing the argument for the existence of real affinity derived from the presence of such bony dorsal shields in both those two orders. The importance of this character might seem the more unquestionable, since no other reptiles and no birds or beasts whatever were known to exhibit a similar structure.

Quite recently, however, Prof. Alphonse Milne-Edwards has described a beast from Africa (Lophiomys) belonging to the Rodent (rat, rabbit, and squirrel)

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order, which has a skull, the temporal fossa of which is similarly enclosed by bony plates.

Fig. 46.—External form of Lophiomys.

This unexpected discovery completely destroys any weight which might be attached to this character as an evidence of genetic affinity. It does so, because it is inconceivable that this Rodent should have directly descended from a common progenitor of frogs and of Chelonians through a line of ancestors which never lost this cranial shield, though the ancestors of all other beasts, all birds, and all reptiles, except turtles, did lose it. It is inconceivable, for if it were true, a variety of the lowest mammals (Marsupials [19] and Monotremes [20] ) must have less diverged from the ancient common stock than have the members of the Rodent order, and nevertheless these lowest mammals exhibit no trace whatever of such a cranial shield. Here then we have an undoubted example of the

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independent origin of structures, so similar that at first sight their similarity might well have been deemed a conclusive evidence of affinity.

Fig. 47.—Lateral view of skull of Lophiomys, showing bony lamellæ behind the orbit.

Here, also, we have a memorable caution against hasty inferences from structural similarities. If this resemblance and that of the dorsal shields are, when taken together, no signs whatever of special genetic affinity—it is difficult to say what structural likenesses are to be deemed unquestionable evidences of a common ancestry.

Passing now to the skeleton of the limbs, we come to a character of great significance, as it is one which serves to distinguish all the limbed species of the frog's class from lower vertebrates. The character is very significant, because all Batrachians, in spite of their numerous and important fish affinities, differ from all fishes, and agree with all higher classes in that they— if they have limbs at all—have them divided into those very typical segments which exist in man; namely, shoulder-bones, arm-bones, wrist-bones, and hand-bones; and into haunch-bones, leg-bones, ankle-bones, and foot-bones respectively. It is difficult, then, to avoid the belief that in the Batrachian class we come

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upon the first appearance of vertebrate limbs, differentiated in a fashion which thenceforward becomes universal.

The bones of the wrist in the frog, again, present a nearer resemblance to those in man than do those of most reptiles, and this is still more the case in some other members of the frog's class, e.g. Salamandra

Fig. 48.—Skeleton of anterior extremity of an eft.
Fig. 49.—Skeleton of posterior extremity of the same.
and other Efts. Nevertheless, there are certain reptiles, and, strange to say, they are once more Chelonians, which agree in this resemblance—as may be seen in the hand of the tortoise— Chelydra serpentina. The bones of the fingers show, moreover, a greater likeness, in certain respects, to those of beasts than to those of reptiles. No finger has a greater number of
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joints than three, while, in some lizards, the fourth digit may have as many as five joints.

Fig. 50.—Dorsal surface of skeleton of right hand of the Tortoise, Chelydra (after Gegenbaur). c, cuneiforme: in, intermedium (or centrale); l lunare; m¹-m5, metacarpals; r, radius; s, scaphoides; u, ulna; 1—5, the five distal carpals, namely—1, trapezium; 2, trapezoides; 3, magnum; 4 and 5, divided unciforme.

In the frog the wrist-bones (called respectively the magnum and unciforme) which support the third, fourth, and the little fingers, are formed together into a single ossicle. The same condition, however, sometimes occurs even in the orang. On the other hand, the single bone which in man and beasts supports both the "ring" and the "little" fingers, may be represented by two ossicles in the frog's class (as e.g. in Salamandra, and in some reptiles (as e.g. in Chelydra).

No member of the frog's class which has an arm at all, bears less than two fingers (as in Proteus) upon it. Thus we meet with a number as small as that which is developed amongst beasts in ruminants, but not so small a number as in the horse.

In the rudimentary condition of its thumb the frog participates in a very common defect, since this member is absent in very many forms. It is so even in creatures as highly organised and as like man in bodily structure as monkeys, since both the spider-monkeys

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of America and certain long-tailed monkeys (Colobi) of Africa, are thumbless.

In man, when standing, the weight of the body is transferred to the limbs by a large bony girdle which, from its basin-like shape, is called the pelvis. This basin consists of the two haunch-bones which meet together in front, but behind are separated by the lower part of the backbone (called the sacrum), to which the haunch-bones are attached, and which forms the hinder portion of the pelvis. The pelvis has a depression, or "socket," on each side, into which fits the head of one of the thigh-bones. Each "haunch-bone" consists of three parts, which are, in

Fig. 51.—Outer side of os innominatum of Man. a, acetabulum; ai, anterior inferior spinous process of the ilium; as, anterior superior spinous process of the ilium; c, crest of the ilium: ip, i lio-pectineal eminence; o, obturator foramen; p, pubis—its horizontal ramus; posterior inferior spinous process; ps, posterior superior spinous process; s, spine of the ischium; t, tuberosity of the ischium.
man, primitively distinct, but afterwards anchylose together, and all three elements (in each haunch-bone) take a share in the formation of the bony thigh-socket or acetabulum. These three elements are named—1, ilium; 2, ischium; and 3, pubis. It is the ilium
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which is adjoined to the sacrum. The pubis, in man, meets its fellow of the opposite side in the middle line in the front of the body. The two ischia (one to each haunch-bone) support man's body when in a sitting posture.

The pelvis of man is often quoted as one of the most peculiar and characteristic parts of his skeleton, and its shape in him is very peculiar. Nevertheless the pelvis as it exists in frogs and toads is a far more exceptional structure. It is so in the extraordinary elongation, yet small vertebral attachment, of the haunch-bones (ilia), as also in the fact that these bones as well as the other pelvic elements (ischia and pubes) are all closely applied to each other in the middle line

Fig. 52—Right side of Pelvis of Frog. il, ilium; is, ischium; p, pubis. The three bones meet at the upper margin of the acetabulum.
Fig. 53.—Dorsal view of pelvis of Frog, showing the narrow ends of the ilia for attachment to the backbone, and also the close approximation of the acetabula.
of the body. Thus these elements form a bony disc, and the two sockets (acetabula) destined, respectively, for the heads of the two thigh-bones, come to be
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closely approximated one against the other. The great elongation and small attachments of the ilia allow the pelvis as a whole to be bent upon the back-bone. Thus the hinder part of the body is movable, and forms, as it were, an additional common root segment for the two limbs.

The skeleton of the ankle as developed in the frog's class presents us with some characters, which, more than even those of the wrist, suggest the passage of the line of affinity directly from Batrachians to mammals leaving both reptiles and birds on one side.

Fig. 54.—Bones of foot of Frog.—a, astragalus; c, os calcis; ac, united portions of these bones; li, extra ossicle of inner side of foot; cb, ossicle representing cuboid and other tarsal bones—1, 2, 3, 4, 5—the five metatarsals.

In the first place we meet in the frog with certain extra ossicles in the inner side of the foot, which present the appearance of a broad rudiment of an extra digit on the inner side of the great toe. Now

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we find a structure very similar in form in animals remote enough from Batrachians, yet rarely do we find such in any intermediate kinds. Thus in certain tree-porcupines the ankle is furnished in like manner—another instance of the independent origin of strikingly similar structures.

Fig. 55—Right foot of Emeu. a, astragalus; d2-d4, second, third, and fourth digits; m, metatarsals anchylosed together except at their distal ends; t, tibia, t2, distal tarsal element.
Fig. 56—Left foot of a Monitor Lizard (Varanus). f, fibula; m¹-m5, the five metatarsals, m⊃1 being that of the hallux; t, tibia; 1, astragalo-calcaneum; 2, cuboides; 3, ecto-cuneiforme.

There are other matters, however, more important than this. It has been remarked that the wrist shows an amount of resemblance to the same part in beasts which is wanting in most reptiles and in all birds.

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The same observation may be repeated with far greater force as regards the ankle.

In all beasts, as in man, the motion of the leg on the foot takes place by means of a joint between the shin-bone of the leg and the small bones of the ankle; and though in some beasts (as in the orang) there is considerable power of motion between the first and the second row of ankle-bones, this is small compared with the mobility of the foot and ankle taken together upon the leg,

In all birds, on the contrary, not only is there no motion between the ankle-bones (as a whole) and the shin-bone, but the two rows of ankle-bones actually anchylose respectively with adjacent parts—the row nearer the leg coming to form one with the shin-bone; the second row coming to form one with the bones of the foot. Thus in birds the motion of the foot on the leg takes place not between the ankle and the shin-bone, but between the two rows of ankle-bones.

The same thing to a less degree takes place in reptiles; the ankle-bones do not indeed anchylose with the shin-bone and foot respectively, but they nevertheless unite with those parts so firmly that motion takes place between the bones of the ankle and not between the whole ankle and the leg.

Now in the frog's class, e.g. in the order Urodela, we meet with a condition which is mammalian rather than reptilian or avian. Motion takes place freely between the leg and the whole tarsus. Moreover, the number and proportions of the ankle-bones themselves far more closely agree with the condition of the same parts exhibited to us by certain beasts than

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it does with that which is possessed by any bird or by most reptiles.

The frogs and toads, however, differ from the Urodela and present us with a peculiar condition of the ankle-bones, in that the two which represent the bones of the first row are so greatly elongated as to give to the limb an additional segment—as it were two "long bones" more.

We should search in vain through every other order of the Batrachian class, through every known group of birds and reptiles, both living and fossil, to find any

Fig. 57.—Elongated tarsus of Lemuroids. Left-hand figure, tarsus of Cheirogaleus; right-hand figure, tarsus of Tarsius. A, calcaneum B. cuboides, C, naviculare.
analogous structure. None of the lowest mammals, no marsupial, no rodent, no insectivorous or carnivorous beast, no hoofed mammal, presents us with anything of the kind. Nevertheless, at almost the other end of the series, in the very highest order, that to which
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man himself belongs, we actually find a similar development.

Amongst the very peculiar beasts which inhabit the island of Madagascar, there are certain small creatures,

Fig. 58—The Maholi Galago.
"Half-Apes," belonging to the genus Cheirogaleus, in which two of the ankle-bones are elongated in a manner similar to that of the frog. The same character is more marked in an African genus of
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half-apes (Galago), and still more so in a third half-ape (Tarsius), from the island of Banca. Now it is absolutely impossible to believe that a special genetic affinity connects together by a peculiarly common descent Half-Apes and Frogs! We are then driven to the conclusion that we have here again a striking similarity of structure in two instances which are quite independent in their origin. That the power of rapid and prolonged "jumping" does not carry with it as a necessary consequence the elongation of ankle-bones, is demonstrated by the fact that in other animals which, to say the very least, jump no less than do these half-apes—as for example in the kangaroos, jumping shrews, and jerboas—it is not bones of the ankle but bones of the foot proper, which take on an augmentation in length.

FOOTNOTES: Chapter 6

[[14]]

. See "Lessons in Elementary Anatomy," Lesson II., p. 22.

[[15]]

See "Elementary Physiology," Lesson V., § 19.

[[16]]

From Νo̓τος, back, and Χορδh̓, chord.

[[17]]

. From i̓δους, a tooth, and εi̓δος, form.

[[18]]

So named from its resemblance to the Greek letter υ.

[[19]]

i.e. Opossums, kangaroos, &c.

[[20]]

The Ornithorhynchus and Echidna.

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