Mivart, St. George Jackson . The Common Frog
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Chapter 7

CHAPTER VII.
THE MUSCLES OF THE FROG.




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We may now pass to the consideration of some points exhibited by another set of structures -- namely, the muscles.

    The muscles of an animal constitute its flesh, which as the most ordinary inspection shows us, is composed of different portions of soft fibrous substance, such portions being separated from one another by interposed layers of membrane. Each such portion, so separated, is a muscle, and is attached at its two ends to two parts (bones or what not), which may be adjacent or more or less distant. The fibres which compose it have the remarkable property of contracting under certain conditions, and, when contracted, the whole muscle is shorter and thicker than before, and the two parts to which it is attached become consequently approximated.

    Muscles may be large expanded sheets of flesh (as in the abdomen), or long and more or less narrow, as in the limbs.

    Muscles are said to be "inserted," or to "take origin from" the parts to which they are attached,


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and they may be so inserted by their own muscular fibres or by the intervention of a tough membrane or a dense fibrous cord called a "tendon."

Fig. 59. -- Superficial Muscles of the Perch. The fin-rays of all the fins are cut off. 1, great lateral muscle, showing the numerous vertical tendinous intersections slightly but variously inflected; 2, Small superficial muscles inserted into the fin-rays of the dorsal and ventral fins; slender longitudinal muscle running (in the interval of the summits of the two great lateral muscles) between the dorsal and caudal fins; 5, similar muscle on the ventral margin, which also appears between the anal and ventral fins; 6, small radiating muscles of the caudal fin; 7, part of the great lateral muscle inserted into the skull; 8 and 9, elevators of the operculum; 10, elevator of the palato-quadrate arch; 11 and 12, muscular mass by which its contraction closes the jaws; 13, superficial muscles of the pectoral fin;14 and 15, muscles of the ventral fin.

    All the motions of an animal are produced by means of the contractions of its muscles pulling the bones, which act as so many levers (of different kinds according to circumstances), and so effecting locomotion.

    These muscular contractions are in life produced by the agency of certain of the nerves proceeding from the nervous centres, i.e. from the brain and spinal marrow, and which carry an influence outwards to the muscles. Other of the nerves so proceeding convey an influence inwards to the nervous centres from an irritated portion of the body's surface. The muscles, however, especially in the frog, may, for a time, be made to contract after death by direct irritation of the nerves themselves.


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After the skeleton, it is the muscular formation of the body which mainly determines its general form and aspect, though occasionally -- and often in the Frog's order -- the voluntary inflation of the lungs will alone produce a vast modification in an animal's appearance.

    The curious and grotesque resemblance which exists between the figure of the adult frog and that of man has been a common subject of remark. It may then be less surprising to some to learn that there is a great degree of resemblance between the muscles of the Rational and of the Batrachian animals; though the much greater gulf which separates the Batrachian than the Reptilian class from mammals may lead others to anticipate a greater divergence than in fact exists.

    The frog, however, in its immature stage of existence, is widely different from the adult in its muscular or myological) furniture, and this for one obvious reason.

    "Muscles" are, as we have shown, par excellence, "organs of motion," and the motions of the tadpole are essentially different from those of the frog.

    The frog, as all know, progresses on land by jumps, and swims through the water by a series of movements which are in fact aquatic jumps. This action is familiar to many of us, not only from observation but also by imitation (the frog being a swimming-master given us by nature), but it is none the less a mode of swimming which is very exceptional indeed.

    The tadpole progresses through the water in a very different manner, namely, by lateral undulations of its


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tail, which is the usual mode of swimming among vertebrate animals -- that made use of by sharks and porpoises, as well as by the overwhelming majority of fishes.

    Studying the life-history of this one animal, then, we become acquainted with a process of direct

Fig. 60. -- Anterior muscles of the Trunk of Man: the pectoralis major of the right side and the left external oblique being removed. 1, pectoralis major; 2, pectoralis minor: 3, subclavius; 4, serratus magnus; 5, internal intercostals; 6, external oblique; 7, internal oblique; 8, linea alba.
transition from the condition of a fish to that of a quadruped, as regards a most important group of organs.

    In ourselves, the back is provided with muscles which extend along its length in a complex series of longitudinal divisions, from the middle line outwards.

    The abdomen of man is inclosed and protected by successive muscular layers laid one upon another, the


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fibres of the successive muscles being differently directed. Thus we have (1) the external oblique (the fibres of which pass obliquely downwards and backwards), (2) the internal oblique (the fibres of which pass obliquely downwards and forwards), (3) the Transversalis (with transverse fibres), and (4) the Rectus abdominus (situated in the middle line of the body, and with fibres directed antero-posteriorly).

Fig. 61 -- Deeper Abdominal Muscles of Man -- the external oblique being removed from the left side of the body, and the internal oblique and part of the rectus also, from its right side. 1, the internal oblique; its outer tendon (2) is cut and reflected from the outside of the rectus to show its deeper tendon (3), which passes within the rectus except towards the pubis; 4, transversalis; 5, its fascin; 6, sheath of the rectus -- near the pubis, the conjoined aponeuroses of the abdominal muscles pass in front of that muscle: 7, pyramidalis; 8, rectus of left side, showing the tendinous intervals, or lineæ transversæ.

    In the frog we also meet with the vast sheets of muscle with oppositely directed fibres (the external and internal oblique), and with a median, antero-posteriorily directed rectus muscle.

    A very different condition exists in fishes, where there is indeed a median antero-posteriorly directed rectus, but where the abdomen and tail are encased


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with a mass of muscular fibres not arranged in super-imposed sheets, but as a series of narrow segments separated from each other by layers of membrane. The edges of these membranous layers, when the skin is removed, appear as a successive series of undulating lines proceeding from the back to the belly.

Fig. 62. -- Tadpole of Bull Frog, partly dissected, to show the muscles of the tail and the branches of the 8th nerve or the vagus. a, great lateral branch giving off -- b, a dorsal branch, and c, the lateral branch (or nervus lateralis); d, branches descending and passing along the branchial arches. The descending branches seen behind the branchial nerves on the side of the belly are not branches of the vagus at all, but spinal nerves, which come out from beneath the muscles and pass down under the nervus lateralis, and without having any communication with it.

    Now the tadpole exhibits a muscular condition quite similar to that of the fish, and in the great persistent larva the axolotl, we find no truly oblique abdominal muscles, but only as it were a hyper-trophied rectus. In other species of the frog's class, which retains a tail throughout life, the marked superimposed lamellæ are distinctly developed, but more or less distinct traces are also retained of the successive membranous partitions separating the muscular segments of both the dorsal and ventral regions.

    Another stage of development may be detected in the tail-muscles of certain reptiles.

    Here the membranous partitions have become drawn out at short intervals from above downwards into a funnel-shaped condition, so that the muscular


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fibres enclosed, assume the forms of cones. Moreover, the apices of the membranes enclosing the cones,
Fig. 63. -- Superficial Muscles of Extensor Side of Leg and of parts of Trunk and Tail of Menopoma. ES, erector spinæ -- directly continued into dorsal half of tail; ELD, extensor longus digitormn pedis; FC, femoro-caudal; GMx, probably rectus femoris; I, muscle resembling iliacus: ILC, ilio-caudal; IP, ilio-peroneal; RF, part of great extensor of thigh; SM and ST, muscles like the semi-membranosus and semi-tendinous.
become denser in substance, and so modified into ligaments.

    We come thus to have a key to the process of development, by which the muscles of the back may be conceived to have arisen.

    The muscles of the back may be conceived as having arisen through increasing obliquity, conical prolongation, and partial detachment (from muscle) of the separating membranous lamellæ; the produced ends becoming condensed into firm tendons directed more or less obliquely forwards.




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    The muscles of the abdomen may be conceived as having arisen through atrophy, in that region, of the

FIG.64. -- Diagram of Caudal Muscles of Right Side or Tail of Iguana, showing how the ventral mass resembles the dorsal part, and how the tendinous intersections of the muscular fibres are drawn out into cones. N, neural spine; H, hypapophysial spine; z, zygapophysis; t, transverse process; 1, dorsal series of cones; 2, upper lateral series of cones; 3, lower series of cones; ventral series of cones.
separating membranes and subsequent splitting up of the muscular mass into super-imposed sheets of differently-directed fibres.

    This filiation between piscine and mammalian myology could hardly have been detected but for the remarkable series of gradations which the frog's class exhibits -- gradations both between species, and

Fig. 65. -- Muscles of the Right Side of the Tongue of Man. 1, stylo-glossus; 2, stylo-hyoid: 3, stylo-pharyngeus; 4, hyo-glossus; 5, genio-hyoid; 6, genio-glossus; 7, lingualis.
between different ages and conditions of one and the same species.

    The muscles connected with the human lingual apparatus are sufficiently complex. One such muscle


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-- the stylohyoid -- passes downwards on each side, from a process of the base of the skull to the corniculum of the os-hyoides or tongue-bone. The tongue-bone of the frog is, as we have seen, relatively far greater than is that of man, and the same may be said of the muscles attached to it, since we have no less than four muscles descending from the skull, and implanted into it, on each side.

    This fact might well be supposed to bear direct relation to the size and mobility of the frog's tongue. The tongue in the frog and toad is singularly different from the tongues of most familiar animals, in that it is not free and movable in front, but behind. These Batrachians take their food by suddenly throwing forwards, out of the mouth, the free hinder end of the tongue. The insect or other small animal struck by it, adheres to it, on account of a viscid saliva with which it is coated. The prey is then suddenly drawn into the mouth and swallowed.

Fig. 66. -- Head of the Frog Phyllomedusa, showing the tongue fixed in front, but free posteriorly.
Here then is the ready explanation of the development of the os-hyoides and its muscles. There is a difficulty, however, in that two toads already described, the Pipa and the African form Dactylethra (Figs. 11 and 12), have no tongue whatever.


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Moreover, there is another toad ( Rhinophrynus) which is even more exceptional in its order than these two; in that its tongue is not free behind, but, like that of ordinary vertebrates, in front (Fig. 13).

    The fact is, that the large tongue-bone of these animals serves, with the muscles attached to it, as much to facilitate respiration as nutrition.

    It has already been said that the frog has no ribs by the elevation and depression of which it may alternately fill and empty its lungs. Neither does it possess that transverse muscular partition, the diaphragm, or midriff, which in man's class is the main agent in carrying on that function.

    The lungs of the frog are inflated as follows: -- The mouth is filled with air through the nostrils and kept shut while the internal openings of the nostrils are stopped by the tongue, and the entrance to the gullet is closed. Then, by the contraction of the muscles attached to it, the os-hyoides is elevated; and every other exit from the mouth being closed, except that leading to the larynx, air is thus driven down the glottis into the lungs.

    Thus, for pulmonary respiration it is necessary to the frog to keep the mouth shut; and in this way, but for the action of the skin, the animal might be choked by keeping its mouth open.

    It has been already stated that the typical segmentation of the limbs is wanting in all fishes, but present in all Batrachians that have limbs at all. Similarly, in all Batrachians that have limbs at all the muscles of those limbs have essentially and fundamentally the same arrangement as in higher animals. In the higher


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animals, as in man, the muscles of the limbs belong to different categories named from the kinds of motion to which their contractions give rise.

    Thus, when two bones are united by a movable joint (as the thigh-bone and shin-bone), muscles which by their contraction tend to make the angle formed by such bones acute are termed "flexors." Those, on the contrary, which tend to open out such an angle are termed "extensors."

    In the fore-arm of man, and allied animals, there are muscles which tend by their contraction to place the hand in a position either of pronation or of supination.

    When the arm and hand hang down, the palm being directed forwards, the position is that of supination, and the bones of the fore-arm are situate side by side. When the arm and hand hang down, but the back of the hand is turned forwards, the position is that of pronation, and the radius crosses over the ulna. When we rest on the hands and knees, with the palms to the ground, the fore-arms are in pronation.

    Muscles which tend to place the fore-arm and hand in the position of pronation are termed pronators; those which, by their contraction, tend to render it supine are called supinators.

    It is somewhat surprising to find in an animal so nearly related to fishes as Menobranchus definite flexors, extensors, pro-and supi-nators essentially like those of higher animals; and these distinctions once established, persist up to man himself with increasing complications.




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    The muscular conformity between the highest and lowest of typically-limbed vertebrates is strikingly shown by the structure of the thigh and leg, the

Fig. 67 -- Muscles of Ventral Surface of Menobranchus. On the right side, superficial muscles; on the left side, deeper muscles, the mylo-hyoidei, pectoralis, and external oblique being removed. Also superficial flexor muscles of right pectoral limb of Menobranchus . B, biceps; CBI and CB2, coraco-brachialis; CHE, cerato-hyoideus externus; E0, external oblique; FL, flexor longus; GH, genio-hyoid; MH¹ and MH², mylo-hyoideus; OH, omo-hyoid; P, P¹ and P², pectoralis: R, rectus; S, subclavius; SH, sterno-hyoid; SL, supinator longus; T, triceps.



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leading muscles of these parts in the frog being so like those of man that the practice of calling them by the same name is abundantly justified.

    The perfection of man's hand has been justly the theme of panegyric, esteemed as widely as it is known. The delicacy and multiplicity of the motions of which it is capable are of course greatly due to the number and arrangement of the muscles with which it is provided.

    One of the most important of these motions is that of the thumb as placed in opposition to the fingers, and effected by a muscle termed opponens pollicis.

    An "opponens" muscle is one which passes from the bones of the wrist to one or other of the bones of the middle of the hand called metacarpals, and the opponens pollicis passes of course, as its name implies, to the metacarpal of the pollex or thumb.

    No other finger of man's hand is furnished with such a muscle except the little finger, which possesses an opponens minimi digit , passing from the wrist to the fifth metacarpal. The same condition obtains in the apes, though in them the opponens of the thumb is smaller and weaker than in man. Though the foot of man is furnished with many muscles, like the hand, yet not one of the toes is provided with an " opponens" or muscle, passing from the bones of the ankle to one or other of the bones of the middle of the foot, which latter are called metatarsals. The same is the case with the apes, except that the Orang-utan has a small "opponens" attached to the great toe.

    This being premised, the foot of the Frog may well excite surprise as to its rich muscular structure. In


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addition to very numerous other muscles on both surfaces, every one of the toes is provided with a separate opponens muscle, each having a muscle which passes from the bones of the ankle to its middle foot bone or metatarsal.

    The question naturally occurs on beholding this prodigality of muscles -- What special purpose is served by the Frog's foot? Surely mere jumping and swimming cannot require so elaborate an apparatus.

Fig. 68. -- Deep muscles of exor surface of Frog's hind foot. (The numbers indicate the digits to which the muscles belong. -- No. 1 indicating the first digit or great toe.) ab, abductors; ad adductor ; fb, flexor brevis; fp, flexores profundi; fph, flexores phalangium: op, opponens muscles; tm, transverse muscles.

    In fact, however, the Frog does make use of his feet for a purpose requiring actions no less dexterous and delicate than nest-building.




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    In 1872 Dr. Günther observed a Frog, busily occupied, and industriously moving its hind legs in a singular manner, On approaching closely he found it had constructed for itself a shelter in the shape of a little bower, formed of dexterously interwoven blades of grass. The circumstances have been kindly transmitted to the author by the observer, in a private letter, as follows: --

    "The `nest-building' Frog was a large example of Rana temporaria or esculenta (I forget which), which I had brought into the garden behind my house. It had taken up its abode in grass, near the edge of a tank, from which the turf sloped abruptly to the level of the garden. When I first disturbed the Frog from its lair, I found that it had lain in a kind of nest, which I cannot better describe than by comparing it to the form of a hare, with the grass on the edges so arranged that it formed a sort of roof over it. Sometimes the animal returned to it, sometimes it prepared a new form close to the old one, which remained visible for several days until it was obliterated by the growing grass.

    "When in its nest, nothing could be seen of the Frog but the head.

    "One day I poked the Frog out of its lair; after two or three jumps it returned to the old spot, and, squatting down on the grass, by some rapid movements of the hind legs it gathered the grass nearest to it, pressing it to its sides, and bending it over its body so as to be partially hidden.

    "In all these operations no material was collected by the animal for its nest, but only the growing grass


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was either pressed down, or arranged so as to form a complete retreat. Unfortunately, the Frog soon disappeared altogether."

    It is very probable that other functions, as yet unnoticed, may be performed by these members, since, though the observation just above related is the first known observation of the kind, yet the

Fig. 69. -- Deeper Muscles of Outer Aspect of Right Pelvic Limb of Parson's Chameleon: the ilio-peroneal cut reflected. A, adductor; B, biceps; D¹ gluteus primus; D², gluteus secundus; D³, gluteus tertius; EL, extensor longus digitorum; F2 and F3, rectus femoris: FC, femoro-caudal; FDI, flexor longus digitorum; FD2, flexor tertius digitorium; G, gracilis; GE, gastrocnemius externus; IP, ilio-peroneal; P, peroneus; S, tibial adductor; SM, semi-membranosus; VE, vastus externus; X, gluteus maximus; y, tendon of femoro-caudal.
manoeuvre recorded is no doubt a constant habit of the animal.




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    Doubtless, also, the very singular actions performed by the male Pipa and Obstetricans are performed by the help of the hinder extremities.

    At the same time that the Frog shows so startling a resemblance in its leg muscles to the higher animals, it shows as striking a difference from the leg muscles of animals with which it is nearly allied, -- namely, with those of its class-fellows, the Urodela.

    In Reptiles we meet with a muscle which takes origin from beneath the joints of the tail, and is

Fig. 70 -- Deeper Muscles of Extensor Surface of Right Leg of Menopoma. B, biceps; EB, extensor brevis; EH, extensor hallucis; ELD, extensor longus digitorum; FC, femoro-caudal: GMd and GMi, muscles like the lesser glutei; GMx and RF, great extensors of the thigh; I, muscle resembling the iliacus; IlC, ilio-caudal; IP, ilio-peroneal; SM and ST, muscles like the semi-membranosus and semi-tendinosus respectively; TA, tibialis anticus.
inserted with the thigh-bone, and which has no certain representation amongst mammals, and is called the femoro-caudal.




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    In the Urodela we also meet with a femoro-caudal, but no such structure exists in the Anoura. This is not so surprising when we recollect the abortive condition of the tail of the Frog. It might, however, have been expected that in the Tadpole, during the co-existence of the tail with the hind legs, and while it thus externally resembles an eft -- such a muscle would transitorily exist. Such, however, is not the case, and the distinction is a very remarkable one.

    In one point, however, the Efts resemble the Frogs, namely, in the greater number and greater complexity as well as the greater size of the muscles of the hind-limbs than of the fore-limbs. It is well known that the Efts make use of their hind-limbs in attaching their eggs to the leaves and branches of aquatic plants; and further observations may show with regard to these animals facts, as to the use of the members, as novel and interesting as the one just cited with regard to the nest-building actions of the Frog.