CHAPTER VII.
THE MUSCLES OF THE FROG.
The Common Frog | ||
7.
CHAPTER VII.
THE MUSCLES OF THE FROG.
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,
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.
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
Studying the life-history of this one animal, then, we become acquainted with a process of direct
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
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
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
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.
The muscles of the abdomen may be conceived as having arisen through atrophy, in that region, of the
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
The muscles connected with the human lingual apparatus are sufficiently complex. One such muscle
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.
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
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.
The muscular conformity between the highest and lowest of typically-limbed vertebrates is strikingly shown by the structure of the thigh and leg, the
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
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.
In fact, however, the Frog does make use of his feet for a purpose requiring actions no less dexterous and delicate than nest-building.
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
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
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
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.
CHAPTER VII.
THE MUSCLES OF THE FROG.
The Common Frog | ||