NA TURE 



YJan. 8, 1874 



common descent, Half-Apes and Frogs ! We are then 

 driven to the conclusion that we have here ngam a strik- 

 ing similarity of structure in two instances which are 

 quite independent in their origin. 



That the po«er of rapid and prolonged "jumping" does 

 not carry with it a-? a necessary consequence the elonga- 

 tion of anVle-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 e.xample in the kingaroos, 

 jumping shrews, and jerboas — it is not bones of the ankle 

 but bones of the foot proper, which take on an augmen- 

 tation in length. 



TIu 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 fle^h, which 

 as the most ordinary inspection shows us, is composed of 

 different portions of soft fibrous substance 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 con- 

 tracting under certain conditions, and, when contracted, 

 the whole muscle is shorter and thicker than before, and 

 the two paits 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, and they 

 may be so inserted either by their own muscular fibres or 

 by the intervention ot a tough membrane or a dense 

 fibrous cord called a " tendon." 



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 

 circumstancesj, 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 intluence 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 irrita- 

 tion of the nerves themselves. 



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 pro- 

 duce 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 

 surpiising 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 sepal ates the liatrachian 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 myo- 

 logical) furniture, and this from one obvious reason. 



" Muscles " arc, as we have shown, par excellence, 

 " organs of motion," and the motions of the tadpole are 

 essentially diflerent from those of the frog. 



The frog, as all know, progresses on land by jumps, 

 and swims through the water by a scries of movements 



which are in fact aquatic jumps. This action is familar 

 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 

 tail, which is the usual mode of swimming among verte- 

 brate 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 transition 

 from the condition of a fish to that of a quadruped, as re- 

 gards a most important group of organs. 



In ourselves, the back is provided with muscles which 

 extend along its length in a complex series of longitu- 

 dinal divisions, from the middle line outwards. 



The abdomen of man is inclosed and protected by 

 successive muscular layers laid one upon another, the 

 I'lbres of the successive muscles being differently directed. 

 Thus we have (i) the external oblique (the fibres of which 

 pass obliquely downwards and backwards, (2) the inter- 

 nal obhque (the fibres of which pass obliquely downwards 

 and forwards), (3) the Transvcrsaus (with transverse 

 fibres), and (4) the Rectus abdomiiius (situated in the 

 middle line of the body, and with fibres directed antero- 

 posteriorly). 



In the frog we also meet with the vast sheets of 

 muscle with oppositely directed fibr<;s (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 anteio posteriorly directed rectus, 

 but where the abdomen and tail are encased with a mass 

 of muscular fibres not arranged in superimposed sheets, 

 but as a series of narrow segments separated from each 

 other by layers of membrane. The edges of these mem- 

 branous layers, when the skin is removed, appear as a 

 successive series of undulating lines proceeding from the 

 back to the belly. 



Now the tadpole exhibits a muscular condition (Fig. 56) 

 quite similar to that ot the fish, and in the great persistent 

 larva the axolotl, we find no truly oblique abdominal 

 muscles, but only as it were a hypertrophied rectus. 



In other species of the frog's class which retain! a tail 

 throughout life, the marked superimposed lamelL-e 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 

 at short intervals from above downwards out into a formal 

 shaped condition, so that the muscular fibres enclosed, 

 assume the forms of cones. Moreover, the apices of the 

 membranes enclosing the cones, become denser in sub- 

 stance, and so moditied into ligaments. 



We come thus to have a key to the process of develop- 

 ment, by which the muscles of the back maybe conceived 

 to have arisen. 



The muscles of thebackmay be conceived as having arisen 

 through increasing obliquity, conical prolongation, and 

 partial detachment (from muscle) of the separating mem- 

 branous lamella; ; the produced ends becoming condensed 

 with 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 

 separating membranes and subsequent splitting up of the 

 muscular mass into super imposed sheets of differently- 

 directed fibres. 



This filiation between piscine and mamma ian myology 

 could hardly have been detected but for the remarkable 

 series of gradations which the frog's class exhibits — 



