542 



MUSCULAR SYSTEM. 



tionate to the violence of the impulse required 

 in moving the tail, and occupying the lateral 

 regions of the body, extend quite from the head 

 to the caudal fin, constituting almost the entire 

 bulk of the animal, and possessing sufficient 

 strength from their combined contractions to 

 scull the fish through the water with surprising 

 velocity, or even to enable the salmon to throw 

 itself up the cataract, that bars its progress up 

 the river, where it is commissioned to lay its 

 eggs. 



During the changes that accompany the de- 

 velopement of the tadpole, which by its meta- 

 morphosis into a frog is literally converted from 

 the condition of a fish into that of a reptile, the 

 transmutation observable in the condition of 

 the muscles acting upon the spine are not less 

 remarkable, than those witnessed in the verte- 

 bral column itself. Whilst in its tadpole state 

 the frog is, as regards its powers of locomotion, 

 strictly a fish, and rows itself about entirely by 

 the movements of its expanded vertical tail 

 exactly as fishes do, but as the limbs of the 

 reptile gradually make their appearance the 

 lateral muscles of the spine that previously 

 formed the bulk of the creature are absorbed 

 and disappear, the hitherto flexible and elon- 

 gated vertebral column becomes short and but 

 little gifted with motion, and its muscles in 

 the same ratio grow feeble and unimportant. 



In the other forms of Reptiles, as well as in 

 Birds and Mammalia, the muscular system 

 acting upon the vertebral chain presents great 

 uniformity of character, the number and strength 

 of the muscular fasciculi being exaggerated, or 

 diminished in different regions in proportion as 

 mobility is permitted, the movements of the 

 spine being generally diminished, and tram- 

 melled in exactly the same ratio as the loco- 

 motive limbs become more perfect and efficient. 



2. The costal muscles form a system apart, 

 quite independent of those connected with the 

 vertebral column, and exactly keeping pace 

 with the developement of the skeleton of the 

 thorax. In Fishes a thoracic cavity cannot be 

 said to exist, inasmuch as the ribs that enclose 

 the viscera seem rather processes fixed to the 

 spine, in order to give a greater extent of sur- 

 face for the attachment of muscles destined to 

 act upon the tail, than properly the representa- 

 tives of the costal elements of the skeleton ; 

 neither do ribs exist in the tadpole, or even in 

 the perfect frog. Even in those Batrachia that 

 are most gifted in this particular, minute corni- 

 cula appended to the apices of the transverse 

 processes of the vertebrae are the only rudi- 

 ments of a costal system of bones, and these 

 muscles are vainly looked for. 



In the Tortoises and Turtles likewise, al- 

 though both vertebral and sternal ribs are 

 present, and so hugely developed that they con- 

 stitute the great bulk of the carapax covering 

 these strange reptiles, such is the immobility 

 of the dorsal shield, and so securely are the 

 ribs conjoined by suture, that any muscular 

 apparatus destined to act upon them would 

 have been obviously superfluous. 



In Serpents, however, the case is widely 

 different ; for in these lithe and limbless crea- 



tures the ribs are made to serve as most impor- 

 tant locomotive agents, and their movements 

 must be proportionably free. Dorsal ribs only 

 are here met with, but these being now move- 

 ably articulated to the sides of the spinal co- 

 lumn, and moreover acting at their opposite 

 extremity upon the ventral scuta, perform the 

 duties of internal legs, and being continued in 

 an unbroken series from the very atlas nearly 

 to the termination of the tail, it is not difficult 

 to imagine the numbers and complexity of the 

 additional muscles now provided, to wield or- 

 gans so numerous and important. 



In Lizards and in Birds the thorax assumes 

 its most complete state of developement, and 

 exhibits both dorsal and sternal ribs articulated 

 to each other and capable of extensive move- 

 ments ; muscles are therefore given to act upon 

 both the dorsal and the sternal series. 



Lastly, in the mammiferous races the ante- 

 rior costal bones are once more removed, their 

 place being occupied by elastic cartilages, the 

 resiliency of which to some extent antagonizes 

 those muscles which act upon the moveable 

 portions of the thorax. 



The sternum, or rather the sternal system of 

 bones, although frequently found to enter 

 largely into the composition of a thoracic ca- 

 vity, seems rather to be in relation with the 

 anterior extremity, and the muscles derived 

 from it principally subservient to the motions 

 of those limbs. Thus in the frog and toad we 

 have a largely developed sternum without either 

 ribs or thorax ; and in the case of Birds, the 

 strict correspondence between the condition of 

 the sternum and the powers of flight is most 

 strikingly exemplified. 



3. Perhaps the most interesting lesson to be 

 derived from such a survey of the muscular 

 system of vertebrate animals as this, is taught 

 by an examination of the hyoid apparatus, and 

 of the muscles connected with it, in the diffe- 

 rent members of the vertebrate series, and also 

 during the different phases of embryonic deve- 

 lopement, in any of the air-breathing or more 

 elevated classes. It is in Fishes that this part 

 of the skeleton exhibits the greatest complexity 

 of structure, and forms a most elaborate frame- 

 work of branchial arches, destined to support 

 the gills, which some writers have been tempted 

 erroneously to consider identical with the thorax 

 of the air-breathing races. The branchial or 

 hyoid organs are in fact substitutes for the tho- 

 racic or pulmonary portion of the skeleton, 

 and in exact proportion as the latter becomes 

 more complete, and better adapted to aerial 

 respiration, does the former shrink in its dimen- 

 sions and become simplified by the obliteration 

 of successive portions, which previously entered 

 into its composition, and a consequent remodel- 

 ling, as it were, of the muscles connected there- 

 with. Thus during the metamorphosis of the 

 tadpole, the branchial arches that before were 

 largely developed, are progressively found to 

 disappear as the lungs assume their office, and 

 the whole hyoid system of bones and muscles 

 changed so as to become adapted to the per- 

 formance of totally different functions. 



The permanent or adult condition of the 



