In the other forms of ltept.les, as well as in Birds and Mainmalia, 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 development 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 vertebra: 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 oarapax 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, hut 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 be.ng occup.ed by elastic cartilages, the resiliency of which to some extent antagonizes those musc'es width act upon the moveable poi tions of the thorax.
The sternum, or rather the sternal system of bones, althou•h 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 pr.ncipa!ly 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 musc'es 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 skeletoa 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 ram 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.