The permanent or adult condition of the hyoid apparatus likewise undergoes a progres sive simplification as we examine it in the more elevated forms of Vertebrata. Thus, ill the bird it still consists of elements, that in the inammiferous animal may be dispensed with. The cornua, the last remnants of the branchial arches, are still largely developed in many qua drupeds, and in fact it is only when we arrive at the human species that we see the pieces composing the hyoid portion of the skeleton reduced to their simplest condition, and the muscles appended thereto correspondently re duced in their number and modified as regards the functions they perform.
4. The muscles that act upon the opercular openings in Fishes are, of course, peculiar to animals possessed of a bronchial respiratory system.
5. The muscles of the limbs exhibit perhaps greater varieties than any others belonging to the animal economy, their existence and rela tive size being entirely dependent upon the kind of progression conferred on any given race or family of the vertebrate creation. It would seem indeed that an inverse proportion always exists between the condition of this system of muscles and those that act upon the trunk— the large developement of the one set rendering the other of secondary importance. Thus in the generality of the osseous Fishes the enor mous bulk of the lateral muscles of the trunk renders any great strength of limb unnecessary, and the muscles moving the pectoral and ven tral fins, the representatives of the arms and legs, are proportionately small and feeble ; but in the Plagiostome Cartilaginous Fishes, the Skates and Rays, the conditions are precisely reversed ; the muscles of the trunk shrink into comparative insignificance, and the enormously developed hands, which here form the great bulk of the body, moved as they are by muscles of corresponding power, form the great agents in locomotion, and by their vigorous flappings raise these creatures from the bottom of the sea, their usual resting-place.
The phenomena attendant upon the growth of the limbs in Amphibious Reptiles beautifully exemplify the same circumstances. In the Lepido-siren, that possesses still the form awl the scales of a fish, although it breathes both with gills and lungs, the legs or fins, for it is difficult to say to which appellation they are best entitled, are of the simplest possible struc ture, each consisting but of a simple, tapering stem, so flexible and feeble that it can scarcely be deemed at all useful for the purposes of lo comotion.
In the Siren lacertina we have still the long and flexible body of an eel, the tail obviously forming the chief, or, indeed, the only effective agent in progression. Nevertheless, seeing that this Amphibian being possessed of lungs can breathe the air, the first sproutings of legs are here manifest. Two rudimentary limbs corres ponding with the anterior pair of other reptiles, and terminated by four extremely imperfect toes, are appended to a feeble scapulary appa ratus, and thus the Siren is allowed to raise its head at least out of the, marsh where it re sides, and obtain a supply of the atmospheric fluid.
The Proieus is, in form, almost as fish-like as the Siren, and its tad is still a strong and muscular oar ; the I iinbs nevertheless are slightly more developed; and besides the imperfectante rior extremities, each of which is terminated by three toes, a rudimentary pelvis and pelvic extremity are now sketched out, the latter pre senting two little toes, but hardly as yet suffi ciently complete to be useful as locomotive organs.
Equally striking examples of the gradual de velopment of locomotive extremities are found in those reptiles whereby the transition is effect ed, between the Ophidian and Saurian types of structure ; talus in the genus Anguis, as for ex ample, in the common English blind-worm (Auguisfragilis), although externally it would appear to be as strictly apodous as the gene rality of other serpents, yet on stripping off the skin, these reptiles are found to possess the first rudiments of limbs, that are afterwards to be made efficient in more highly gifted genera ; a little pelvis is distinctly discernible, imbedded in the muscles towards the hinder part of the body ; and, in font, a sternum, scapula, and clavicle, may all be perceived hidden beneath the integument, although no traces of legs or feet are as yet to be detected.
In other serpents more nearly approximated to the quadrupedal Saurians, as in the genus named Bimanes ( Clurotes,Cuv.) in addition to the scapulary apparatus, two short anterior extremities armed with toes, moved by tole rably complete muscles, are met with, whilst the hinder legs are wanting. In Sipes, on the contrary, it is the pelvic pair of legs that are developed, the place of the anterior being only indicated by the existence of the frame-work and muscles of the shoulder. Lastly, in the Saurians and Tortoises the quadrupedal type is fully adopted, and the muscles of the limbs assume an importance proportionate to the duties they have to perform.