Still mure interesting is it to watch the daily growth of the muscles that make their ap pearance, as the legs of the Frog are slowly formed, budding, as it were, from the sides of the Tadpole, and vicariously taking the office of those, that previously constituted the loco motive apparatus ; the vertebral system of mus cles, whereby the tail of the aquatic animal is moved, being entirely obliterated as the limbs advance to maturity.
6. The masticatory muscles, or those con nected with those movements of the lower jaw that are concerned in the preparation of food, present great uniformity of arrangement throughout all the Vertebral orders, and ob viously constitute a distinct and isolated group, the development of which is in exact relation with that of the rest of the manducatory ap paratus.
7. The tegumentary system of muscles, al though only represented in the human body by a few detached and isolated remnants, con stitutes among many of the lower animals a very important part of their economy, either destined to act upon the skin itself or upon cuticular structures of very diversified shape, which are occasionally developed in different regions of the body, and not unfrequently ap propriated to the performance of important duties. In those quadrupeds that have their backs covered with strong spines, such as the Echidna, the Porcupine, and the Iledgehog, the cutaneous muscles, usually named panni culas carnosus, are met with in their most com plete form, since in these creatures every quill or spine is moved by muscular bands con nected with its base, that serve to erect or de press it at pleasure. The crests and other moveable appendages to the skin met with among Birds, are equally furnished with the means of motion by strengthening particular parts of the muscular apparatus in question.
In Reptiles, on account of the nature of their corneous integument, the muscles of the skin are but slightly developed, or indeed most generally are not to be detected. But in Fishes they once more present themselves, under a novel and most important aspect. The azygos tins in these aquatic Vertebrata are, as is else where shewn (vide Prsc cs),derivations from the exo-skeleton, and consequently all the slips of muscle, that act upon the individual rays of the dorsal, caudal, or anal fins, however ano malous their nature may appear without such a key to their real character, are obviously merely portions of the tegumeutary system of muscles here elevated into an importance not witnessed in other animals, where the exo skeleton is less decidedly appropriated to the purposes of locomotion.
8. The muscles whereby vocal sounds are modulated, are equally entitled to be looked upon as a distinct and superadded system only conferred upon certain races of Vertebrata, and that under very various conditions. In Fishes these muscles are of course absolutely wanting, and even amongst the air-breathing Reptiles they are so imperfectly developed as scarcely to be regarded as vocal organs. But in Birds and Manimalia they assume a higher form, and are variously located and more or less nume rous in exact proportion as the voice is per fected. In Birds, indeed, the vocal muscles are principally placed at the thoracic extremity of the trachea, but in Quadrupeds and in Man, at the opposite end, the whole machinery being thus so completely altered that even analogies between the different sets of muscles are not easily pointed out.
9. The diaphragm is an apparatus exclu sively conferred upon the Mamrniferous Ver tebrata, since in these only is the thoracic cavity separated from the abdomen by a mus cular septum.
10. The muscles of the tongue must like wise be regarded as forming a distinct group, increasing in complexity and extent of motion, in proportion as the organ to which they belong assumes greater importance, either as an in strument for the prehension of food, or as an agent in mastication.
11. The ocular system of muscles may be divided into those which act on the eyeball, and those employed in moving the palpebral appendages. I' he former when complete sist of four ;Teti, two oblapi, and the rhourtoid or suspensary muscle, which not unfrequently is distinctly divided into four. The rieli are invariably met with and present few variations worthy of notice. The obliquus superior in Mammals passes through a pulley, which is not the case in other Vertebrata, while the ehoanoid muscle is principally met with in Quadrupeds.
The muscles of the eyelids are most per fectly developed in Birds, in which distinct muscles are appropriated to the movements of the upper and lower eyelid as well as to the nictitating membrane, which in the feathered races has a proper set of muscles appo.nted to draw it over the eye not met with in other classes.
12. The muscles of the auditory apparatus become fully developed only in the matnintfe rous ear, where four little muscles are inva riably found connected with the ossicu!a audi tus, as in the human subject. The muscles appointed for the movements of the external ear are, however, in many Quadrupeds much more numerous than in Man; in tlict, in the human ear they merely exist, in a rudimentary condition.
13. The nasal apparatus has likewise a sys tem of muscles of its own, although the in stances in which it is met with in anything like a complete state of developement are com paratively rare. In Fishes, Reptiles, and Birds these muscles, indeed, can hardly be said to exist; and even in the generality of Mammals they are feeble and unimportant. It is only in the proboscidean species, that the nasal muscles assume their full complexity, and the trunk of the elephant is in modern times the only ex ample, wherein the anatomist can contemplate them.
14. The muscles of the generative system are only found to exist, as a distinct set, in the Mammalia, as in these alone is the urethral canal complete, and a perfect ejaculatory ap paratus given.
Thus, therefore, we may learn from this short survey, that so far from finding in the human frame the fullest and most elaborately con structed examples of the various divisions of the muscular system, or, in other words, a ty pical condition of that part of the animal economy, the human anatomist, in many in stances, has only an opportunity of examining the vestiges or rudiments of organs, that in the lower animals attain to a far more complete developement.
(T. Rymer Jones.)