MUSCULAR SYSTEM, (COMPARATIVE ANATOMY op).—The muscular system of ani mals, as the term is generally understood, is composed of masses or fasciculi of highly irri table filaments, by the contractions of which the movements of the body, whether voluntary or involuntary, are effected, and the arrange ment of these moving powers, their size and strength, forms and general disposition, must of course vary ad infinitum in the different classes of animals, in conformity with the varie ties of their external form, or the innumerable kinds of apparatus conferred, for special pur poses, upon particular tribes or even species of living beings. Of these detailed accounts are elsewhere given in those articles which treat of the structure and anatomy of each class entering into the composition of the animal kingdom. Nevertheless, it has been deemed advisable, upon the present occasion, to collect together in one view the principal facts connected with general myology, and to state, as briefly and concisely as the nature of the subject will allow, those grand physiological points which an extended review of the comparative organi zation and development of the muscular system reveals to the anatomical observer, in order to concatenate the leading facts recorded in other pages of this work.
It may be laid down as an axiom universal in its application, that the condition of the muscular system in any race of animals must be dependent upon, or at least in strict conlimmity with, the development of the nervous apparatus, by the influence of which muscular movements are excited, controlled, directed, am! associated.
Thus, as we advance from lower to more elevated forms of living creatures, it is easy to perceive that in exact proportion as the nervous system makes its appearance, and becomes pro gressively more elaborately organized, the mus cles themselves become developed and assume a perfection of structure and precision of move ment adapted to the increasing exigencies of the animal economy; nay,it has now been satisfacto rily establ ished that even among thehighest races of the animal creation, during the progress of embryo development, the most intimate relation is observable between the state of the nervous centres and the condition of the nascent muscles as they become gradually formed and perfected. In the lowest Zoophytes where nervous fibres of any kind are not perceptible, even under the most rigid microscopical examination, the con tractile tissue of the body is equally diffused and devoid of aggregation into filaments or fasciculi of mascular fibre, and precisely under the same conditions the first rudiment of the vertebrate embryo, being as yet entirely devoi of nerves, is also destitute of distinct mus cies, the movements of which could only b associated and rendered efficacious by means nervous intercommunication ; and this corn plete want of aggregation of the elements o muscular tissue is as remarkable throughout al the Arnie division of the animal world as it in the nervous matter entering into the compo sition of their bodies, which, although its pre sence is not to be detected by our senses, is rea sonably supposed to exist in a diffused state even in the lowest tribes of animated beings.
As soon as nervous threads become apparent, and long before ganglionic masses of neurine are developed to such an extent as to entitle them to be regarded as centres of innervation, the muscular tissue, in like manner, assumes a different and far more perfect character ; the elementary molecules composing muscular fibre are then distinctly visible, and assuming a defi nite arrangement become grouped in longitu dinal series, exhibiting contractile bands and fascicles placed in precise directions, and capa ble of effecting movements of a more decided character than could possibly be exercised in creatures deprived of nervous cords, whereby the contractions of numerous muscular fasciculi might be associated and made to cooperate for the accomplishment of a given purpose. Moreover, it must be obvious that, in that great division of the animal creation which is characterized by the existence of nervous fila ments, the ganglionic centres being as yet imperceptible, or at least where any have been detected, in a very rudimentary state of de velopment (the NEMATONEURA of Professor Owen); such a condition of the nervous appa ratus involves, as a necessary consequence, Important circumstances connected with the general economy of the beings so organized as regards their means of relation with the external world. Having as yet no ganglia developed sufficiently important, from their size or situa tion, to merit the title of brains, or tit to be regarded as consuming a common sensorium, whereunto information derived from remote parts of the body may be conveyed, localized instruments of sensation would be as yet super fluous, and consequently, with the exception of the generally diffused sense of touch which, from its extreme delicacy, seems in these lowest forms of existence to supply to a certain extent the deficiency of other means of perception, instruments of sensation are not as yet conferred. The presence of a localized organ of sense, analogous to an eye or an ear, must obviously be useless to a creature pos sessed of no sensorial centre. to which informa tion, derived through the medium of that sense, may be transmitted, and organs of the higher senses are, therefore, as yet entirely wanting throughout the NEMAT0hEUROSE division of the animal kingdom,* as, a .fortiori, they are necessarily deficient throughout the ACItITE classes. In like manner the existence of external locomotive members, moved by any powerful or elaborately constructed muscular apparatus, is not to be expected in animals that possess not ganglia capable of presiding over voluntary muscular motion. Limbs, there fore, properly so called, are not as yet de veloped; and, if in some of the most perfect Epizea, the rudiments of such structures be come apparent, it is only because the animals possessing them are so nearly allied to the Articulate, in their general structure, that the nervous ganglia in them are beginning to be developed, and thus they can only be looked upon as the transition steps leading by an almost imperceptible gradation from one great type of animal organism to another of a more elevated character.