MUSCULAR SYSTEM. 



541 



In the PTEROPOD MOLLUSCA, we likewise 

 find the entire body enclosed in a muscular 

 bag, forming what is called the visceral sac, 

 but the locomotive organs present themselves 

 under a different aspect. These consist of two 

 muscular flaps or wings, appended to the op- 

 posite sides of the neck, which form, in fact, 

 two oars or paddles, wherewith the little Ptero- 

 pods row themselves about from place to place, 

 or gambol gaily among the waves, which some- 

 times, in the northern oceans, swarm with count- 

 less multitudesof them. The lateral fins in ques- 

 tion were regarded by Cuvier as being likewise 

 subservient to respiration, an opinion, however, 

 which Eschricht satisfactorily confutes. The lat- 

 ter writer, moreover, points out a little circum- 

 stance worth recording, namely, that the wings 

 are not distinct and separate organs as at first 

 they would appear, but that the muscles mov- 

 ing them pass continuously in a crucial di- 

 rection through the neck of the animal from 

 one wing to the opposite, so as to convert the 

 whole apparatus into an exact representation of 

 the double paddle used by the Greenlander, in 

 rowing his kajac, or canoe, over the very seas 

 frequented by the Pteropods, in such abun- 

 dance. 



In the Carnivorous CEPHALOPODA the mus- 

 cular sac composing the body, the parietes of 

 the head, and the long and flexible arms with 

 their curiously constructed sucking cups ap- 

 pended, are all made up of variously disposed 

 contractile fibres ; but these are too fully and 

 well described in another place to require more 

 than a passing notice in this general survey. 

 (See CEPHALOPODA.) 



Arrived at the vertebrate division of the ani- 

 mal series, we at once find the moving powers 

 assuming a complexity of arrangement and 

 precision of action, proportional to the elabo- 

 rate construction of the internal osseous, or car- 

 tilaginous skeleton, which now forms the frame- 

 work of the body, and must be regarded as 

 entering into the composition of several dis- 

 tinct systems of organs appointed to different 

 offices, and physiologically independent of 

 each other. Each of these systems, or sets of 

 muscles, indeed, is developed for special pur- 

 poses, and so far are they from progres- 

 sively presenting themselves, in a gradually 

 improving condition, as we rise from lower to 

 more elevated orders of Vertebrate animals, 

 that the physiologist must be prepared to ex- 

 pect every irregularity in this respect ; impor- 

 tant organs, or sets of organs, that in the lowest 

 Vertebrata are found to be most elaborate and 

 complex in their structure, are not unfre- 

 quently either wholly or partially obliterated, as 

 we ascend the scale of animal life, and others 

 equally important to the animals possessed of 

 them, are only met with in certain races, that 

 are endowed with peculiar habits or capabi- 

 lities. 



But, what is still more startling to the ana- 

 tomist, who has confined his dissections to the 

 examination of the muscular system as it exists 

 in mature or complete animals, and has con- 

 sequently been accustomed to describe as being 

 permanent and invariable the origins and in- 



sertions of every muscle, that he meets with, 

 the study of embryogeny reveals to the philo- 

 sophical enquirer a series of changes in pro- 

 gress, as relates to the arrangement or even the 

 existence of various parts of the animal eco- 

 nomy, involving changes as remarkable, in all 

 the muscular apparatus connected therewith. 



In order, therefore, fully to lay before the 

 reader, with as much brevity as perspicuity 

 will allow, phenomena so important as those 

 which next offer themselves to our notice, it 

 will be advisable, first, to enumerate the prin- 

 cipal systems of muscles that enter into the 

 composition of a completely formed Vertebrate 

 creature, premising that each may be but feebly 

 developed in proportion to the rest, and many 

 of them, indeed, absolutely wanting in a given 

 animal, and afterwards to examine separately 

 the varieties of arrangement met with in the 

 animal series in relation to each, and likewise 

 the metamorphoses that accompany embryonic 

 development. 



Without overburdening with detail this in- 

 teresting enquiry, or unnecessarily multiplying 

 divisions of the muscular apparatus, we shall 

 content ourselves with grouping all the muscles 

 of a Vertebrate animal, as belonging to one or 

 other of the following systems, each of which 

 will demand separate examination. 



1. Vertebral system, muscles directly acting 

 upon the spine arid cranial vertebrae. 



2. Costal system, muscles moving the ribs 

 and parietes of the thorax and abdomen. 



3. Hyoid system, muscles acting upon the 

 os hyoides and branchial arches. 



4. Opercular system, muscles moving the 

 operculum of fishes. 



5. Muscles of the limbs. 



6. Muscles acting upon the lower jaw and 

 serving for mastication. 



7. Teg u mentary system, muscles acting upon 

 the skin and its appendages. 



8. Vocal system, muscles of the voice. 



9. Diaphragm. 



10. Lingual system. 



1 1 . Ocular system, muscles moving the eye- 

 ball and its appendages. 



12. A ural system, muscles acting upon the 

 ossicles of hearing and moving the external ear. 



13. Nusal system, muscles acting upon 

 moveable parts of the nose. 



14. Generative system, muscles attached to 

 the apparatus of generation. 



1. The muscular apparatus peculiarly appro- 

 priated to the movements of the vertebral chain 

 of bones presents its maximum of developement 

 in the osseous fishes, in which animals loco- 

 motion being principally accomplished by the 

 lateral sweepings of the broadly expanded ver- 

 tical tail, every arrangement has been made to 

 increase the depth of the spinal column and to 

 extend the surface presented by the superior 

 and inferior spinous processes to the greatest 

 possible degree, not only by lengthening inor- 

 dinately those processes themselves, but like- 

 wise by appending to their extremities addi- 

 tional pieces deiived apparently from the 

 exo-skeleton. The muscles destined to act 

 upon the flexible spine of the fish are propor- 



