LOCOMOTION OF FISHES. 251 



tympanic pedicle of Osseous Fishes into several partly ovcrlap]>ing 

 ])ieccs adds to its streno-th^ and by permitting a slight elastic 

 Ijending of the whole diminishes the liahility to fracture. The 

 enormous size, moreover, of the tympano-mandibular arch, and of 

 its diverging appendages, contributes to ensure that proportion of 

 the head to the ti'unk which is best adapted for the progressive 

 motion of the fish through the water. But without the admission 

 and appreciation of these pre-ordained adaptations to special exi- 

 gencies in the skeleton of Fishes, the superior strength and 

 complex developement of the tympanic pedicle and its ajipendages 

 Avould be inexplicable and unintelligible in this lowest and first 

 created class of Vertebrate animals. 



All writers on Animal Mechanics have shown how admirably 

 the whole form of the fish is adapted to the element in which it 

 Ha'cs and moves : the viscera are packed in a small comjiass, in a 

 canity brought forwards close to the head ; and whilst the conse- 

 quent abrogation of the neck gives the advantage of a more fixed 

 and resisting connection of the head to the tnmk, a greater pro- 

 portion of the trunk behind is left free for the developement and 

 allocation of the muscidar masses which are to move the tail. In 

 the caudal, which is usually the longest, portion of the trunk, 

 transverse processes cease to be developed, whilst dermal and 

 intercalary spines shoot out from the middle line above and below, 

 and give the A'crtically extended, compressed form, most efficient 

 lor the lateral strokes, by the rapid alternation of which the fish 

 is propelled forwards in the diagonal, Ijetwccn the direction of 

 those forces. The advantage of the bi-concave form of vertebra 

 with intervening elastic capsules of gelatinous fluid, in effecting a 

 combination of the resilient with the muscular j^iowcr, is still more 

 obvious in the Bony Fishes than in the Shark. 



The normal character of Ichthyic myology shows itself in the 

 ■\-ast proportion of the vegetatively-repeated myocommas, corre- 

 sponding with the vertebral segments, as compared with the 

 su})eradded system of muscles subservient to the action of their 

 diverging appendages : but this condition, which, inasmuch as it 

 deviates so little from the fundamental type, throws so nnich light 

 upon the essential nature and homologies of the muscles of the 

 Vertelu-ata, is not less admirably and expressly adapted to the 

 habits and medium of existence of the Fish. Tlie interlocked 

 myocommas of the trrmk constitute, physiologically, two great 

 lateral muscular masses, adapted by their attachments, and 

 especially by those of the anterior and posterior ends, to bend 

 vio'orously from side to side, with the whole force of their alter- 



