LOCOMOTION OF FISHES. 251 



tympanic pedicle of Osseous Fishes into several partly overlapping 

 pieces adds to its strength, and by permitting a slight elastic 

 bending of the whole diminishes the liability 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 trunk 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 appendages 

 would be inexplicable and unintelligible in this lowest and first 

 created class of Vertebrate animals. 



All writers on Animal Mechanics have shown how admirably 

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

 lives and moves : the viscera are packed in a small compass, in a 

 cavity 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 trunk, a greater pro- 

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

 allocation of the muscular 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 vertically extended, compressed form, most efficient 

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

 is propelled forwards in the diagonal, between 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 power, is still more 

 obvious in the Bony Fishes than in the Shark. 



The normal character of Ichthyic myology shows itself in the 

 vast proportion of the vegetatively-repeated myocommas, corre- 

 sponding with the vertebral segments, as compared with the 

 superadded 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 much light 

 upon the essential nature and homologies of the muscles of the 

 Vertebrata, is not less admirably and expressly adapted to the 

 habits and medium of existence of the Fish. The interlocked 

 myocommas of the trunk constitute, physiologically, two great 

 lateral muscular masses, adapted by their attachments, and 

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

 vigorously from side to side, with the whole force of their alter- 



