NERVOUS SYSTEM OF FISHES. 207 



level with the side of the head, offers no impediment to those move- 

 ments. This form of cornea diminishes the capacity of the aqueous 

 chamber ; but the aqueous humour is needed only to float the free 

 boi-der of the iris ; and to make up for the small quantity of that 

 humour, the convexity and refractive power of the lens are increased. 

 To compensate for the deviation from the spherical form of the 

 eyeball produced by the flattening of its fore-part, and the con- 

 sequent weakening of the power to resist external pressure, the 

 sclerotic capsule is cartilaginous or bony. 



This beautiful chain of adjustments and interdependencies cannot 

 but I'aise the rightly constituted mind to the contemplation of the 

 attributes of that Creative Intelligence herein so strikingly displayed. 



Organ of Hearing. — The cartilaginous capsules of the acoustic 

 oi'gans are precociously developed in all flshes : in the Myxinoids 

 and Ammocetes they retain their primitive exterior position at the 

 sides of the base of the proper cranium (^Jig. 24. 16) ; they are 

 less conspicuous in the Lamprey {fig. 26. 16) ; they become in- 

 volved in the thick cartilaginous walls of the cranium in the Plagi- 

 ostomes ; and, in Osseous Fishes, are walled up externally either by 

 the surrounding cranial bones, or by a special ossification of the 

 exterior part of the capsule itself, forming an ' os petrosum,' as, e. g. 

 in the Cod {fig. 30. le). In the dry-skull the ear-chamber appears 

 as a large lateral compartment of the cranial cavity, and is formed as 

 described in p. 102. 



In the Myxinoids the membranous labyrinth is a simple annular 

 tube, lined by vibratile cilia, filled with fluid, and supporting the 

 ramifications of the acoustic nerve. In the Ammocete and Lamprey 

 the labyrinth is specially attached to its cartilaginous capsule, and 

 consists of a ' vestibule ' and two ' semicircular canals,' each of which 

 dilates, at its origin, into an ' amj^ulla,' which has some processes 

 from its inner surface. The two canals again communicate with the 

 vestibule, where they cross each other: the two divisions of the acoustic 

 nerve first surround the ampulla? before they spread over the rest of 

 the labyrinth. 



In all other Fishes the membranous labyrinth consists of a vesti- 

 bule and three semicircular canals ; the vestibule dilating into one 

 or more ' sacculi,' separated by a constriction, or by a narrow canal 

 fi-om the ' alveus communis,' and containing, besides the fluid called 

 ' endolymph/ two or more masses of carbonate of lime, called ' oto- 

 lites.' * These are compact and crystalline in Osseous Fishes. 



* Figures of these bodies will be found in i.xvi. iii. i)l. 35. j in lxxi. lxviii. and 

 in i.xxii., with microscopic figures of the crystals. 



