342 ANATOMY OF VERTEBRATES. 



depressor muscle, o. Of the muscles of the eyeball, p marks the 

 rectus superior ; q the rcctus inferior ; r the rectus externus ; a 

 the obliquus inferior : the rectus interims and obliquus superior 

 are likewise present. The letter x marks the insertion of the 

 choanoid muscle or retractor of the eyeball, which consists of four 

 portions surrounding the optic nerve, v. Counting these with the 

 other muscles of the eyeball and lids, there are not fewer than 

 thirteen ; and the eye of the Crocodile has its special skeleton as 

 well as muscles, represented by the super-palpebral ossicle. 



In both lieptiles and Fishes the range of gradations of dioptric 

 structures is very great ; and the number of species in which the 

 eye is a mere passive recipient of the stimulus of light, and unfit 

 for sight, or the discernment of outward objects, is greater in the 

 air-breathing than in the water-breathing Hcematocrya. 



66. Organ of Hearing in Fishes*- -The cartilaginous capsules 

 of the acoustic organs are precociously developed in all Fishes : in 

 the Myxinoids and Ammocetes they retain their primitive exterior 

 position at the sides of the base of the proper cranium, fig. 58, 16; 

 they are less conspicuous in the Lamprey, fig. 60, 16 ; they 

 become involved in the thick cartilaginous walls of the cranium in 

 the Plagiostomes ; and, in Osseous Fishes, are walled up exter- 

 nally either by the surrounding cranial bones, or by a special 



227 



Otocrane and labyrinth of Perch, xxiu. 



ossification of the exterior part of the capsule itself, forming an 

 6 os petrosum/ as e. g. in the Carp, fig. 83, 16, and Perch, figs. 

 85, 84, 16. In the dry skull the ear-chamber appears as a large 

 lateral compartment of the cranial cavity, fig. 227, o ; and is 

 formed as described in p. 115. 



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,' 



