312 



ANATOMY OF VERTEBRATES. 



depressor muscle, o. Of the muscles of the eyeball, p mai'ks the 

 rectus superior ; q the rectus inferior ; r the rectus externus ; s 

 the obliquus inferior : tlie rectus internus 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 eyeljall 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 Keptiles and Fislies the range of gradations of dioptric 

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

 eye is a mere passive recii>ient of the stimulus of light, and uniit 

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

 air-breathing tlian in the water-breathing H<zmatocrya. 



§ 66. Or<jan of Hcarbuj 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 

 ]iositi(jn at the isides of the base of the proper cranium, fig. 58, lo; 

 they arc less cons])icuous in the Lamprey, fig. 60, 16 ; they 

 l)CComc involved in the tliick cartilaginous walls of the cranium in 

 the Plaglostomes ; and, in Osseous Fishes, are Availed up exter- 

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



Otocranc aiij labjriiilh tif rt'i-rh. xxiir. 



ossification of the exterior part of the ca]isule itself, forming an 

 ' OS pctrosum,' as c. g. in tlie Carp, fig. 83, i6, and Perch, figs. 

 85, 84, 10. In the dry skull the ear-chamber appears as a large 

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

 formed as described in p. 115. 



In the INIyxmiiids tlie membranous labyrinth is a simple annular 

 tube, lined by vibi'atile cilia, filled with fluid, and supporting 

 the ramifications of the acouslic uerAc. In tlie Ammocete and 

 Lam]M-ey the labyrintli is specially attached to its cartilaginous 

 capsule, and consists of a ' vestibule ' and two ' semicircular Janals,' 



