262 THE ANATOUy OF VERTEBRATED ANIMALS. 



this muscle necessarily tends to draw the tendon of the^y- 

 ramidalis away from the optic nerve. A tubercle is some- 

 times developed from the sclerotic above the entrance of the 

 optic nerve, and prevents the tendon of the jiyramidalis from 

 shifting forward and inward. 



The eyeball is always moved by iour recti and two obliqui. 

 The superior oblique does not pass over a pulley. The Ghe- 

 lonia and most Lacertllla have a more or less completely de- 

 veloped retractor, or.choanoid, muscle. 



A ring formed of bony plates is developed in the fore-part 

 of the sclerotic in Ziaoertilia, Ghelonia, Ichthyosauria, Dl- 

 cynodontia, Pterosauria, and Aves, but not in Ophidia, Plesi- 

 osauria, or Crocodllia. 



The iris and the tensor choroidel contain striated muscu- 

 lar fibres. 



A pecten is very generally developed. It attains a large 

 size, and becomes much plaited, in most Aves. 



Only Crocodllia and ^yes possess a rudiment of an exter- 

 nal ear. 



The OpJiidia and the AinpMsboenoidea have no tympanic 

 cavity. In some Ghelonia, in Sphenodon, and in the Chamrc- 

 leons, the tympanic membrane is covered by the integument, 

 but a tympanic cavity exists. In Ziacertilia, the tympanic 

 cavities communicate by wide openings with the pharynx ; 

 but in Ghelonia, Grocodilia, and Aves, the communicating 

 passages, reduced in size, become Eustachian tubes. In the 

 Ghelonia, these curve backward, downward, and inward, 

 round the quadrate bones, and open separately on the roof of 

 the mouth. In the Grocodilia there are, ;is has been described 

 above (p. 319), three Eustachian tubes — one median and two 

 lateral. In Aves, there is but one Eustachian aperture, an- 

 swering to the median of the Grocodilia; and, as in the latter 

 group, each Eustachian tube usually traverses the osseous 

 base of the skull, to join with its fellow in the common aper- 

 ture. 



The stapes is a columelliform bone, the outer end of which 

 is attached to the tympanic membrane, where the latter is 

 developed ; but lies among the muscles when there is no tym- 

 panic cavity. 



All Sauropsida possess a fenestra rotunda, as well as a 

 fenestra ovaUs, and all have a cochlea, which is never coiled 

 spirally, and is more rudimentary in the Ghelonia than in 

 other groups. Three semicircular canals, an anterior and 



