262 THE ANAT0M7 OF VERTEBEATED ANIMALS. 



this muscle necessarily tends to draw tlie tendon of the py- 

 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 pyramidalis from 

 shifting forward and inward. 



The eyeball is always moved by four recti and two ohliqui. 

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

 Ionia and most Lacertilia 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 Lacertilia^ Ghelonia^ Ichthyosauria^ Di- 

 cynodontia, JPterosauria^ and Aves, but not in Opliidia^ Plesi- 

 osauria^ or Crocodllia. 



The iris and the tensor choroidei 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 Aves possess a rudiment of an exter- 

 nal ear. 



The Ophidia and the Ainphishoenoidea have no tympanic 

 cavity. In some Chelonia, in Sphenodon, and in the Chamne- 

 leons, the tympanic membrane is covered by the integument, 

 but a tympanic cavity exists. In Lacertilia^ the tympanic 

 cavities communicate by wide openings with the pharynx ; 

 but in Chelonia^ Crocodllia^ and Aves, the communicating 

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

 Chelonia, these curve backward, downward, and inward, 

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

 the mouth. In the Crocodilia there are, as has been described 

 above (p. 219), three Eustachian tubes — one median and two 

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

 swering to the median of the Crocodilian and, as in the latter 

 grouj), 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 bene, 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 ovalis, and all have a cochlea, which is never coiled 

 spirally, and is more rudimentary in the Chelonia than in 

 other groups. Three semicircular canals, an anterior and 



