306 THE ANATOMY OF VERTEBEATED ANIMALS. 



from the wall of the orbit, but from the upper surface 

 01 the sclerotic itseK, whence it passes backwards and 

 ends in a fibrous sheath which incloses the tendon of 

 the pyramidalis. The contraction of this muscle necea- 

 saiily tends to draw the tendon of the pyramidalis away 

 from the optic nei-ve. A tubercle is sometimes developed 

 from the sclerotic above the entrance of the optic nerve, 

 and prevents the tendon of the pyramidalis from shifting 

 foiTvards and inwards. 



The eyeball is always moved by four recti and two 

 obliqui. The superior oblique does not pass over a pulley. 

 The Chelonia and most Lacertilia have a more or less com- 

 pletely developed retractor, or choanoid, muscle. 



A ring formed of bony plates is developed in the fore 

 part of the sclerotic in Lacertilia, Chelonia, Ichthyosauna, 

 Dicynodontia, Pterosauria and Aves, but not in Ophidia, 

 Plesiosauria, or Crocodilia. 



The iris and the tensor clioroidei contain striated mus- 

 cular fibres. 



A pecten is very generally developed. It attains a large 

 size, and becomes much plaited, in most Aves. 



Only Crocodilia and Aves possess a rudiment of an exter- 

 nal ear. 



The Ophidia and the Amphisbcenoidea have no tympanic 

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

 leons, the tympanic membrane is covered by the integu- 

 ment, but a tympanic cavity exists. In Lacertilia, the 

 tympanic cavities communicate by wide openings with the 

 pharynx; but in Chelonia, Crocodilia, and Aves, the com- 

 mimicating passages, reduced in size, become Eustachian 

 tubes. In the Clidonia, these curve backwards, downwards, 

 and inwards, round the quadrate bones, and open separately 

 on the roof of the mouth. In the Crocodilia there are, as 

 has been described above (p. 2-55), three Eustachian tubes 

 — one median and two lateral. In Aves, there is but one 

 Eustachian aperture, answering to the median of the Croco- 

 dilia ; and, as in the latter group, each Eustachian tube 



