DR KNOX on the Comparative Anatomy of the Eye. 53 



the base of the iris with the anterior edge of the annulus albus, 

 and which may either be considered as a peculiar body, or a por- 

 tion of the choroid tunic immediately subjacent : this dark colour- 

 ed body forms the inner surface of a cavity, which will be more 

 minutely described afterwards : lastly, the annulus albus itself con- 

 nected posteriorly with the outer layer of the choroid coat. The 

 anatomy of these parts shall be described more minutely whilst 

 speaking of the various classes of Animals. - 



As it is in birds that some of these parts are most distinctly 

 and readily made out, I shall here briefly describe the general 

 anatomy, drawn from very numerous specimens, of almost all the 

 natural families of that class ; but I shall dwell more particular- 

 ly on the eye of the cassowary, not because there is in it any pe- 

 culiarity, but because the magnitude of the eye-ball permits the 

 minutest parts to be satisfactorily demonstrated. Near the pos- 

 terior edge of the annulus albus, the choroid is firmly fixed all 

 round the eye-ball to the inner layer of the sclerotic. At this 

 point, the external layer of the choroid may be considered either 

 as terminating, or what, in favourable specimens, may be partly 

 demonstrated, as passing forward united to the inner layer, to 

 form the ciliary body. Anterior to this is another adhesion, be- 

 tween the anterior expansion of the annulus albus and the scle- 

 rotic ; this adhesion may be supposed to proceed onwards, and 

 to terminate in the cornea by a thin broad membrane, gene- 

 rally of a dark colour. 



Anterior to the principal insertion of the annulus albus into 

 the sclerotic, and lying on a strong membranous expansion, 

 stretching between the annulus albus and cornea, are found the 

 ciliary nerves, which here resemble a plexus, being broad, large, 

 and ribbon-like ; they completely surround the eye-ball, and 

 send numerous distinct branches to the iris and to the annulus 

 albus *. I had already remarked, during the course of my dissec- 



* See Plate III.Fig. 1. 



