414 DISSECTION OF THE PIGEON 



3. The structure of the eye. 



Place the eye under water, and divide it by an equatorial 

 incision into two halves. 



The general structure of the eye is the same as 

 that of other vertebrates ; but the following points 

 are of special interest. 



a. The pecten is a vascular pigmented fold, which pro-. 



jects into the cavity of the eye from its orbital 

 surface, ventral to the entrance of the optic 

 nerve. It is folded longitudinally like a fan ; 

 and its function is unknown. 



b. The lens is, as in terrestrial animals generally,' 



much flatter than that of the dog-fish. 



c. The sclerotic plates are a ring of small flat bones, 



overlapping one another, and surrounding the eye, 

 just behind the junction of the cornea and the 

 sclerotic. 



B. The Ear. 



1. The tympano-Eustaehian passag^e. 



This is the modified by o- mandibular cleft of the 

 embryo, corresponding to the spiracle of the dog-fish. 

 Its connection with the ear is of a purely secondary 

 character, and it forms no part of the essential organ 

 of hearing. 



a. The external auditory meatus is the outer part of 



this passage, from the exterior to the tympanic 

 cavity. The external aperture is a circular hole 

 on the side of the head, behind and below the 

 posterior border of the orbit. 



Cut awa/y the lower jaw. Gut away the external meatus 

 carefully with scissors, so as to expose the tympanic mem- 

 brane. 



b. The tympanic membrane is a thin transparent 



septum, separating the external meatus from the 

 tympanic cavity. 



