456 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 foUowing 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 Uke 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-Eustachian passage. 



This is the modified hyo-mandibular cleft of the 

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

 Its connection with the ear is of a purely secondary 

 character. It serves to keep the air in the tympanic 

 cavity at the same pressure as outside. 



a. The external auditory meatus is the outer part of 



this passage, from the exterior to the tympanic 

 membrane. The external aperture is a circular 

 hole on the side of the head, behind and below 

 the posterior border of the orbit. 



Gut away the lower jaw. Cut aiuay 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. 



