238 ELEMENTABY BIOLOGY. [XIII. 



if the water, in which the eye is being dissected, 

 be poured away. 

 /. Lining the posterior chamber of the eye is the 

 retina, which, from the action of the water, will 

 probably now be somewhat cloudy: in the un- 

 altered state it is perfectly transparent, allow- 

 ing the choroid (3. g) to be seen through it. 

 With the point of a microscope needle gently 

 raise the retina from the black membrane (cho- 

 roid) beneath it. It will be found that this can 

 be readily done except at one point (answering to 

 the bUjid spot of our own eyes), which, by turning 

 the eye-ball over will be seen to be opposite the 

 point of entrance of the optic nerve (2. /3). 



g. The choroid coat of the eye-ball is now exposed. 

 It is a dark pigmented membrane, of loose floccu- 

 lent texture, which can be readily detached, by 

 needles, from the sclerotic which lies outside it. 



b. The Ear. 



1. The frog has no external ear, its tympanic membrane, 

 as already mentioned (A. 2. a), being exposed on each 

 side of the head. 



a. Note the arrangement of the tympanic mem- 

 brane : it is smoothly stretched over a hard ring. 



b. Dissect away the outer or integumentary layer of 

 the tympanic membrane. Beneath it will be 

 found a transparent membrane, formed by the 

 iibrous and mucous layers, with an opaque white 

 patch about its middle. 



c. Cut through these layers of the tympanic mem- 

 brane along their margins: the tympanic cavity 

 will then be laid open. 



