﻿IlS ELEMENTARY BIOLOGY. [CHAP. 



y. The retina ; a thin filmy layer, greyish and 

 transparent, internal to p. It stops short at 

 the base of the iris (ora serratd] ; internally it 

 passes into the optic nerve. 



If the vitreous humour be much disturbed this 

 layer will be puckered and torn, or otherwise 

 displaced. 



S. The conjunctiva and eyelids (cf. Sect. C. 3. <-/.). 

 The former can only be distinguished from the 

 cornea with difficulty. 



Section b. 



i. Examine the inner half from within, under water. 



a. The retina. Gently raise it ; it will be found to 

 adhere firmly at the point of entrance of the 

 optic nerve (blind spot}. Note the position of 

 same. 



/?. The choroid ; strip off the whole retina and ex- 

 amine this, noting its texture and great vas- 

 cularity. 



y. The sclerotic; examine this in like manner, 

 after having removed fi. 



ii. Examine the outer half from within, and note 

 especially the relations of the lens and iris. 



4. The auditory organ. 



a. Examine the tympanic membrane from the outside 

 (cf. Sect. A. a. (3.). Dissect off its outer or 'tegu- 

 mentary layer'; there will thus be brought into 

 view : 



a. The annul us tympanicus ; a cartilaginous rin 

 supporting the edge of the membrane. 



