CHAPTER XI. 



CHEMICAL HISTORY OF CERTAIN OF THE PERIPHERAL 

 TERMINATIONS OF THE NERVOUS SYSTEM AND OF 

 THE ACCESSORY STRUCTURES CONNECTED WITH 

 THEM, THE TISSUES AND MEDIA OF THE EAR, 

 THE TISSUES AND MEDIA OF THE EYE. 



Introduc- DIRECTLY or indirectly all the nerve fibres of the 



tor y- organism are connected centrally with nerve centres, of 



which we have examined the chemical history, so far as it is at 

 present knovva to us. Peripherally nerve fibres either commence in 

 certain special end-organs capable of being influenced by movements 

 in the external medium and of transmitting the influence through 

 the nerves to the nerve centres (afferent nerve fibres), or they ter- 

 minate in structures of which the immense majority are concerned in 

 bringing about changes in the position of different organs of the body, 

 and changes in the relation of the organism to the medium which it 

 inhabits (efferent nerve fibres}. Fibres of the latter class terminate by 

 peculiar end-organs in the contractile tissues which have formed the 

 subject matter of Chapter IX. 



In the present chapter there remains to be discussed the chemi- 

 cal history, so far as it is known, of the peripheral nervous end-organs 

 which are connected with afferent nerves, though unfortunately it is 

 only in connection with the eye that any detailed information is 

 available. For reasons of expediency we shall consider not merely the 

 chemical facts relating to the actual nervous structures, but also those 

 relating to the accessory apparatus with which they are connected. 



SECT. 1. THE TISSUES AND MEDIA OF THE EAR. 



The organ of hearing of vertebrates, reduced to its simplest form, 

 consists of a membranous sac of greater or less complexity, termed the 

 membranous labyrinth, on the inner surface of which are situated 



