PART III. 

 FUNCTIONS OF THE NERVOUS SYSTEM. 



NERVES. 



THE organs of the nervous system of which the functions 

 are known are (i) Reflex centres ; (2) End-organs ; and 

 (3) Conducting organs. The most important conducting 

 organs are nerves. A nerve is made up of fibres, each of 

 which consists of axis cylinder, medullary sheath and nu- 

 cleated sheath. The medullary sheath is divided into 

 lengths by septa at equal intervals, but the axis cylinder 

 is continuous. The axis cylinder consists chiefly of pro- 

 teid, the medullary sheath of material for the most part 

 soluble in ether. 



Living nerve exhibits in itself three properties which 

 appear to be characteristic (i) that when injured so as 

 to produce solution of continuity of its fibres, the injured 

 part is electrically negative to the uninjured, (2) that 

 when a nerve is excited, this electrical property is modified, 

 the modification thus produced characterising the state of 

 excitation, and (3) that this state can be propagated along 

 the fibre in both directions. With reference to the state 

 of excitation, two inferences are allowable, viz., (i) that the 

 electrical change exists in uninjured nerves, although it is 

 imperceptible, and (2) that it is associated with a chemical 

 change. 



The state of excitation is capable of being propagated 

 from the nerve originally excited to excitable end-organs, 

 namely, in the case of efferent nerves, to muscle or gland, 

 and in the case of afferent nerves to centres. In this way 



