NER VE-IMP ULSES. 67 



cerebral ganglia, break up into many branches, and are dis- 

 tributed to the anterior part of the body. 



Besides the main ganglia of the central system, there are many smaller 

 ganglia in various parts of the body. Of these the most important are the 

 pharyngeal ganglia 3 to 5 in number which lie on the wall of the 

 pharynx on each side just within the pharyngeal collar. They are con- 

 nected with the latter by fine branches, and send minute nerves out upon 

 the walls of the pharynx. This series of ganglia is often inappropriately 

 called the sympathetic system. 



Physiology of the Nervous System. Nerve - impulses. 

 What is the origin and nature of a nerve-impulse? Under nor- 

 mal conditions the impulse is set up as the result of some dis- 

 turbance, technically called a stimulus, acting upon the end of 

 the fibre. A touch or pressure upon the skin, for example, acts 

 as a stimulus to the nerve-fibres ending near the point touched 

 that is, it causes nerve-impulses to travel inwards along the fibres 

 towards the central system. The nerves may be stimulated by 

 a great variety of agents : by mechanical disturbance, as in the 

 case just cited, by heat, electricity, chemical action, and in 

 special cases by waves of light or of sound, and upon this prop- 

 erty of the nerves depends the power of the worm to receive as 

 afferent impulses impressions from the outer world. But, besides 

 this, nerve-fibres may also be stimulated by physiological changes 

 taking place within the nerve-cells, which may thus send out 

 efferent impulses to the various organs and so control their ac- 

 tion. 



Regarding the precise nature of the nerve-impulse we are ignorant, but 

 it is probably a chemical or molecular change in the protoplasm, travelling 

 rather rapidly along the fibre, like a wave.* We know that the nature 

 of the impulse is not in any way dependent upon the character of the stimu- 

 lus. The stimulus can only throw the nerve into action ; and this action 

 is always the same whatever be the stimulus as the action of a clock 

 remains the same whether it be driven by a weight or by a spring. 



Co-ordination. The activities of the various organs are co- 

 ordinated by a chain of events which in its simplest form is known 

 as a reflex action, and which lies at the bottom of most of 

 the more complicated forms of nervous action. Its nature is 



* In the frog the nervous impulses travel at the rate of about 28 metres per 

 second ; in man it is considerably more rapid. 



