PSYCHIC NERVE-CENTRES. 305 



which carry impulses to the centres and those which carry 

 them out from the centres. The former are called afferent 

 or sensory fibres and the latter efferent or motor. 



The posterior roots of the spinal nerves contain only 

 afferent, the anterior only efferent, nerve-fibres. 



Classification of Nerve-Centres. Nerve-centres are of 

 three kinds: (1) Automatic centres, which, without being 

 excited by the action of any sensory nerve or by the Will, 

 originate in themselves stimuli for efferent nerves. (2) Re- 

 flex centres, which act quite independently of the Will and 

 of consciousness, but are aroused by the action of a nervous 

 impulse conveyed to them by a sensory nerve, and in turn 

 excite one or more efferent nerves. (3) Conscious or psy- 

 chic centres, whose activity, however aroused, is accom- 

 panied by some kind of mental activity; as feeling, or 

 willing, or reasoning. 



The Psychic Nerve-Centres lie in the fore-brain, and 

 mainly in the gray matter of its convolutions. If the cere- 

 bral hemispheres of a pigeon be destroyed and all the rest of 

 its body left intact, the animal can still control its muscles 

 so as to execute many movements, but it gives no sign of 

 consciousness. Left to itself it will stand still until it dies; 

 corn and drink placed before it arouse in it no idea of eat- 

 ing; it will die of starvation surrounded by food. Yet it 

 can move all its muscles, and if food is placed in its mouth 

 will swallow it. If its tail be pulled it will walk forward; 



What fibres are found in the posterior spinal nerve-roots? What 

 in the anterior? 



Name the main varieties of nerve-centres. What is done by 

 automatic centres? What by reflex? What is the characteristic 

 property of the psychic centres? 



Where are the psychic nerve-centres located? What may be ob- 

 served in a pigeon whose cerebral hemispheres have been destroyed? 



