cxxxii NERVOUS SYSTEM. 



iial stimulus, and do not manifest themselves in outward acts. To this class 

 also belong sensation and volition. In the exercise of sensation the mind be- 

 comes conscious, through the medium of the brain, of impressions conducted 

 or propagated to that organ along the nerves from distant parts ; and in 

 voluntary motion a stimulus to action arises in the brain, and is carried 

 outwards by the nerves from the central organ to the voluntary muscles. 

 Lastly, emotion, which gives rise to gestures and movements varying with 

 the different mental affections which they express, is an involuntary state of 

 the mind, connected with some part of the brain, and influencing the 

 muscles through the medium of the nerves. 



The remaining functions of the nervous system do not imply necessary 

 participation of the mind. In the production of those movements, termed 

 reflex, excited, or excito-motory, a stimulus is carried along afferent nerve- 

 fibres to the brain or spinal cord, and is then transferred to efferent or 

 motor nerve-fibres, through which the muscles are excited to action ; and 

 this takes place quite independently of the will, and may occur without 

 consciousness. The motions of the heart, and of other internal organs, the 

 contraction of the coats of the blood-vessels, as well as the invisible changes 

 which occur in secretion and nutrition, are in a certain degree subject to the 

 influence of the nervous system, and are undoubtedly capable of being modi- 

 fied through its agency ; though, with regard to some of these phenomena, it is 

 doubtful how far the direct intervention of the nervous system is necessary 

 for their production. These actions, which are all strictly involuntary, are, 

 no doubt, readily influenced by mental emotions ; but they may also be 

 affected through the nerves in circumstances which entirely preclude the 

 participation of the mind. 



The nervous system consists of a central part, or rather a series of con- 

 nected central organs, named the cerebro-spinal axis, or cerebro- spinal centre ; 

 and of the nerves, which have the form of cords connected by one extremity 

 with the cerebro-spinal centre, and extending from thence through the body 

 to the muscles, sensible parts, and other organs placed under their control. 

 The nerves form the medium of communication between these distant parts 

 and the centre. One class of nervous fibres, termed afferent or centripetal, 

 conduct impressions towards the centre, another, the efferent or centrifugal, 

 carry motorial stimuli from the centre to the moving organs. The nerves 

 are, therefore, said to be internuncial in their office, whilst the central organ 

 receives the impressions conducted to it by the one class of nerves, and im- 

 parts stimuli to the other, rendering certain of these impressions cognisable 

 to the mind, and combining in due association, and towards a definite end, 

 movements, whether voluntary or involuntary, of different and often of dis- 

 tant parts. 



Besides the cerebro-spinal centre and the nervous cords, the nervous 

 system comprehends also certain bodies named ganglia, which are connected 

 with the nerves in various situations. These bodies, though of much smaller 

 size and less complex nature than the brain, agree, nevertheless, with that 

 organ in their elementary structure, and to a certain extent also in their 

 relation to the nervous fibres with which they are connected ; and this cor- 

 respondence becomes even more apparent in the nervous system of the lower 

 members of the animal series. For these reasons, as well as from evidence 

 derived from experiment, but which is of a less cogent character, the ganglia 

 are regarded by many as nervous centres, to which impressions may be 

 referred, and from which motorial stimuli may be reflected or emitted ; but 

 of local and limited influence as compared with the cerebro-spinal centre, and 



