PROPERTIES OF NERVOUS SYSTEM. 227 



tricity, or any other similar agent, or upon the organs of special sense 

 (the eye and ear, the nose and tongue), by light or sound, by odorous 

 or sapid bodies, these impressions, in the healthy and wakeful state of 

 the Nervous system, are felt as sensations ; that is, the mind is rendered 

 conscious of them. Now there can be no doubt that the mind is imme- 

 diately influenced, not by the impression in the remote organ, but by a 

 certain change in the condition of the brain, excited or aroused by that 

 which has originated elsewhere. For if the communication with the 

 brain be cut oft', no impression on the distant parts of the nervous system 

 is felt, notwithstanding that the mind remains perfectly capable of re- 

 ceiving it. The mind, then, is only rendered conscious of external objects, 

 by the influence which they exert upon the brain, or upon a certain part 

 of it, which, being the peculiar seat of sensation, is called the sensorium. 

 Hence we recognise, in the process by which the mind is rendered con- 

 scious of external objects, three distinct stages ; first, the reception of 

 the impression at the extremities of the sensory nerve ; second, the 

 conduction of the impression, along the trunk of the nerve, to the 

 sensorium ; third, the change excited by it in the sensorium itself, 

 through which sensation is produced. Here, then, the change in the 

 condition of the nervous system commences at the circumference, and 

 is transmitted to the centre ; and the fibres which are concerned in this 

 transmission are termed sensory. 



391. On the other hand, when an emotion, an instinctive impulse, or 

 an act of the will, operates through the central organs to produce a 

 muscular contraction, the first change is in the condition of the vesicular 

 substance of those organs. The influence of this change is transmitted 

 by the motor nerves to the muscles, among which they are distributed ; 

 and the desired movement is the result. Here, too, we have at least 

 three stages ; first, the origination of the change by an impression act- 

 ing on the central organ ; second, the conduction of that change along 

 the motor nerves ; and third, the stimulation of the muscles to contrac- 

 tion. But the operation here commences at the centre ; and the effects 

 of the change in the brain are transmitted to the circumference, by a 

 set of nervous fibres which are termed motor. The complete distinct- 

 ness of these two classes of fibres was first established by Sir C. Bell. 

 It is best seen in the nerves of the head, of which some are purely sen- 

 sory, and others purely motor ; but it may als6 .be clearly proved to 

 exist at the roots of the spinal nerves (although their trunks possess 

 mixed endowments), the posterior being sensory, wjailst the anterior are 

 motor. 



392. But although sensations can only be felt through the brain, and 

 voluntary motions can only be produced by an action of the mind through 

 the same organ, yet there are many changes in the animal body, in 

 which the nervous system is concerned, which yet do not involve the 

 operation of the brain, being produced without our consciousness being 

 necessarily excited, and without any act of the will, or even in opposi- 

 tion to its efforts. Of these actions, the spinal cord of Yertebrata, and 

 its prolongation within the cranium, are the chief instruments ; in the 

 Invertebrate animals, they are performed by various ganglia, which are 

 usually disposed in the neighbourhood of the organs to which they 



