SENSATION. 



509 



the mind be unimpaired. But unless the 

 .mind is conscious of the irritation excited 

 we cannot say that a sensation has taken 

 place. The person on whom the injury is 

 inflicted may be comatose, or in a profound 

 sleep, or under the influence of intoxicating 

 or anaesthetic agents, and consequently his 

 perceptive powers are in abeyance. Never- 

 theless, the same physical changes take place, 

 whatever be the state of the mind, and all the 

 physical phenomena, which may flow from or 

 succeed to those which are capable of excit- 

 ing sensation, may ensue upon them, and yet 

 true sensation will not take place, unless the 

 mind perceives and takes cognisance of the 

 physical change induced. 



It must then be regarded as a cardinal 

 point in reference to the acceptation of the 

 term Sensation in Physiology, that an action 

 of the mind is necessarily involved, that act 

 being of the nature of a recognition or per- 

 ception of the physical changes associated 

 with the sensation. 



The true organ of sensation is the organ of 

 the mind the brain, and especially that part 

 of the brain which constitutes the centre of sen- 

 sation, and which extends into the spinal cord, 

 forming the posterior horn of its grey matter. 

 When an impression is made upon a nerve or 

 nerves which communicate directly or indi- 

 rectly with any part of this centre, a sensation 

 is excited, provided the intracranial portion 

 of it be in a normal state, and provided also 

 the connection between the cranial and spinal 

 portions be complete and uninterrupted. 



Sensations depend, as to their nature, on 

 that of the excitant, and nerves are adapted 

 to receive impressions from various agents, 

 ponderable or imponderable. The mechanical 

 qualities of bodies, heat, cold, electricity, light, 

 sound, &c., are capable of exciting their ap- 

 propriate sensations, which the mind soon 

 learns to appreciate and distinguish. Sensa- 

 tions thus distinguished receive the appella- 

 tion of pleasurable or of painful, according as 

 they are agreeable or the reverse. These 

 sensations are infinitely varied in kind and in 

 degree. It is impossible, a priori, to deter- 

 mine how a pleasurable or a painful sensation 

 may be excited. Nor will the experience of 

 one person be always a guide for another, in- 

 asmuch as a sensation which may be agree- 

 able to one, may be painful or disagreeable 

 to another. 



Physiologists distinguish sensation as com- 

 mon and special : the former being that which 

 is excited by ordinary mechanical or chemical 

 stimuli ; the latter is excited by special sti- 

 muli, and is exemplified in the special senses 

 of vision, hearing, smell, taste, and touch. 

 The nerve of vision does not, when irritated, 

 communicate simply a feeling of pain or of 

 pleasure ; its chief effect is to excite the sen- 

 sation of a flash of light. When the electric 

 stream passes through the retina, a sensation 

 is caused similar to that which the sudden 

 presentation of a luminous object would pro- 

 duce. In like manner the mechanical or 

 electrical stimulation of the other nerves of 

 pure sense will create, not pain, but a feeling 



closely allied to that which would be excited 

 by the application of the stimulus proper to 

 each. This is remarkably illustrated by the 

 effects of mechanical or electrical stimulation 

 of the nerve of hearing and of the nerve of 

 taste. Mechanical impulses against the tym- 

 panum occasion the sense of a dull sound, 

 and the electric current developes a musical 

 note. Galvanic excitation of the gustatory 

 papillae of the tongue causes a peculiar sour 

 taste, and, as Dr. Baly has pointed out, the 

 mechanical stimulation of them by a sharp 

 tap with the fingers, occasions a taste some- 

 times acid, sometimes saline. 



The nerves which minister to specia 

 sensation, differ from the nerves of common 

 sensation in no essential point of their ana- 

 tomy, except in their mode of organisation 

 at the periphery of the body. Each of them 

 has, probably, likewise some peculiarity of 

 connection with the brain : this is obvious as 

 regards the olfactory and the optic nerves ; 

 less so as regards the nerves of taste, touch, 

 and hearing. The physiological peculiarity of 

 these nerves is then, in all probability, due to 

 their central and peripheral organisation ; and 

 especially, perhaps, to the latter, which, doubt- 

 less, renders them peculiarly susceptible of the 

 influence of those delicate physical agencies 

 to which each of them is exposed. 



The nerves and organs of special sensa- 

 tion, especially those of touch, are so com- 

 prehensive in their objects, that it would 

 almost seem that little was left for the so- 

 called nerves of common sensation. 



These latter nerves, nevertheless, serve 

 many important objects ; they doubtless ex- 

 cite in the mind many feelings, agreeable or 

 disagreeable, of pain or of pleasure, or even 

 feelings neutral as regards pain and pleasure, 

 which could not be developed through the 

 nerves of special sense. The consciousness of 

 the integrity of our limbs and of the general 

 framework of our bodies, is secured, in a great 

 measure, through the instrumentality of these 

 nerves. Injuries to various parts disturbances 

 in their nutrition, as inflammations, ulcera- 

 tions, &c. are made known to the mind by 

 the painful sensation excited through these 

 nerves. The sensibility of organs and textures 

 i. e. the degree to which affections of these 

 parts are capable of inducing corresponding 

 affections of the mind depends upon the 

 number of these nerves which are distributed 

 to them the degree of sensibility being in 

 proportion to the number of the nerves. 

 Hence these nerves of common sensation ex- 

 ercise a conservative influence over the several 

 textures and organs to which they are dis- 

 tributed, and serve to afford warning of the 

 approach or of the existence of danger. 



What some have called the muscular sense, 

 i. e. the knowledge which we have of the state 

 of our muscles, is generally attributed to these 

 same nerves. As the sensibility of the muscles 

 is doubtless due to these nerves, we may 

 reasonably impute to them the faculty of 

 informing the mind of the state and degree of 

 contraction or relaxation of the muscles, and 

 thus of contributing to that power of adjust- 



