258 ox THE SENSES. 



nute images fall at two difiFerent places on the retina, a nervous current will be 

 directed from either point along the respective fibres to the brain, and each will 

 produce its separate, and not a blended, sensation. Thus the mind, from two 

 impressions, when different fibres are excited, acquires two sensations, and from 

 three impressions three separate sensations ; but the divided sensation conveys 

 no immediate perception of a relative position. The place at which the three 

 particles of light struck the retina, the situation of the ends of the irritated 

 nerves, forms no part of the sensation ; from the mere sensation we obtain no in- 

 telligence whether the rays of light struck the retina in a right line or angu- 

 larly, or how widely separated are the points from which they proceeded. At 

 the knowledge of these relations of space the mind first arrives by more circuit- 

 ous processes. We must imagine that the sensations which the individual nerve 

 fibres produce are distinguit^hed from one another, in some manner which cannot 

 be clearly indicated ; that the sensation of blue light, for instance, always bears 

 with it, according to the fibre excited, a certain definite token, which, while it is 

 constant for that one fibre, is distinguishable from the token of every other 

 fibre. It is from these tokens that the mind learns to form for itself represent- 

 ations of relative position ; or, in other words, learns to refer every such token 

 to a determinate place in its instinctive representation of space, and consequently 

 to assign every sensation, which is accompanied by this token, to the corre- 

 sponding place in the space-picture. When, therefore, as was above supposed, 

 three separate sensations of light arise simultaneously from three impressions, 

 each of the impressions bears with it that localizing token, according to which 

 the mind forms for itself a representation or idea of the relative situation and 

 distance of the points from which the irritating rays proceed. If it be the 

 image of a candle flame which falls on the retina, the separate sensations 

 which arise are as many in number as the ends of the nerves irritated, and these 

 separate sensations disclose through their localizing tokens that the exciting im- 

 pressions lie near one another in the form of a flame. In what manner the mind 

 learns to refer the visual sensations with the attending representations of posi- 

 tion to objects exterior to the body and from which the rays proceed, to give 

 them ohjcctivity, in a word, has been shown above. It is needless to say that 

 the mind originally knows nothing of the image on the retina as an object of 

 sight, nor is aware of its presence ; scientific research has first shown its exist- 

 ence and causative relation to the perceptions of light. 



Thus much we have thought proper to say, by way of example, respecting 

 the purpose and structure of the apparatus adapted to the exterior terminations 

 of the nerves of sensation ; it would detain us too long to dwell at present upon 

 other provisions of this sort, si\cli as that by which the undulations of sound are 

 converted into a suitable irritant for the nerves of hearing, &c. We return from 

 this digression upon the general physiology of the nerves and senses to a more 

 particular consideration of the sense of feeling, with a hope that the foregoing 

 discussion may have rendered what we have to say upon that and other special 

 topics more easily intelligible. 



Nearly all parts of our body are furnished with nerves of feeling — that is, 

 with nerves whose excitation, through its effect on the cells at the interior ter- 

 mination of the nervous fibres, produces Some one of the above recited sensa- 

 tions of feeling; and indeed there is one quality of such sensations which ail 

 these nerves are capable of generating, and which is therefore characteristic of 

 them — the sensation, namely, of pain. Hence, would we ascertain, respecting 

 any branch of nerves in the body, whether it contains any fibres of sensation, 

 v/e have only to search for it in a living animal and observe whether the irrita- 

 tion of it is followed by indications of uneasiness, (outcry, attempt at flight,) 

 a kiua of experiment which, in the eyes of the laity, has rendered the ideas of 

 physiologist and cruelly almost inseparable, but with as little justice, on the 



