CH. III.] VIS NERVOSA OF INTERNAL IMPRESSIONS. 253 



tion ; or is reflected downw wards, and develops indirect nerve- 

 actions of the external impression ; or else, both results take 

 place : at the same time, however, the irritation made by the 

 needle is transmitted downwards to the terminating fibrils of 

 the nerve, as a non-conceptional internal impression, and excites 

 nerve-actions in the parts to which the terminating fibrils are 

 istributed. 



485. An internal impression, unconnected with conceptions, 

 acts downwards from the point where the impression is made 

 along the twigs and branches of the nerves, and if there be no 

 obstacle to its course, operates in those mechanical machines 

 connected with the fibrils. It may also (as when produced by 

 conceptions) be communicated to other nerves and to other 

 mechanical machines, if the fibrils be interwoven in ganglia 

 beneath the point irritated (124). 



486. As the action is the same in the mechanical machines, 

 whether the internal impression be produced by conceptions or by 

 other irritants, so also is its transmission; so that, just as internal 

 impressions from conceptions are transmitted along the trunks 

 of the nerves to the terminal fibrils, without being commingled 

 with each other, although passing along the same trunk, as, for 

 example, the spinal cord, or principal branch of a nerve (125), 

 so also the non-conceptional internal impressions have their 

 independent course, and are not confused by or commingled with 

 each other ; whence it is inferred, that every internal impression 

 takes place in a special fibril, running an independent course to 

 a distinct mechanical machine, as has been previously shown (125). 

 If we knew which particular fibrils were excited by a certain 

 conception, when it produced a certain movement of a mecha- 

 nical machine, as a sentient action, we might be able to excite 

 the same movement as a nerve-action, by irritating the special 

 fibrils, and thus causing an internal impression, either in the 

 spinal cord or in the branches given off from it. It is not 

 possible, however, to perform this experiment, because we can- 

 not distinguish the various efferent fibrils on which a special 

 conception acts; and because the irritation of a single efferent 

 fibril would probably require much more delicate instruments 

 than we possess. 



487. The non-conceptional internal impressions will be as 

 little interrupted in their course to the terminal fibrils, by ex- 



