CH. I.] NERVE-ACTIONS IN GENERAL. 203 



the cortical substance which surrounds it (159), it follows, that 

 the principal seat of the primary vis nervosa must be specially 

 in the nerves. 



373. Nevertheless, it cannot be denied, that the medullary 

 substance of the brain itself has some share in the vis nervosa. 

 We cannot, however, determine by experiment, whether an ex- 

 ternal impression which reaches the brain but is not felt, does 

 not develop some animal action through the medullary matter 

 of the brain, although it causes no material external sensation 

 (46, iii). For, although it excites the most energetic movements 

 through the nerves, yet the same phenomena occur when the 

 head is wholly severed from the body (357), consequently the 

 external impression is changed into an internal lower down, 

 probably in the ganglia (48), to produce the nerve-actions, for 

 it would appear as if the condition absolutely requisite to the 

 change of the external impression into an internal in the brain 

 and its reflection is, that it develops therein a material external 

 sensation (25). Nevertheless, there are cases in which it must 

 remain doubtful whether an external impression does not produce 

 purely nerve-actions through the medullary substance of the 

 brain, for when the latter is injured in an animal, the body is 

 convulsively agitated (Haller^s ^ Physiology,^ § 368). It has 

 been already shown (132), that this may constitute sentient 

 actions of an external sensation of pain, because the medullary 

 substance also transmits external impressions to the origins of 

 the fibrils of the irritated nerve, or of the fibrils of the cerebral 

 medulla, and can excite therein material external sensations 

 which may act reflexly by means of their internal impression 

 on the brain, and through it on the nerves in the mechanical 

 machines. But if a similar external impression on the medul- 

 lary substance should not be felt, and yet excite movements 

 (convulsions), would not these be no other than mere nerve- 

 actions of an external impression on the brain, and must not 

 the brain, consequently, possess a vis nervosa of external impres- 

 sions in addition to its animal-sentient forces ? It is difficult to 

 determine, whether external impressions be felt or not, and in 

 the latter case, whether they produce the same movements as 

 in the former. Although indeed many injuries of the brain 

 are not at all painful or manifest, and foreign bodies may lodge 

 therein for a lengthened period without the knowledge of the 



