CH. III.] VIS NERVOSA OF INTERNAL IMPRESSIONS. 265 



applied above the point of injury, cannot excite a nerve-action 

 in the machines to which the nerve is distributed. Nevertheless, 

 the brain and conceptive force are not necessary to nerve-actions, 

 although in many cases nerve-actions cannot take place without 

 the brain; not, however, because the cerebral forces or the 

 conceptive force are requisite, but because it contains the 

 origins of the nerves, and without it non-conceptional impres- 

 sions cannot be applied to that portion of the nerves. If the 

 brain be excited by some general stimulus, in which the roots 

 of all or the greater number of the motor nerves participate, its 

 influence must be extended to the greater part or the whole of 

 the muscular system, as occurs with internal impressions caused 

 by conceptions (164, v). The same doctrine applies to the 

 spinal cord. Consequently, in experiments made on animals 

 when the brain and spinal cord are irritated mechanically, some- 

 times partial, sometimes general convulsions are excited (359). 



512. The nerve-actions from the two kinds of internal im- 

 pressions are liable to the impediments that have been already 

 indicated (428 — 431, &c.), and hence it is not every internal 

 impression which can excite nerve-actions in the muscles (50Q). 



513. Unfelt external impressions in the muscles can be 

 reflected in the muscles themselves, and then cause nerve- 

 actions in them by means of internal impressions (447); so 

 that various movements of the limbs and the viscera, which 

 appear to be nerve-actions of external impressions only, are at 

 the same time nerve-actions of an internal impression induced 

 by external impressions (448, 423). The excited motion of the 

 heart and intestinal canal may be taken as examples (446). 



514. A direct nerve-action of an external impression may 

 at the same time, or at another time, or in another animal, be 

 indirect, or the nerve-action of internal impressions, or a sen- 

 tient action of external sensations, or of other conceptions, nay, 

 may even be volitional; and vice versa. Consequently, those 

 go too far who maintain that the irritability of muscles is their 

 only or principal motor force. It would rather appear, that 

 although the greater number of muscular actions can be directly 

 excited by external impressions, in the natural condition of 

 animals endowed with a brain and conceptive force, yet 

 internal impressions, whether primary or originating in a re- 

 flected external impression, may co-operate therewith ; so that the 



