CH. 111.] VIS NERVOSA OF INTERNAL IMPRESSIONS. 263 



system [the animal machines] ; both kinds of impressions are 

 transmitted in the same way through the nerves, and move the 

 mechanical machines with which they are incorporated (360). 

 We have now to show, by facts, in what structures this class of 



trve-actions are produced in the normal condition. 

 504. Conceptions act on the brain and nerves, so as either 

 put mechanical machines into motion or not (117). In the 

 latter case, they are manifested specially in the brain " and 

 sensitive nerves. It is already established by experiment, so 

 far as the difl&culty of investigating this profound subject will 

 allow, that there are also other stimuli than conceptions, which 

 excite nerve-actions in and limited to the brain itself, by means 

 of their internal impressions (374). It has also been shown 

 (377, 378), that internal impressions not caused by conceptions, 

 excite in the purely sensitive nerves, as nerve-actions, the same 

 changes which the internal impressions of conceptions produce, 

 as sentient-actions, inasmuch as both induce imperfect external 

 sensations (148, 377). Hence it follows, that the internal im- 

 pressions not caused by conceptions, are subject to the same 

 law of deflexion, and excite the same changes in the nerves, as 

 the internal impressions derived from conceptions {vide § 151). 

 The question is, however, so recondite, that it is difficult to 

 institute experiments in regard to it. 



505. Conceptions act on the mechanical machines (155) 

 either directly through the brain, or through the nerves. Now, 

 as non-conceptional internal impressions develop changes in the 

 brain, they can also thereby put mechanical machines into 

 motion. It is properly, however, the vessels, and especially the 

 blood-vessels, that experience this change (156 — 159), enabling 

 them to alter the cerebral circulation and the vital movements 

 generally. It is not, however, possible to show clearly, that 

 non-conceptional internal impressions cause the same changes 

 in the cerebral capillaries, since it is impossible to determine 

 whether a certain stimulus of the brain acts through the nerve- 

 fibrils in the brain, according to § 132; or directly on the 

 membranes and mouths of the capillaries of the brain, accord- 

 ing to § 392 ; or whether in either of these two cases it acts 

 as an external impression by means of direct or indirect (418, 

 419) nerve-actions, or through the sentient action of an external 

 sensation (132) ; or (much more probably) as a primary internal 



