260 



ANIMAL FORCES. 



[II. 



the brain and the conceptive force. When a decapitated frog 

 leaps from pinching of its toe, the action is a nerve-action of 

 an internal impression derived from a reflected external im- 

 pression .(419j 415, ii). The external impression in this case 

 can pass uninterruptedly from the point of irritation in the toe 

 to the point of injury in the spinal cord, where it is reflected, 

 and passes uninterruptedly back again, and thus the same 

 nerve-action is caused by it, as if a primary internal impression 

 had excited the spinal cord (494). 



496. A non-conceptional internal impression can excite nerve- 

 actions under the following conditions only : 



i. The medulla of the trunk of the nerve must receive an 

 internal impression in one direction downwards from the brain, 

 independently of any conception (121, 4SS). 



ii. This internal impression must be transmitted downwards, 

 along the trunk and its branches through the efferent fibrils, to 

 the mechanical machines which have to perform the nerve-action. 



iii. The mechanical machines must be capable of performing 

 that nerve-action, which the received internal impression can 

 effect in virtue of its nature ; just as is necessary in sentient 

 actions excited by conceptional internal impressions (130, ii, 

 129, iv). 



The conditions under which those nerve-actions of internal 

 impressions take place, that are caused by a reflected external 

 impression, have been already stated (419, 422). 



497. The nerve-actions of non-conceptional internal impres- 

 sions are quite independent of the brain and the conceptive 

 force ; nevertheless, at another time, or in other animals, they 

 may be sentient actions of impressions caused by conceptions 

 (364, i); and this applies especially to indirect nerve-actions of 

 external impressions (423). Thus, the volitional movement of 

 a limb (a sentient action) may occur as the nerve-action of a 

 primary internal impression, when an acrid humour irritates the 

 trunk of the nerve going to the limb ; and although this acrid 

 humour, as in gouty diseases, may cause pain, yet this external 

 sensation of pain is not the motor force of the limb, but the 

 concurrent internal impression (484). See a previous paragraph 

 for examples of this class (423) . 



498. A nerve-action of a primary internal impression cannot 

 take place, or will be prevented : 



