256 ANIMAL FORCES. [ii. 



chanical machines. For all conceptions are primarily produced 

 by external sensations, that is, by means of external impres- 

 sions (65); and by means of these identical impressions, when 

 reflected ere they reach the brain, can those movements be 

 excited, as simply nerve- actions of non-conceptional impressions, 

 which are sentient actions of conceptions; or they may act 

 concurrently with conceptions (421-2). This great truth will 

 be amply established, by what will be laid down in this and 

 the following chapter, as to the vis nervosa of non-conceptional 

 internal impressions. 



490. Nevertheless, there are other irritants that act as non- 

 conceptional impressions. When the medulla of a nerve is 

 irritated, as by pricking, pinching, cutting, &c., the same move- 

 ments take place in the mechanical machines to which the 

 nerve is distributed, as if a conceptional internal impression j 

 had produced them; or as if they were the indirect nerve-action 

 of an external impression. Thus, a decapitated frog retracts 

 its leg when its spinal cord is irritated, just as it did when, 

 during life, it willed to make a spring, and as also, if decapitated, 

 when its toe is pinched (359). So also a muscle contracts, when 

 its nerve is cut through. (Haller^s ' Physiology,^ §§ 403, &c.) 

 Suppose that instead of this artificial irritation, some materies 

 so irritates a nervous trunk in the direction downwards from 

 the brain, that the muscles which the nerve regulates are 

 thereby put into motion; the resulting movements are then 

 nerve-actions of a non-conceptional internal impression, which 

 are not in the first place indirect nerve-actions of an external 

 impression, but are produced by 2i primary internal impression. 

 Such irritants, while exciting movements in a direction down- 

 wards from the brain, may be also felt ; but in this case 

 sensation contributes nothing to the movements which are j 

 excited at the same time by the primary internal impression. % 



491. Observation teaches, that this class of nerve-actions 

 occurs, but the most distinct are usually contra-natural. Various 

 contractions, spasms, convulsions, and cramps of muscles and of 

 limbs, occur in disease, without being induced by a conception 

 or an external sensation, or even by an external impres- 

 sion on the motor nerves, and are therefore neither sentient 

 actions, nor indirect nerve-actions of external impressions, 

 and have no other origin, than that some acrid irritant matter 



