228 ANIMAL FORCES. [ii. 



i. If the meclianical macliines which ought to be its seat, 

 are not endowed with nerves. 



ii. If their nerves be not irritated, or only so irritated that the 

 nerve-medulla does not receive thereby an external impression 

 (418, ii). Something more than mere irritation is required to 

 this end, as a strong excitant fails to excite the nerve-actions, 

 if- it be not such an one as stimulates the nerve suitably. 

 (Compare 413, 414.) 



iii. If the mechanical machines be unfit, at the point of 

 impression, to manifest the nerve-actions to which they are 

 stimulated (418, iii). In such cases, the impression has no 

 direct action ; but leaving the machines unchanged, acts upon 

 them indirectly, and causes an indirect nerve-action, or, if it be 

 felt, a sentient action. Thus, a stimulus applied to a muscle, 

 excites no contraction, if it be already affected with spasm, 

 although it may be felt, and excite spasmodic movements in 

 many other muscles. (See §§ 208, 464.) 



425. An indirect nerve-action from an external impression 

 cannot arise, or will be prevented, — 



i. If the external impression do not reach to the point, 

 where it can be so turned as to be reflected (422, i), as when 

 the nerve is tied, or divided. 



ii. If the external impression, although it reach this point, 

 be not changed into a non-conceptional impression, or, in other 

 words, at least partly deflected from and hindered in its course 

 to the brain ; as when there is induration, or other disease, of 

 the ganglia, or plexuses, or of points of branching. 



iii. If the transmission of the reflected impression downwards 

 to the machines be prevented, as by ligature ; it being under- 

 stood that the machines themselves are capable of the actions 

 (422, iii). 



426. Since we can prevent the action of irritants and of ex- 

 ternal impressions, and hinder their course, reflexion, and trans- 

 mission downwards, so also nature herself regulates them ; and 

 the impressions do not act blindly and necessarily, being pre- 

 vented in various ways, and guided so as to excite the mechanical 

 machines for specific objects, just as various external sensations 

 are prevented by various means, to the end that the conceptive 

 force be not excited by every irritant applied to the body 

 (47 — 51). It is of importance to comprehend all the modes 



