

226 ANIMAL FORCES. [ii. 



in so far as it is independent of that transmission^ its direct nerve , 

 action. The conditions requisite to the production of this are — • 



i. That the mechanical machine be endowed with nerves 

 (160, 417). 



ii. That its nerves be so touched, that an external impression 

 be communicated to their medulla (414). 



iii. That the mechanical machines be capable at the point of 

 impression of such movement as the impression can excite. 



419. The direct nerve-actions are to be distinguished from 

 the indirect, which are induced either remotely from the point 

 of impression, or, if they arise in the machines whose nerves 

 are irritated, are not excited through those nerves, but through 

 some other, or else through the efferent fibrils of the irritated 

 nerve (127) . The conditions under which they arise, are more 

 numerous than the preceding, and to understand them the 

 following must be premised. 



420. Take a nerve which pursues a direct course from 

 the brain to a limb, and penetrates tissues, without giving 

 off a single branch to any organs or structures in its course, 

 having neither ganglia nor plexuses, and instead of ending in 

 numerous fibrils, gives off one only to a simple muscular fibre. 

 If the latter be irritated with the point of a needle, and the 

 nerve thus receive an external impression, the fibril contracts, 

 and so a direct nerve-action is produced, whether the im- 

 pression be transmitted upwards or not (161, 204). But sup- 

 pose it to be so transmitted, what then happens ? Since there 

 are no ganglia or plexuses, in which the impression can be 

 reflected, and since there are no branches whatever given off, 

 the tissues it penetrates cannot be excited to action; but the 

 impression must go on to the brain, in which, if it reach the 

 brain, it causes sensation, and can then be reflected from the 

 brain as the internal impression of an external sensation, and 

 thus again move the fibrils it moved before (188, 127). This 

 result is, however, a sentient action, and not a nerve-action. 



421. When, therefore, an external impression on a nerve ex- 

 cites, in addition to its direct nerve-actions, a movement in remote 

 machines, or in the same machine, to which it is distributed by 

 fibrils of nerves distinct from that which received the impression^ 

 or by efferent fibrils ; or, in other words, when it develops indirect 

 nerve-actions, it follows that either the nerve itself is deflected, 



