I 



CH. HI.] VIS NERVOSA OF INTERNAL IMPRESSIONS. 261 



i. If an irritation, although applied to the medulla of a nerve 



the direction downwards from the brain, excites no internal 

 impression therein. Thus, in disease, there are often traces of 

 putrid humours collected about the points where the trunks of 

 nerves occur, without any motions resulting in the muscles; 

 either because the putrid fluids do not penetrate to the medulla 

 of the nerve, but remain externally to it, or else because they 

 are of a kind which excites no internal impression in it (492) . 



ii. If the transmission of a primary internal impression is 

 prevented by some impediment, so that it cannot reach the 

 mechanical machines which it regulates (496, ii). Thus, an 

 acrid humour in the axilla may irritate the nerves, which regulate 

 the forearm and hand, and so excite convulsive movements 

 therein. But if there be a tumour at the elbow-joint com- 

 pressing the nerves, or if some external pressure be made on 

 them (as when a limb goes to sleep), then the transmission of 

 the primary internal impression from above downwards is 

 prevented, and no nerve-actions take place. 



iii. If the mechanical machines are rendered incapable of 

 the requisite nerve-action. Thus, when an acrid humour irri- 

 tates the nerves, a bandaging of the muscles of the convulsed 

 limb, will prevent their action, and so the patient may obtain 

 sleep and rest. (Compare also § 425.) 



499. There must be natural obstacles to the development of 

 the nerve-actions of non-conceptional internal impressions, as 

 well as of sentient actions (136 — 138). Those already enume- 

 rated as preventing the indirect nerve-actions of external im- 

 pressions, act also in preventing the nerve-actions of primary 

 internal impressions (428, vi; 429). The following is a short 

 statement of some others. 



500. A natural impediment to the production of certain 

 nerve-actions from primary internal impressions, occurs — 



i. When a nerve, by nature, is but slightly or not at all 

 susceptible of certain primary natural stimuli (492, 498, i) ; 

 consequently, if it do receive an impression, the latter is too 

 weak to be propagated to the mechanical machines. It is quite 

 certain that there are stimuli, which, when applied to the 

 medulla of the nerve in its normal condition, do not excite 

 nerve-actions. The nerves are in close relation with many 

 tissues, and their medulla is penetrated by capillaries and fluids. 



