262 ANIMAL FORCES. [ii. 



all which must often necessarily act as active stimuli, but they 

 seem to act as such, for the most part, extraordinarily and 

 abnormally only, otherwise they would incessantly stimulate 

 our limbs to movements. 



ii. When certain internal impressions on the nerves are not 

 naturally transmitted to certain mechanical machines (498, ii). 

 This hindrance is principally constituted by the ganglia and points 

 of division of the nerves (137, 429), which sometimes so receive 

 internal impressions of a certain kind, that they go forwards ; 

 but also, sometimes, so that the nerves are not excited inter- 

 nally by them, and consequently the impressions have no in- 

 fluence on the mechanical machines innervated by the nerves. 



iii. When the mechanical machines are as yet naturally 

 incapacitated for the animal movement which a certain internal 

 impression can communicate to it, or becomes so incapacitated. 

 This impediment is observed in young animals, or in the very 

 old, which are excited to certain movements simply by their 

 blind instincts, in virtue of the vis nervosa (439) . 



501 . Habit, and the frequent repetition of internal impres- 

 sions, can act as impediments to this class of nerve-actions, in 

 the way already laid down (430, 431). 



502. Since the nerve-actions of non-conceptional impressions 

 are, for the most part, indirect nerve-actions of external im- 

 pressions (which determine the sensibility of animals), the 

 vis nervosa of internal impressions has also its influence in 

 determining the irritability and vital constitution of animal 

 bodies. 



SECTION II. ON THE VIS NERVOSA OF INTERNAL IMPRESSIONS 



IN PARTICULAR. 



503. Nerve-actions of internal impressions, whether the latter 

 be primary internal impressions, or reflected external impres- 

 sions changed into internal, can be developed in all the organs 

 that are capable of sentient actions ; for the whole diff'erence 

 of the two consists in this, — that in the development of the 

 latter, the nerves are excited by conceptions, and solely at their 

 origin in the brain ; of the former, by other stimuli applied to 

 the whole nervous system, except the terminal fibrils (121, 483). 

 Both kinds of stimuli consist in the same changes in the nervous 



