SECT. III.] PROPERTIES OF THE VIS NERVOSA. 391 



vis nervosa and stimulus, generally considered, — As effects are 

 proportionate to their causes, so the operations of the nervous 

 system are proportionate to the vis nervosa and the vis stimuli. 

 The operations of the nervous system_, for example, will be the 

 more powerful and extensive in proportion as the vis nervosa is 

 more active and the stimulus efficient : and contrarily in 

 proportion as the vis nervosa is less active and the stimulus 

 feebler, in that proportion will the operations of the nervous 

 system be more languid. A less energetic stimulus is suffi- 

 cient for a more active vis nervosa, just as a more powerful 

 stimulus will compensate for a less active vis nervosa ; so that 

 in both cases, the effect on the operations of the nervous system 

 may be equal. The vis nervosa is not, however, indifferent to the 

 kind of stimulus, for it is more readily excited by one than by 

 another, although they may appear to be equally forcible ; nay, 

 it sometimes responds more actively to apparently a very mild 

 than to a very powerful stimulus. Thus the heart and intes- 

 tinal canal, according to Haller,^ are thrown into more powerful 

 contractions by inflated air than by water, or any poison ; and, 

 on the other hand, a drop of water getting into the trachea 

 excites a violent cough, while the air is insensibly inspired and 

 expired through it. I shall adduce many such examples 

 hereafter as illustrations of idiosyncrasy. 



iv. Under what circumstances the vis nervosa is increased. — 

 It is evident that the stimulus may be greater or less, longer 

 or shorter, more or less general, or quite local; and the same 

 is true of the vis nervosa. This, in fact, differs in degree 

 according to the difference of age, sex, temperament, climate, 

 the condition of the body as to health or disease, and other 

 circumstances, and in a portion of the nervous system as well 

 as in the whole, which it will suffice to prove by a few examples. 



a. In the first place, the vis nervosa is generally greater in 

 childhood than adult age ; for a slight stimulus at that age will 

 act violently upon the nervous system, which scarcely affects 

 the nerves in more advanced years, a truth abundantly proved 

 by the testimony of celebrated men. Young animals are 

 the more sensitive,^ and organs which in the newly-born are 

 irritable, become insensible through age,^ and languid in motion 



' El. Phys., torn, iv, p. 575. =» Haller, Ibid., p. 456. 



3 Whytt apiid Haller, Ibid., p. 184. 



