392 THE NERVOUS SYSTEM IN GENERAL, [ch. ii. 



and sensation. Sensation is more acute in the young man 

 than in the aged.^ The pupil is more contractile in the infant, 

 less readily acted on by light in the aged. And the same 

 principle illustrates the cause of senile impotence. That the 

 sensibility of the genital organs is diminished with age is 

 proved by the fact, that the seminal emissions so readily excited 

 in youth, cease to take place about the fiftieth year or some- 

 what later, even in able and strong men. Since the female 

 sex has a more excitable nervous system, it is established as a 

 general rule that the feeble, or rather the tender organisms, 

 feel more acutely than the robust.^ Observations also show, 

 that the amount of vis nervosa varies with the climate, since 

 those who inhabit hot climates indulge more in ease and 

 pleasure than the inhabitants of colder regions ; Montesquieu^ 

 thinks we may distinguish climates by the degrees of sensibility, 

 just as we distinguish by degrees of latitude. Often in diseases, 

 the sensibility or vis nervosa of the whole nervous system, is 

 increased in a very remarkable manner ; whence it happens, I 

 think, that we cannot then bear a slight degree of cold in the 

 atmosphere, on account of the shiverings and unpleasant sen- 

 sations excited through the whole body, that in health we 

 should not notice. Thus, also, a moderate draught of wine 

 greatly increases the fever of a fever-patient, but which, if 

 taken by a person in health, would produce no change what- 

 ever in the pulse. For the same reason it happens, that in 

 hemicrania, gout, or any painful aifection, we are impatient, and 

 cannot endure any noise or light, or a variety of objects. All 

 nerves that have become too sensitive can no longer tolerate 

 even the most common impressions.* If it were not altogether 

 superfluous, many other instances of increased vis nervosa in 

 the whole nervous system might be adduced. 



h. Frequently an increased degree of vis nervosa is observed 

 in a portion only of the nervous system, and not in the whole ; 

 in the animal organs alone, or only in the sensorium commune, 

 or in one or other of the nerves. Thus, I imagine, there is an 

 increased degree of vis nervosa in the delirious and maniacal, 



' Haller, El. Phys., torn, iv, pp. 293, 294. 

 « Battle apud Haller, El. Phys., torn, iv, p. 459. 



^ Esprit, de Loix. Vid. La Roche, * Analyse de Fonctions du Systeme Nerveux/ 

 torn. i. 



*' Tissot. von Nerv., 2 Band, ii Th., § 77, s, 165. 



