Illusions of the senses. 375 



by the laws of the association of ideas. But even the sense 

 of touch, which has been generally regarded as the least lia- 

 ble to fallacy, is not exempt from this sourceof error, as is 

 proved by the well known experiment of feeling a single 

 ball, of about the size of a pea, between two fingers which 

 are crossed; for there is then a distinct perception of the 

 presence of two balls instead of one. 



But limited as our senses arc in their range of perception, 

 and liable to occasional error, we cannot but perceive, that, 

 both in ourselves, and also in every class of animals, they 

 have been studiously adjusted, not only to the properties and 

 the constitution of the material world, but, also, to the re- 

 spective wants and necessities of each species, in the situa- 

 tions and circumstances where it has been placed by the gra- 

 cious and beneficent Author of its being. , 



If the sensorial functions had been limited to mere sensa- 

 tion and perception, conjoined with the capacity of passive 

 enjoyment and of suffering, the purposes of animal existence 

 w^ould have been but imperfectly accomplished; for, in or- 

 der that the sentient being m^ay secure the possession of 

 those objects which are agreeable and salutary, and avoid or 

 reject those which are painful or injurious, it is necessary 

 that he possess the power of spontaneous action. Hence, 

 the faculty of Voluntary Motion is superadded to the other 

 sensorial functions. The muscles which move the limbs, 

 the trunk, the head, and organs of sense, — all those parts, in 

 a word, which establish relations ^vith the external world, 

 are, through the intermedium of a separate set of nervous 

 filaments, totally distinct from those which are subservient 

 to sensation,^' made to communicate directly with the senso- 

 rium, and are thereby placed under the direct control and 

 guidance of the will. The mental act of volition is doubt- 

 less accompanied by some corresponding physical change in 

 that part of the sensorium, whence the viotor nerves^ or 



* On this subject I must refer the reader to the researches of Sir Charles 

 Bell, and Magcndie, who have completely cstablisiicil the disitniction be- 

 tween these two classes of nerves. 



