76 CEREBRAL FORCES. [i. 



141. Although all operations through the nerves of impres- 

 sions in the brain are movements (122)^ it is not a necessary 

 consequence, that these movements arise in proper mechanical 

 machines, as muscles, glands, viscera, &c., for they may occur in 

 nerves of sensation which are not distributed to these machines 

 (14) ; or may, even in motor nerves, be only movements of the 

 vital spirits, which are not manifested by visible movements; 

 yet they are as certainly operations of the cerebral impression 

 acting through the nerves (sentient actions, § 123), and as 

 certainly cause important phenomena in the animal economy 

 (as we shall shortly see), as those which are manifested by 

 visible movements in the animal machines. (This shows inci- 

 dentally the correctness of the division, § 117). 



Actions of the Material Ideas in the Nerves exclusively , and 

 when not extended to Mechanical Machines. 



142. After the general consideration of the actions of the 

 material ideas on the nerves, we have now to investigate their 

 special action in the nerves exclusively, without reference to 

 their extension to the mechanical machines. Indeed all kinds 

 of nerves are subject to this influence of the material ideas in 

 the same way. But while internal impressions are distinctly 

 manifested by the movements of mechanical machines, there 

 are only shght traces of movements in the brain, in the nerves 

 themselves, (no action in a mechanical machine following,) or 

 in purely sensitive nerves, with which no mechanical machines 

 are connected. They are best observed in the nerves of the 

 external senses which simply feel. 



143. Every nerve in animals endowed with sensation receives 

 external impressions, some of which at least are transmitted 

 onwards to the brain, where they produce, at the origin of the 

 nerve in the brain, a material external sensation (45, 124) ; or, 

 in other words, an internal or cerebral impression, which must 

 directly excite sentient actions in the nerve itself, even if it only 

 feels, provided it be a nerve capable of so receiving an internal 

 impression ; and it must be so capable, if the impression is to 

 be propagated outwards from the brain (129, ii, iii.) That every 

 nerve, whether purely sensitive, or both sensitive and motor, 

 must possess this capability, although it may not be able to 



