370 INNERVATION. [c H AP . XI . 



In many instances,, where pain has been excited by a physical cause, 

 there can be no doubt it has been continued long after the cessation 

 of its exciting cause, by the attention of the patient having been 

 directed to it. It is probable,, that in such cases the perceiving 

 parts of the brain (so to speak) become habituated to a certain 

 condition of the centre of sensation, produced by the original ex- 

 citing cause of the pain. 



Nerves are implanted only in those parts of the encephalon which 

 are capable of physical nervous actions : the convulsions of the 

 brain, the corpus striatum, the optic thalamus, and the cerebellum, 

 are capable only of mental nervous actions. In every change of 

 these latter, the mind is either the excitor or the excited ; the con- 

 ditions of the nerves involve them only through the influence of 

 the centres in which the nerves are implanted; and they affect 

 the nerves only through the same medium. Matteucci's experi- 

 ments as to the effects of electricity on the different parts of the 

 brain, shewed that, as long as the current was confined to those 

 parts which are capable only of mental actions, no apparent effect 

 was produced. But when the poles of the battery had penetrated 

 to the base of the brain so that the current might pass through 

 the deeper seated parts, then the animal cried out with pain, and 

 strong convulsions were produced. 



Those parts in which physical nervous actions take place, (al- 

 though capable of partaking in the mental actions,) require the 

 excitation of physical stimuli in order to develop their peculiar phe- 

 nomena, and thus have frequent remissions in the active performance 

 of their functions in the frequent absence of the ordinary stimuli. But 

 the ever-active mind keeps up a constant and proportionately rapid 

 train of changes in those parts which are more especially connected 

 with mental actions : hence these parts, requiring repose, fall at 

 certain periods into that peculiar and inscrutable state called sleep ; in 

 which, whatever be the condition of the mind itself, the brain either 

 refuses, or is slow to respond to its stimulation, or to convey im- 

 pressions to it. In deep sleep we are completely unconscious, and 

 may remain for a considerable time motionless. But as the ac- 

 customed period of repose approaches to its termination, the sleep 

 becomes lighter, a degree of consciousness returns, and mental 

 changes take place, which, whether incoherent or connected, con- 

 stitute what are familiarly known as dreams. In lighter sleep, 

 it cannot be said that there is complete want of consciousness ; nor 

 is the mind, although comparatively quiescent, in complete repose. 

 The readiness with which, at times, some persons, during sleep, 





