SPECIAL PHYSIOLOGY. 57 



These considerations afford the clew of the mechanism by 

 which hallucinations are prodaced, whose cause is located in 

 the encephalon and gives rise to certain sensations attributed 

 by the patient to the periphery. 



This explains also the associated sensations ; an external 

 sensation arriving at a nerve-centre can there produce an 

 excitation sufficient to radiate towards the neighboring cen- 

 tres ; these will then give us sensations identical with those 

 we should have experienced had the excitation been produced 

 on those nerves that make communication between these cen- 

 tres and the periphery. In this way a foreign body introduced 

 into the ear may produce as an associated sensation a feeling 

 of tickling in the back part of the -throat, and perhaps even 

 coughing and vomiting. These associations are caused on 

 account of the nearness of the central gray nucleus of the 

 trifacial and of the nucleus of the glosso-pharyngeal and 

 pneumogastric, from which excitations perceived by the 

 former radiate towards the latter. 



There are examples of associated sensations still more start- 

 ling that seem due to the same mechanism : in certain per- 

 sons an irritation on the foot between the third and fourth 

 toe produces a sensation of tickling in the sub-umbilical 

 region of the abdomen; an irritation on the skin of the 

 scrotum will give rise to pains in the right hypochondriac 

 region, etc. 



Memory and Volition. Finally, the sensations present in 

 addition to the preceding this peculiar fact, that they can be 

 stoiedup in the cerebral organs; the impressions are fixed 

 there to reappear at a later time ; in this way are caused those 

 phenomena designated under the name of memory. The 

 sensations thus reserved in a latent condition reappear by a 

 mechanism analogous to that of the associated sensations, and 

 this revival of a sensation can bring on a number of others 

 similar or analogous : as, one idea calling up another, and 

 what is called association of ideas. 1 



1 The cell in the spinal cord is also susceptible of preserving up 

 to a certain point the impression which has been produced by a 

 centripetal nerve, though generally the former retains nothing after 

 having brought its peculiar reflex action. Thus a certain habit of 

 reflex actions is brought about, which terminates in happening 

 more readily and regularly. In fact, the spinal cord can be edu- 

 cated; we need only cite the example of persons who play upon 

 musical instruments, who finally attain the faculty of executing a 

 musical piece or tune almost without any conscious volition, and 



