146 THE SENSES. 



It is by means of impressions so received and conducted to it that 

 the mind is able to control the body and to take cognizance of the 

 external world. 



Into what classes may the sensations be arranged? 



Common sensations and special sensations. These last are com- 

 monly called " the senses." 



What is meant by the common sensations? 



Such perceptions as cannot be distinctly located in any organ or 

 set of organs, such as fatigue, hunger, thirst, satiety. Besides this, 

 there are some sensations which involve certain organs which must 

 be classed under this head ; thus inclinations to cough or to sneeze 

 or to vomit are common sensations, and, similarly, to urinate or 

 defecate. Many of these sensations occupy a border-line between 

 common sensibility and the special sense of touch, such as itching 

 and tickling. 



Is pain a common sensation? 



It is, but is very closely allied to the sense of touch. The two 

 may be differentiated, however. If one touch a sharp instrument, 

 he may perceive its shape and condition, but if the pressure be 

 increased the ability to perceive its form is lost, and instead the 

 sensation of pain is established. The relation of the two is curi- 

 ously shown in partial anaesthesia by drugs, as when one takes 

 nitrous-oxide gas for the extraction of a tooth, and is able to feel 

 the operation and to know what has been done, without in the least 

 feeling pain. 



What is the real seat of the senses ? 



The brain or sensorium. The organ of the mind, which perceives 

 the thing which the organ of sense has taken an impression from, 

 is the fundamental structure in the necessary chain. 



What is hallucination? 



It is the perception of an object as a real presence without the 

 presence of the object to justify the perception ; that is, it is an 

 act of the brain which refers its action to an organ of the senses. 

 Thus, in delirium tremens a person may perceive many curious and 

 uncanny things, which his mind hears and sees and feels, but which 

 his senses could not take cognizance of, because they are only 

 " creatures of the mind." 



