630 THE SENSES 



effort made by the muscles of the calf. But the muscle sense will be dis- 

 cussed further, page 703. 



It is manifestly impossible to draw a very clear line of demarcation 

 between some of these senses. 



Special Senses. The special senses include Touch, Temperature 

 (Heat and Cold), Taste, Smell, Hearing, Sight. 



The most important distinction between common and special sensations 

 is that by the former we are made aware of certain conditions of various 

 parts of our bodies, while from the latter is gained a knowledge of the ex- 

 ternal world. This difference will be clear if we compare the sensations of 

 pain and touch, the former of which is a common, the latter a special, sensa- 

 tion. "If we place the edge of a sharp knife on the skin, we feel the edge 

 by means of our sense of touch; we perceive a sensation, and refer it to the 

 object which has caused it. But as soon as we cut the skin with the knife, 

 we feel pain, a feeling which we no longer refer to the cutting knife, but which 

 we feel within ourselves, and which communicates to us the fact of a change 

 of condition in our own body. By the sensation of pain we are neither able 

 to recognize the object which caused it nor its nature. " 



It is important in studying the phenomena of sensation clearly to under- 

 stand that the sensorium, or seat of sensation, is in the brain, and not in the 

 particular organ through which the sensory impression is received. In com- 

 mon parlance we are said to see with the eye, hear with the ear, etc., but in 

 reality these organs are only adapted to receive stimuli which produce changes 

 that are, through their respective nerves, conducted to the sensorium, to give 

 rise to sensation. 



Hence, if the optic nerve is severed, vision is no longer possible. Although 

 the image falls on the retina as before, the sensory impulse can no 

 longer be conveyed to the sensorium. When any given sensation is felt, all 

 that we can with certainty affirm is that some part of the brain is excited. 

 The exciting cause may be some object of the external world, producing an 

 objective sensation; or the condition of the sensorium may be due to some 

 excitement within the brain itself, in which case the sensation is termed sub- 

 jective. The mind habitually refers sensations to external causes; and hence, 

 whenever they are subjective we can hardly divest ourselves of the idea of an 

 external cause, and an illusion is the result. 



Sensory Illusions. Numberless examples of such illusions might 

 be quoted. As familiar cases may be mentioned humming and buzzing in 

 the ears caused by some irritation of the auditory nerve center. These 

 stimuli may even be interpreted as musical sounds, or voices, sometimes 

 termed auditory spectra. So-called optical illusions in which objects are 

 described as seen, although not present, may be caused by changes going 

 on in some part of the visual apparatus beyond the eye. Such illusions are 

 most strikingly exemplified in cases of delirium tremens or other forms of 



