THE SENSES. 547 



traction or relaxation of the various muscles and groups of muscles, and 

 thus obtain the information necessary for their adjustment to various 

 purposes standing, walking, grasping, etc. This muscular sensibility 

 is shown in our power to estimate the differences between weights by the 

 different muscular efforts necessary to raise them. Considerable delicacy 

 may be attained by practice, and the difference between 19 oz. in one 

 hand and 20 oz. in the other is readily appreciated. 



This sensibility with which the muscles are endowed must be care^ 

 fully distinguished from the sense of contact and of pressure, of which 

 the skin is the organ. When standing erect, we can feel the ground 

 (contact), and further there is a sense of pressure, due to our feet being 

 pressed against the ground by the weight of the body. Both these are 

 derived from the skin of the sole of the foot. If now we raise the body 

 on the toes, we are conscious (muscular sense) of a muscular effort made 

 by the muscles of the calf, which overcomes a certain resistance. 



(2.) Special Sensations. Including the sense of touch, the special 

 senses are five in number Touch, Taste, Smell, Hearing, Sight. 



Difference between Common and Special Sensations. The most im- 

 portant 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 we gain our knowledge of the external 

 world also. 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 

 sensation. "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 com- 

 municates 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." 



General Characteristics : Seat. In studying the phenomena of sen- 

 sation, it is important clearly to understand that the Sensorium, or seat 

 of sensation, is in the Brain, and not in the particular organ (eye, ear, 

 etc.) through which the sensory impression is received. In common 

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

 reality these organs are only adapted to receive impressions which are 

 conducted to the sensorium, through the optic and auditory nerves re- 

 spectively, and there give rise to sensation. 



Hence, if the optic nerve is severed (although the eye itself is per- 

 fectly uninjured), vision is no longer possible; since, although the image 

 falls on the retina as before, the sensory impression can no longer be 

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

 we can with certainty affirm is that the sensorium in the brain is excited. 



