658 HANDBOOK OF PHYSIOLOGY. 



mate the differences between weights by the different muscular efforts 

 necessary to raise them. It must be carefully 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. 



The most important distinction between common and special sensa- 

 tions 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 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." 



In studying the phenomena of sensation, it is important clearly to 

 understand that the sensorium^ or seat of sensation, is in the brain, and 

 not in the particular organ through which the sensory impression is re- 

 ceived. 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, being conducted to the sensorium, through their re- 

 spective nerves give rise to sensation. 



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

 since, although the image falls on the retina as before, the sensory im- 

 pression 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 subjective. The mind habitually re- 

 fers sensations to external causes; and hence, whenever they are subjec- 

 tive we can hardly divest ourselves of the idea of an external cause, and 

 an illusion is the result. 



Numberless examples of such illusions might be quoted. As familial 



