THE ORGANS OF SENSE. 109 



known as syringomyelia, in which there is pressure on the conducting 

 paths in the spinal cord owing to distension of the central canal. In 

 these cases also there may be loss, for example, of the senses of pain 

 and temperature, while that of pressure is retained. 



The cutaneous senses are therefore independent and are subserved 

 by different nerve fibres, but the further question as to whether each 

 sense has a specific nerve ending cannot be so decisively answered. 

 Special functions can, however, be ascribed to some of the nerve endings 

 in the skin with a reasonable degree of probability. 



The fact that any stimulus applied to the cornea gives rise to pain 

 indicates that interepithelial fibrillar nerve endings are associated with 

 the pain sense. The skin areas most sensitive to pressure are those, 

 like the palmar surface of the fingers, where the touch corpuscles of 

 Meissner are most abundant. Where hair is present the pressure spots 

 immediately surround the point of emergence of the hair. It may 

 therefore be concluded that the touch corpuscles in hairless regions and 

 the fibrillar nerve endings around the hair follicles are the peripheral 

 terminations of the pressure sense fibres. Finally, the position of the 

 Pacinian corpuscles and of those of Golgi and Mazzoni and of Ruffini 

 makes it clear that these structures can only be affected by deep 

 pressure. 



THE DISTRIBUTION OF THE CUTANEOUS SENSORY 

 NERVE ENDINGS. 



Reference has already been made to the fact that the cornea is 

 richly supplied with nerves for pain, and that the skin of the palmar 

 surface of the fingers is markedly endowed with the pressure sense. 

 The distribution of the various sensory nerves is therefore unequal, and 

 investigations have been made, especially in connection with the pressure 

 sense, to determine (1) the degree of pressure which can be detected at 

 different parts of the skin surface, and (2) the relative acuteness in 

 different areas of tactile discrimination, that is, of the power of appreci- 

 ating two separate pressure stimuli applied simultaneously. 



(1) Von Frey's method of estimating the degree of pressure on the 

 skin which can be appreciated is to use hairs of different thickness, the 

 pressure required to cause each to bend being known. In this way he 

 found that the skin of the nose and lips and the mucous membrane of 

 the tongue are most sensitive to pressure, while the skin of the region 

 of the loins has a very low degree of sensitivity. The minimum 

 stimulus which could be detected in different skin areas is shown in 

 the following table : 



