CUTANEOUS SENSATIONS 495 



strength of the stimulus is increased a point is suddenly reached at which 



the sensation evoked is painful. Moreover in parts of the body tactile and 



temperature sense are entirely wanting, though painful impressions can be 



easily evoked. The best example of this is seen in the cornea, minimal 



stimulation of which evokes pain, but nothing which can be regarded as 



a tactile sensation. The specific quality of pain sensation is shown more- 



jover by the fact that in many cases of disease the sense of pain may be 



/abolished without the sense of touch. Such a patient is said to suffer 



I from analgesia, but not anaesthesia. When pricked on an analgesic part 



the patient can say that he is pricked, but has no objection to any amount 



of repetition of the stimulus, since the sensation is entirely devoid of painful 



character. 



THE WORK OF HEAD ON CUTANEOUS SENSIBILITY 

 In a long series of researches on man Head has shown that three different 

 classes of sensations may be evoked by stimuli applied to the surface of 

 the body. In order to study the functions of the afferent nerves Head 

 has investigated not only the condition of patients, the subjects of accidental 

 division of cutaneous or other nerves, but also (in conjunction with Rivers) 

 the effects of nerve section on himself. In the first place, it is necessary to 

 differentiate deep sensibility from cutaneous sensibility proper. After 

 desensitisation of any given area of the skin it is still possible in this area 

 to appreciate deep pressure and pain, and the localisation of the situation 

 of the pressure is fajjjly accurately carried out. On the other hand, the 

 sensations of light toucn^ as well as of temperature and the pain evoked 

 by a light pin prick, are absent. The sensations of pressure, as well as of 

 deep pain oi^pressure pain, are therefore carried by the nerves of deep 

 sensibility. These nerves are not the cutaneous nerves, but are derived 

 from the sensory elements in the muscular nerves. To the fingers, for instance, 

 they run in the tendons of the muscles. Simultaneous division, as by a 

 circular-saw cut, of th"e cutaneous nerves and tendons to the fingers will 

 abolish deep as' well as superficial sensibility. Deep sensibility must there- 

 fore be classified, anatomically at any rate, with the ' organic sensations ' 

 of muscular effort and of position, which will be dealt with in a subsequent 

 section. 



Cutaneous sensibility proper Head divides into two categories, namely, 

 protopathic and epicritic sensibility. These two forms of sensibility may 

 be studied separately on an area of skin, which has been desensitised by 

 section oTitifcutaneous nerves, during the process of regeneration of thrs.> 

 nerves. 



PROTOPATHIC SENSIBILITY returns to the skin at an interval of 

 seven to twenty- six weeks after the nerve section. At this time it is 

 possible to appreciate in the area under investigation the sensation of pain, 

 and to recognise roughness of an object rubbed on the skin. Localisation 

 is still somewhat diffuse and inaccurate, so that the sensation evoked by 

 stimulation of the protopathic area may be referred to some adjoining normal 



