CUTANEOUS SENSATIONS 635 



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- 

 over 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 

 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. In the case of the skin the sense organs concerned in pain 

 appear to be the free intra- epithelial nerve endings. Pain is found to differ 

 somewhat from the other skin sensations in being much more uniformly 

 distributed, more difficult to locate accurately, and more hardy. Thus 

 while most sense organs are rendered less sensitive by cutting off blood 

 supply, pain at first reacts more violently. 





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 fairly accurately carried out. On the other hand, the 

 sensations of light touch, 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 or 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 the 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. l 



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 of its cutaneous nerves, during the process of regeneration of these 

 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 



