552 PHYSIOLOGY 



temperature spots are insensitive to a low grade of stimulus. As the 

 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 moreover 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. 



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. Simul- 

 taneous 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 therefore 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, protopaihic 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 



