THE ORGANS OF SENSE. Ill 



placed between them in this position (Aristotle's experiment), the 

 sensation produced will be of two peas, because the individual has no 

 previous experience of a single stimulus applied in this way. 



PROTOPATHIC AND EPICRITIC SENSIBILITY. 



The investigations of Head have made it possible to subdivide 

 cutaneous sensations into two main groups, protopathic and epicritic. 

 This classification is based upon a study of the order of return of the 

 cutaneous sensations during regeneration after division of nerves. 

 Head observed the recovery of sensation in patients in whom nerves 

 had been accidentally divided, and also had one of his own cutaneous 

 nerves divided in order to study the matter subjectively. He found 

 that after section of a cutaneous nerve all cutaneous sensations were 

 lost, but that there remained the sensations of deep pressure and of 

 pain due to deep pressure, which depend on stimulation of the afferent 

 fibres in the muscles. During regeneration of the divided nerve, the 

 first sensations to return are those of pain, of heat for temperatures 

 above 38 C., and of cold for temperatures below 24 C. These are the 

 protopathic sensations, and they return from seven to twenty-six weeks 

 after the division of the nerve. Tactile localisation and discrimination, 

 the accurate localisation of pain in the skin, and the sense of heat and 

 cold between 37 C. and 25 C., are recovered in from one to two years 

 after the nerve section, and constitute the epicritic sensations. 



Protopathic sensations are "affective" in character, the sensa- 

 tion being intense, prolonged, and usually disagreeable ; they are 

 obviously associated with the dpfcnajvp. mechanism of the body. 

 The seat of the protopathic sensations is believed to be located in the 

 thalamus. Epicritic sensation, on the other hand, is on a higher 

 plane, and is essential for the development of manual dexterity. It 

 may be looked upon as the latest and highest evolution of the tactile 

 sensory mechanism, and the seat of sensation is in the cortex of the 

 cerebral hemispheres. 



SECTION II. 

 THE SENSES OF TASTE AND SMELL. 



The sense of taste is localised in the mucous membrane of the 

 mouth and fauces, especially in that covering the tongue, and the end- 

 organs concerned are widely distributed in the mucous membrane. 

 The principal distribution of the nerves of taste, however, is in connec- 

 tion with the vallate papillae which are found at the base of the tongue. 

 Structures called taste bulbs lie in the stratified squamous epithelium 



