446 PHYSIOLOGY 



The term ' tactile and motor sensibility ' is very inadequate as describing 

 the complex afferent impressions which proceed from all parts of the body 

 to the brain. They may perhaps be better grouped under the term * somatic 

 impressions,' and include three main classes, viz. : 



Exteroceptive . . From the surface of the body 



Enteroceptive . . From the viscera 



I'roprioceptive . . From the muscles and joints, aroused by 



changes occurring within the organism 



itself. 



Of these, the exteroceptive are the most important in giving information 

 as to the external world, and predominate among those impressions which 

 reach and affect consciousness. The enteroceptive under normal conditions 

 very rarely rise to the conscious level. The proprioceptive impressions are 

 also for the most part unconscious, yet those, which do reach consciousness, 

 play a great part, in conjunction with the exteroceptive, in forming the 

 basis of our schema of the material world. 



We find as Head has shown a constant regrouping of somatic impres- 

 sions as we trace them from their origin, at or near the surface of the body, 

 through the spinal cord and nerve tracts to the cerebral cortex. At the 

 periphery these impressions are divided into superficial and deep sensations, 

 and the former again into the epicritic, which determine localisation, dis- 

 crimination and the finer. gradations of pressure, heat, and cold, and the proto- 

 /xithic, comprising pain, the coarser degrees of heat and cold, and tactile 

 s-Mise with defective localisation. 



When these various impulses reach the cord, they are regrouped, so 

 that the pain, heat and cold, and tactile sensations are collected each in a 

 separate bundle, with no distinction between the coarser kinds of tactile 

 sense and the finer qualities involved in discrimination and localisation. 

 This grouping persists as far as the thalamus, and even beyond the thalamus 

 a similar grouping is observed in the sub-cortical white matter through 

 which the tracts run from the thalamus to the sensory cortex. Lesions at 

 any part of these paths may therefore affect one or more of these qualities 

 of sensation separately. 



On arrival at the cortex cerebri all these different kinds of sensation are 

 poured into the grey matter to form the basis of the schema of the external 

 world and the relations thereto of the individual. The cortical type of loss 

 of sensation differs therefore profoundly from the loss produced by a lesion in 

 any other part of the sensory tracts. It may occur as the result of lesions 

 of the pre- and post-central convolutions, of the internal part of the superior 

 parietal lobule and of the angular gyri. The chief feature of this cortical 

 loss of sensation is a defect, not in one or other of the different sensations 

 which have been described, but in the appreciation of the meaning of these 

 sensations, i.e. the loss appears to be rather psychical than physiological. 

 Thus, it is not a question of recognition of touch, pain, heat and cold, but of 

 certain discriminating faculties which can be classed as : (a) recognition of 



