SENSORY PATHS 285 



cut. Head therefore believes that in the nerve-trunks 

 three forms of sensation are carried^protopathic, epicritic 

 and deep. 



For the investigation of the Central Paths taken by 

 afferent impulses two methods have been used. The first 

 is the examination of patients suffering from partial injury 

 to the spinal cord. For touch and pressure this is, indeed, 

 the only method, but for pain there is in addition a second 

 method, based upon Sherrington's Pseudoaffeetive Reflexes. 

 An animal is deprived of its cerebrum and a sensory nerve 

 stimulated. It cannot, of course, feel pain, but the reflex 

 arcs subserving the bodily expression of the emotions are 

 intact. There are snarhng movements of the face, move- 

 ments of the limbs and an elevation of blood-pressure. 

 The occurrence of these changes when a nerve is stimulated 

 denotes that pain would have been felt had the cerebrum 

 been present. Different columns of the cord are divided, 

 and the effect upon the transmission of the sensory impulse 

 noted. 



Within the cord there is a complete regrouping of sensa- 

 tions. There is no longer a distinction between protopathic 

 and epicritic, nor between superficial and deep sensations. 



Sensations of light touch and dee.]) pressure pass upwards 

 on the same side for a variable distance, then cross over 

 gradually and continue their upward course in the anterior 

 columns. Arriving at the optic thalanii, they are con- 

 tinued in a fresh relay of fibres to the cortex. The part 

 of the cortex concerned is the pre-central (motor) area, 

 and probably the adjacent post-central area. When these 

 areas are irritated, tinghng sensations are felt. Conscious 

 sensations of passive movement are located in the motor 

 area. 



Sensations of 'pain and of temperature of all kinds are 

 beheved to decussate immediately on entering the cord 

 and to pass up in the anterolateral region, eventually 

 reaching the optic thalamus. 



