800 CUTANEOUS SENSATIONS. [BOOK in. 



tary movements, has at the same time special connections with 

 cutaneous and other sensations, and in both relations may be 

 mapped out into areas corresponding to parts of the body. 



But this matter of the sensations is one of even greater com- 

 plexity than that of voluntary movements. This is shewn among 

 other things by the fact that the sensations with which we are 

 now dealing may be profoundly affected by operations on parts 

 of the cortex other than the above region. In the dog, for in- 

 stance, according to many observers, removal of almost any part 

 of the cortex impairs cutaneous sensations, the amount and 

 duration of the effect being broadly proportionate to the extent 

 of cortex removed, though operations on the frontal region have 

 the least effect. Other observers again have found that in the 

 monkey removal or destruction of the gyrus fornicatus (Figs. 122, 

 124) on the mesial surface of the brain, ventral to the calloso- 

 marginal sulcus which forms on the mesial surface the ventral 

 limit of the motor region (an operation of very great difficulty), 

 has brought the whole of the opposite side of the body to a con- 

 dition which has been described as an anesthesia, that is a loss 

 of all cutaneous tactile sensations, and an analgesia, that is a 

 loss of sensations of pain, the condition being accompanied by 

 little or no impairment of voluntary movements and, though 

 apparently diminishing as time went on, lasting until the death 

 of the animal some weeks afterwards. Again, removal of the 

 continuation of the gyrus fornicatus into the gyrus hippocampi 

 has in other instances led to a more transient anesthesia also of 

 the whole or greater part of one side of the body. And it is 

 asserted that removal of no other region of the cortex interferes 

 with cutaneous and painful sensations in so striking and lasting 

 a manner as does the removal of parts, or of the whole of this 

 mesial region. These contradictory results shew how complex 

 and difficult the subject is. 



506. We may now attack the problem in a different way, 

 and instead of beginning with the cortex begin with afferent 

 impulses started along afferent nerves from their peripheral 

 endings, and attempt to trace them centralwards. And first 

 we may call to mind what anatomical guidance we possess. 



We have seen ( 452) that the fibres of posterior roots, the 

 channels of afferent impulses, end in the spinal cord in at least 

 two main ways. One set are continued on, not broken by any 

 relays, in the median posterior tract, and by this tract represen- 

 tatives of all the spinal nerves are connected with the gracile 

 nucleus in which the median posterior column ends. The other 

 fibres of a posterior root end in the grey matter of the cord not 

 far from their entrance ; we have reason to think that they are 

 brought into contact by arborescent endings collateral or ter- 

 minal, with the bodies or processes of certain cells in the grey 

 matter. Putting aside as foreign to our present subject the 



