CHAP. XI.] DISPOSITION OF NERVES FOB REFLEX ACTIONS. 335 



from which the nerves of the upper extremities arise, as nearer the 

 seat of lesion, than upon the lumbar segment. But another explana- 

 tion appears to us equally satisfactory, and more accordant with 

 other phenomena. A certain disposition of the nerves upon the tegu- 

 mentary surface is as necessary for the development of reflex actions as 

 of sensations; and these movements will be more or less easily mani- 

 fested, according as this organization of the nerves on the surface 

 is more or less perfect. 



That disposition of the cutaneous nerves which renders the sur- 

 face easily excitable by ti dilation seems most favourable to the 

 development of these actions. Hence, there is no place where they 

 are more readily excited than in the lower extremities by stimulat- 

 ing the soles of the feet or the intervals between the toes, both of 

 which situations are highly susceptible of titillation. At the isthmus 

 faucium the slightest touch on the surface excites a movement of 

 deglutition; and this touch, at the same time, produces a very 

 peculiar sensation of tickling, quite distinct from that which may be 

 excited at other parts of the pharynx, or mouth. When this part 

 of the mucous membrane is in a state of irritation as an effect of 

 coryza, this tickling sensation is present, and repeated acts of swal- 

 lowing are provoked. 



Two facts may be stated here, which illustrate the position we 

 have laid down respecting the necessity of a certain disposition of 

 the nerves on the tegumental surface, for the development of reflex 

 actions. The first is one which has been noticed by Volkmann, and 

 which we had ourselves repeatedly observed, namely, that in frogs, 

 and other animals, reflex actions are readily excited by stimu- 

 lating the feet ; but irritating the posterior roots of the spinal 

 nerves, which supply those parts, is not sufficient for this purpose. 

 In experiments repeatedly made upon the posterior roots of the 

 nerves we have very rarely seen movements excited whilst they 

 have been subjected to irritation, and the recorded statements of 

 all modern experimenters agree in the main with this statement. 

 The second fact is this : in the male frog the development of a 

 papillary structure on the skin of the thumb seems to have reference 

 to the excitation of the physical power of the cord, to enable 

 the animal to grasp the female without the necessity of a prolonged 

 exercise of volition. Stimulating the fingers will scarcely produce 

 reflex actions, but the slightest touch to the enlarged thumb will 

 cause the animal to assume the attitude of grasping. If the papillae 

 be shaved off the thumb, its power of exciting these actions is 

 instantly lost. 



