944 CUTANEOUS SENSATIONS. 



1. The object in contact excites a certain number of tactual sensa- 

 tions of definite intensities and imbued with " local signs." These com- 

 ponent sensations are combined, either (1) simultaneously, or (2) suc- 

 cessively as well as simultaneously. 2. The successive component 

 sensations can be acquired by actively moving the tactual surface, to increase 

 or renew the simultaneously-given groups of sensations, and in so doing 

 the muscular sense is combined with the tactual in obtaining criteria for 

 judging the shape and size of the object by the distances moved. 3. The 

 tactual perceptions can be accompanied by visual. 4. The tactual per- 

 ceptions can be accompanied by both muscular and visual. 



The man blind from birth, on whom Cheselden performed his his- 

 toric operation, on first obtaining his sight described the visual field as 

 fiat and its contents as " seeming to touch his eyes." Children cured of 

 congenital cataract at the age of 6 and 7, when, in spite of blindness, 

 well acquainted through their other senses with the form and number 

 of objects, and able to remember, recognise, and describe them, have still, 

 on first obtaining vision, to touch objects in order to know what and 

 how many they are. When shown a box, a coin, two fingers, etc., they 

 do not recognise what they are, nor their squareness, roundness, or that 

 the fingers are two (not one or three) ; after touching them over (tactual 

 and muscular sensations), they recognise the species, form, and number 

 at once. 1 Judgment of direction and distance has similarly to be trans- 

 lated into visual signs from tactual and muscular. 2 The child for many 

 days after recovering sight, if in a strange room, feels its way between 

 the furniture as though in a dark place. Faulty naming of objects 

 seen, that are perfectly correctly named when touched, is said to be 

 traceable in such children for some years. 



The degree of importance of tactual sensation for guidance and 

 execution of movement is very various. To enervate the foot induces no 

 obvious defect in the stepping of the horse. A bird, in which the 

 cutaneous nerves of the foot have been paralysed by section, will still 

 grasp the perch, and sleep balanced on the enervated foot. 3 On the other 

 hand, Bell found the prehensile movements of the lips of the horse were 

 abolished by section of the sensory nerves to the face. A monkey, in which 

 the fifth cranial nerve of one side has been cut, and sensation lost there, 

 keeps, in drinking from a bowl, moving round it toward the side away 

 from the side of section. 4 The guiding sensations for drinking with the 

 lips are therefore almost exclusively supplied by touch. 



Hering has devised a cylinder aisthesiometer for testing delicacy of 

 judgment of surface form by touch. A set of reels (usually twelve), 

 each about 8 cm. long and 15 mm. in diameter, are closely wound 

 with silvered wire, each reel with a differently-sized wire, the coarsest 

 wire being 1 mm. thick. One reel has an absolutely smooth polished 

 turned surface. The test is carried out by finding with which of the 

 reels the sensation of roughness or smoothness begins or ends. To 

 the finger-tips of many persons, whose hands are not much employed 

 for rough labour, the coils of wire of 11 mm. thickness give a dis- 

 tinctly " uneven feel." 



1 W. Uhthoff, Ztschr.f. Psychol, u. Physiol, d. Sinncsorg., Hamburg u. Leipzig, 1898, 

 Bd. xiv. S. 200. 



2 Francke, Btitr. z. Avgenh., Hamburg u. Leipzig, 1894, Bd. xvi. S. 1 ; Raehlmann, 

 Ztschr. f. Psychol, u. Physiol, d. Sinnesorg., Hamburg u. Leipzig, 1895, Bd. viii. S. 401. 



3 A. Chauveau, Brain, London, 1891, vol. xiv. 



4 Sherrington, Phil. Trans., London, 1892. 



