234 



OF SENSATION, AND THE ORGANS OF THE SENSES. 



We now proceed to consider in more detail the functions of the several Organs 

 of the Senses, and shall commence with that of the most general character. 



II. Sense of Touch. 



316. By the sense of Touch, as commonly understood, is meant that modifi- 

 cation of the common sensibility of the body, of which the cutaneous surface 

 is the especial seat. It derives its peculiar powers simply from the large 

 amount of sensory nervous fibres, which are distributed in its substance ; and 

 especially through the terminations (or rather the origins) of these in the 

 papilla, which are little elevations of the surface of the cutis, easily percepti- 

 ble by the aid of a lens, and each chiefly composed of a vascular plexus sur- 

 rounding the extremity of the nervous fibril. The number of these papillae 

 within any given area, pretty closely corresponds with the degree of sensibility 

 of that part of the surface ; thus we find them most abundant on the hands, 

 especially towards the points of the fingers, and on the lips and tongue. In 

 some animals, especially those of the Feline tribe, the long vibrisse (commonly 

 termed whiskers) evidently minister to sensation ; and it has been demon- 

 strated that their pulps are largely supplied with nerves from the fifth pair. 

 Some interesting observations have been made by Prof. Weber, on the sensi- 

 bility of different parts of the skin. His mode of ascertaining this, was to 

 touch the surface with the legs of a pair of compasses, the points of which 

 were guarded with pieces of cork ; the eyes being closed at the time, the legs 

 were approximated to each other, until they were brought within the smallest 

 distance, at which they could be felt to be distinct from one another. The 

 following are some of the results of the experiments. With the extremities 

 of the fingers and the point of the tongue, the distance could be distinguished 

 most easily in the longitudinal direction ; on the dorsum of the tongue, the 

 face, neck, and extremities, the distance could be recognized best when the 

 arms were placed tranversely. 



Point of middle finger - of aline 

 Point of tongue - - ^ of a line 

 Palmar surface of third finger 1 line 

 Red surface of lips - 2 lines 



Palmar surface of middle finger 2 

 Dorsal surface of third finger 3 

 Tip of the nose - -3 

 Dorsum and edge of tongue 4 

 Part of the lips covered by skin 4 

 Palm of hand - - 5 



Skin of cheek - - 5 



Extremity of great toe - 5 

 Hard palate - - 6 



Dorsal surface of forefinger 7 

 Dorsum of hand - - 8 



Mucous membrane of gun's 



Lower part of forehead 



Lower part of occiput 



Back of hand 



Neck, under lower jaw 



Vertex 



Skin over Patella 



Sacrum 



acromion 



Dorsum of foot 

 Skin over sternum 

 Skin beneath occiput 

 Skin over spine, in back 

 Middle of the arm 

 thigh 



9 lines 



10 



12 



14 



15 



15 



16 

 1 



18 



18 



20 



24 



30 



30 



30 



It is curious that the distance between the legs of the compasses seemed to be 

 greater (although really so much less), when it was felt by the more sensitive 

 parts, than when it was estimated by parts of less distinct sensibility. As a 

 general fact, it seems that the sensibility of the trunk is greater on the median 

 line, both before and behind, and less at the sides. Differences of tempera- 

 ture, and the weight of bodies, were, according to Prof. Weber's observations, 

 most accurately recognized at the parts which were determined to be most 

 sensible by the foregoing method of inquiry. 



317. As already stated ( 308), the only idea communicated to our minds by 

 the sense of Touch, when exercised in its simplest form, is that of Resistance ; 

 but when the sensory surface and the substance touched are made to change 



