ICO THE SENSE OF TOUCH IN ANIMALS 



ceased to operate. It is known tliat in a disease such as that 

 called locomotor ataxy, or tabes dorsalis, the sensory tracts of 

 the spinal cord are damaged and this contributes the main cause 

 of the inabihty felt by the sufferer to stand firm when his eyes 

 are shut, two of the four great afferent impressions to the cere- 

 bellum being thus for the time cut off. Such persons also find 

 greater difficulty in walking at night than in a good hght. It 

 has also been shown in the case of brainless frogs, that if the 

 skin of the hind legs be stripped off, they are unable to ascend an 

 inclined plane in a reflex maimer, which they can do when the 

 skin is intact. This use, then, of the tactile sense in the pre- 

 servation of man's balance in walking and standing is of funda- 

 mental importance to liis well-being and success in the struggle 

 of life. 



The value of the roughened surface of the foot with its papillary 

 ridges can hardly have been great, even in the days when the 

 foot of man was naked ; for these low, soft ridges could do Uttle 

 to prevent shpping on such surfaces as grass, sand, rock, wet 

 or dry, and from the time when he began to protect his feet with 

 coverings, this small value would be still further reduced. Whereas 

 the increased dehcacy of the tactile sense conveyed by the ridges 

 would be most useful in intimating to his higher centres the 

 important varieties of surface injurious or otherwise, with which 

 his foot would be in contact. 



These two main uses to which the tactile sense in man is put 

 are then : 



(1) Discriminative sensibility. 



(2) Maintenance of equilibrium. 



(1) The first is the only one which the human hand exercises, 

 the second that which the human foot alone exercises. 



In the lower Primates and still lower Mammals, the hand and 

 foot both exercise the first of these, though in a lower degree 

 than man, whereas the second is exercised by the hand and foot 

 in a nearly equal degree. 



The discriminative sensibihty of the hand and foot becomes 

 of necessity less important to lower Mammals than it is to man, 

 the varieties of surfaces touched and judgments concerning them 



