692 SENSATION. [CH. L. 



specialised kind. The most frequent kind of sensory end-organ is 

 made of what is called nerve-epithelium ; certain epithelial cells of 

 the surface of the body become peculiarly modified, and grouped 

 in special ways to receive the impressions from the outer world ; 

 these send an impulse into the arborisations at the termination of 

 the axis-cylinders of the nerves which envelop the cells. One 

 of these varieties of nerve-epithelium we have already made the 

 acquaintance of, in the hair-cells of the semicircular canals ; we 

 shall find other kinds in the hair-cells of the cochlea, in the rods 

 and cones of the retina, &c. 



Pain is due to an excessive stimulation of the other sensory 

 nerves, but there is some evidence that it may be a distinct sen- 

 sation. Thus in some cases of diseases of sensory channels, tactile 

 sensation may be intact, but sensitiveness to pain absent, and 

 vice versa ; see also p. 658. 



The other essential anatomical necessities for a sensation, the 

 channels to the brain with their numerous cell-stations on the 

 road, and the parts of the brain to which these tracts pass, we 

 have already dwelt upon. Some of these points we shall, how- 

 ever, be obliged to retiirn to, especially in connection with vision. 

 But here it is sufficient to insist on the necessity of the presence 

 not only of the end-organ, but also of the nervous tracts and 

 centres. Blindness, for instance, may not only be due to disease 

 of the eye, but also to disease of the optic nerve, or of the parts 

 of the brain to which the optic nerve passes. 



A small stimulus, or a small increase or decrease in a big 

 stimulus, will have no effect ; a light touch, a feeble light, a 

 gentle sound, may be so slight as to produce no effect on the 

 brain. The smallest stimulus that produces an effect is called 

 the lower limit of excitation or the liminal (from limen, a 

 threshold) intensity of the sensation. The height of sensibility or 

 maximum of excitation is a stimulus, so strong that the brain 

 is incapable of recognising any increase in it; a bright light, 

 for instance, may be so intense that any increase in its brightness 

 is not perceptible. Between these two extremes we have what 

 is called the range of sensibility. Most of our ordinary sensations 

 fall somewhere about the middle of the range, and Weber's or 

 Fechner's law is a law that regulates the proportion between the 

 stimulus and the sensation, and which is operative for this region 

 of the range of sensibility. In general terms it may be stated 

 that sensations increase as the logarithm of the stirmili ; or, in 

 order that the intensity of a sensation may increase in arith- 

 metical progression, the stimulus must increase in a geometrical 

 progression. 



