716 SENSATION [CH. LI. 



chiefly associated with general sensibility, that vague kind of sensa- 

 tion which cannot be put under any of the special headings taste, 

 sight, hearing, touch, and smell. The nerve-endings of the nerves of 

 special sense are usually end-organs of a 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, etc. 



Pain is due to an excessive stimulation of the other sensory 

 nerves, but there is some evidence that it may be a distinct sensation. 

 Thus in some cases of diseases of sensory channels, tactile sensation 

 may be intact, but sensitiveness to pain absent, and vice versd ; see 

 also p. 668. 



The other essential anatomical necessities for a sensation are the 

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

 and the parts of the brain to which these tracts pass. 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 

 law (as expanded by Fechner) 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 stimuli; 

 or, in order that the intensity of a sensation may increase in 

 arithmetical progression, the stimulus must increase in a geometrical 

 progression. 



A definite example will help us to understand these mathemati- 

 cal terms a little better. We will select our example from the 

 sense of vision, because the intensity of the cause of visual sensa- 



