152 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



viscera, but to the skin, or, strangely enough, to the 

 air. But even now a full knowledge of the visceral 

 sensations is lacking. Yet we know enough to realize 

 that while the visceral sensations in general do not 

 rise into consciousness (or do so only vaguely), they 

 are capable of influencing the emotions and, at times, 

 of arousing very definite mental states. Finally, 

 an extremely significant source of sensory impulses 

 in the brain is the pathways of the special sensory 

 nerves, the olfactory, visual, auditory, and gustatory. 

 When we realize the extent of the sensory mecha- 

 nism and the variety of its forms of sensibility, it is 

 easy to understand how disease, often slight in its 

 development, may so implicate the sensory mecha- 

 nism as to send abnormal stimuli to the brain itself, 

 yet without exciting definite pain. Chronic affec- 

 tions of the sensory system, involving the special 

 senses, the skin, the muscles, or the viscera, often 

 serve to excite excessive reactions in the sensory re- 

 cording apparatus in the brain. If the reactions are 

 painful, they are so obtrusive as to invite remedial 

 interference. But very commonly the stimuli are 

 not of a painful nature. They are on this account 

 none the less effective in causing emotional or mental 

 disturbances. Vague sensations of discomfort, often 

 inconstant and not easily described or localized, 

 make their way into conscious life and serve to de- 

 prive the individual of comfort. The incubus of 

 such stimulation may be, in time, sufficient to inter- 

 fere with the smooth working of the finer cerebral 



