96 THE POPULAR SCIENCE MONTHLY. 



pardon me, you were ignorant, but you did not know it. On the con- 

 trary, you thought you knew a great deal, and were quite satisfied with 

 the particularly absurd metaphysical notions which you were pleased to 

 call the teachings of common sense. You thought that your sensations 

 were properties of external things, and had an existence outside of your- 

 self. You thought that you knew more about material than you do 

 about immaterial existences. And if, as a wise man has assured us, 

 the knowledge of what we don't know is the next best thing to the 

 knowledge of what we cio know, this brief excursion into the province 

 of philosophy has been highly profitable. 



Of all the dangerous mental habits, that which schoolboys call " cock- 

 sureness " is probably the most perilous ; and the inestimable value of 

 metaphysical discipline is, that it furnishes an effectual counterpoise to 

 this evil proclivity. Whoso has mastered the elements of philosophy 

 knows that the attribute of unquestionable certainty appertains only to 

 the existence of a state of consciousness so long as it exists ; all other 

 beliefs are mere probabilities of a higher or lower order. Sound meta- 

 physic is an amulet which renders its possessor proof alike against the 

 poison of superstition and the counter-poison of nihilism ; by showing 

 that the affirmations of the former and the denials of the latter alike 

 deal with matters about which, for lack of evidence, nothing can -be 

 either affirmed or denied. 



I have dwelt at length upon the nature and origin of our sensations 

 of smell, on account of the comparative freedom of the olfactory sense 

 from the complications which are met with in most of the other senses. 



Sensations of taste, however, are generated in almost as simple a 

 fashion as those of smell. In this case, the sense-organ is the epithelium 

 which covers the tongue and the palate ; and which sometimes, be- 

 coming modified, gives rise to peculiar organs termed " gustatory 

 bulbs," in which the epithelial cells elongate and assume a somewhat 

 rod-like form. Nerve-fibers connect the sensory organ with the sen- 

 sorium, and tastes or flavors are states of consciousness caused by the 

 change of molecular state of the latter. In the case of the sense of 

 touch there is often no sense-organ distinct from the general epider- 

 mis. But many fishes and amphibia exhibit local modifications of the 

 epidermic cells which are sometimes extraordinarily like the gustatory 

 bulbs ; more commonly, both in lower and higher animals, the effect 

 of the contact of external bodies is intensified by the development of 

 hair-like filaments, or of true hairs, the bases of which are in immediate 

 relation with the ends of the sensory nerves. Every one must have 

 noticed the extreme delicacy of the sensations produced by the contact 

 of bodies with the ends of the hairs of the head ; and the " whiskers " 

 of cats owe their functional importance to the abundant supply of nerves 

 to the follicles in which their bases are lodged. What part, if any, the 

 so-called "tactile corpuscles," "end-bulbs," and "Pacinian bodies" play 



