74 INTRODUCTION TO NEUROLOGY 



windows so as to look out directly into new fields; but we have 

 increased the range of vision through the old windows, much as a 

 telescope brings remote objects near and as a periscope enables 

 the observer to see around a corner. To the development of 

 the cerebral cortex we owe the acquisition of these new powers 

 which have opened to us the realms of electric vibrations, 

 ultra-violet rays, and many other natural phenomena to which 

 our unaided sense organs are quite insensitive. 



Children in the kindergarten are taught that there are five 

 senses. In reality, there are more than twenty different senses. 

 Some of the sense organs are stimulated by external objects and 

 hence are termed exteroceptors ; others are stimulated by internal 

 excitations of the visceral organs and are termed inter oceptors. 

 Still further classifications have been suggested, to which refer- 

 ence will be made shortly. Here we must first consider the 

 criteria in accordance with which the various senses are dis- 

 tinguished. 



The analysis and classification of the senses is by no means 

 so simple a task as one might at first suppose. It is true that 

 ordinarily we do not confuse a thing seen with a sound heard; 

 but, on the other hand, we do constantly confuse savors with 

 odors, and it often requires refined physiological experimentation 

 to determine whether the organ of taste or the organ of smell is 

 the source of the sensory excitation in question. Most of the 

 common "flavors" of food are, in reality, odors and are perceived 

 by the organ of smell only. A bad cold which closes the pos- 

 terior nasal passages makes "all food taste alike" for this reason. 

 In reality, as we have already seen, there are only four tastes 

 recognized by the physiologists, viz., sweet, sour, salty, and 

 bitter. 



Confusion has arisen in the attempts to analyze these two 

 senses from the fact that different physiologists have used differ- 

 ent definitions of a "sense." One author, who defines these 

 senses in terms of the physical agents which excite them, says 

 that taste is stimulated by liquids and smell by vapors, and that, 

 accordingly, aquatic animals, whose nostrils are filled with water, 

 have by definition no sense of smell. Other authors separate 

 these senses according to the organ stimulated, the excitation of 

 the nose being smell, that of the taste-buds being taste, regard- 



