GENERAL ORGANOLOGY. 



149 



periences gained through our sensory organs. We thus 

 know the external world only in so far as it is accessible to 

 the senses, directed and carefully controlled by the power 

 of judgment. If things exist outside of ourselves which 

 have no influence upon our senses, we can form no con- 

 ception of them. It follows from this proposition that we 

 can gain knowledge of the natural capacity of the sensory 

 organs of animals only by analogy with our own experi- 

 ences. We must extend to the whole animal kingdom 



Hff 



w 



FIG. 73. Cross-section of the human spinal cord. (From Wiedersheim.) Black represents 

 the gray, white the white substance of the cord ; Cc, central canal, surrounded by the 

 anterior and posterior commissures i C and C') \ Sa, Sp, anterior and posterior sulcus ; 

 VW, HIV, anterior and posterior nerve-roots; VH, HH, anterior and posterior horn of 

 gray matter ; y, S, //, anterior, lateral, and posterior columns of white matter. 



the distinction of five senses, viz., touch (including the sense 

 of pressure, and of temperature), smell, taste, hearing, and 

 sighl, which has become intrenched in human physiology. 

 A priori, however, it cannot be denied that sensations occur 

 in animals which we do not experience; in following out 

 this course of thought, we are led to the establishment of a 

 sixth sense. A conception of this kind, however, must re- 

 main to us a meaningless abstraction, since it is impossible 

 for us to conceive of the character of a sense which we lack. 

 Anatomy gives Insufficient Knowledge of Sensory 

 Organs. A further, and still more important, reason for 

 our very fragmentary knowledge of animal sensory life is 

 the fact, that, in regard to the physiological meaning of the 

 sensory apparatus, it is only seldom that we can depend 

 upon experiments, and consequently we must base our 



