GENERAL ORGANOLOGT. 



125 



III. Sensory Organs. 

 Sensations of the Lower Animals. — What we know of the 

 character of the external world is founded upon experiences gained 

 through our sensory organs. We thus know the external world 

 only in so far as it is accessible to the senses, controlled by the 

 judgment. If things exist outside of ourselves which have no 

 influence upon our senses, we can form no conception of them. 

 It follows from this proposition that we can gain knowledge of the 



Fig. 76.— Cross-section of the human spinal cord. (From Wiedersheim.) Black repre- 

 sents the gray, wiiite the white substance of the cord ; Tc, central canal, sur- 

 rounded by the anterior and posterior commissures (C and CO; 'S'a, N/A anterior 

 and posterior fissures ; VW^ HW, anterior and posterior nerve-roots : VH, HH^ 

 anterior and posterior horns of gray matter; V, S, H, anterior, lateral, and pos- 

 terior columns of white matter. 



natural capacity of the sensory organs of animals only by analogy 

 with our own experiences. Hence the distinction of five senses, 

 touch, taste, smell, hearing, and sight, based upon human 

 physiology has been extended to the whole animal kingdom. A 

 priori, however, it cannot be denied that sensations may occur in 

 anim.als which we do not experience; following out this course of 

 thought has led to the idea of a -sixth sense,' which, however, 

 must remain 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 fragmen- 

 tary knowledge of animal sensations is the fact that, in regard to 

 the physiological meaning of the sensory apparatus, it is seldom 

 that we can depend upon experiments, and consequently we must 

 base our conclusions upon structure. But the anatomy of many 

 sensory organs, like those of smell and taste, is by no means so 

 characteristic that it alone is sufficient to determine the physio- 

 logical significance. 



Tactile Organs. — The skin of animals functions as a tactile 

 organ, usually over the whole area, although not everywhere with 



