STRUCTURE OF SENSIFEROUS ORGANS 293 



are a few of the metaphysical topics which are 

 suggested by the most elementary study of 

 biological facts. But, more than this, it may be 

 truly said that the roots of every system of 

 philosophy lie deep among the facts of physiology. 

 No one can doubt that the organs and the 

 functions of sensation are as much a part of the 

 province of the physiologist, as are the organs and 

 functions of motion, or those of digestion; and yet 

 it is impossible to gain an acquaintance with even 

 the rudiments of the physiology of sensation 

 without being led straight to one of the most 

 fundamental of all metaphysical problems. In 

 fact, the sensory operations have been, from 

 time immemorial, the battle-ground of philoso- 

 phers. 



I have more than once taken occasion to point 

 out that we are indebted to Descartes, who hap- 

 pened to be a physiologist as well as a philosopher, 

 for the first distinct enunciation of the essential 

 elements of the true theory of sensation. In 

 later times, it is not to the works of the philoso- 

 phers, if Hartley and James Mill are excepted, 

 but to those of the physiologists, that we must 

 turn for an adequate account of the sensory 

 process. Haller's luminous, though summary, 

 account of sensation in his admirable " Prima3 

 Linea?," the first edition of which was printed in 

 1747, offers a striking contrast to the prolixity 

 and confusion of thought which pervade Reid's 



