SENSATION AND SENSIFEROUS ORGANS. 91 



hand, looking at an odorous substance, or rubbing it on the skin, or 

 holding it to the ear, does not awaken the sensation. Thus, it can be 

 readily established by experiment that the perviousness of the nasal 

 passages is, in some way, essential to the sensory function ; in fact, 

 that the organ of that function is lodged somewhere in the nasal pas- 

 sages. And, since odorous bodies give rise to their effects at consider- * 

 able distances, the suggestion is obvious that something must pass from 

 them into the sense-organ. What is this something which plays the 

 part of an intermediary between the odorous body and the sensory 

 organ ? 



The oldest speculation about the matter dates back to Democritus 

 and the Epicurean school, and it is to be found fully stated in the fourth 

 book of Lucretius. It comes to this : that the surfaces of bodies are 

 constantly throwing off excessively attenuated films of their own sub- 

 stance ; and that these films, reaching the mind, excite the appropriate 

 sensations in it. 



Aristotle did not admit the existence of anv such material films, 

 but conceived that it was the form of the substance, and not its mat- 

 ter, which affected sense, as a seal impresses wax, without losing any- 

 thing in the process. While many, if not the majority, of the school- 

 men took up an intermediate position, and supposed that a something 

 which was not exactly either material or immaterial, and which they 

 called an " intentional species," effected the needful communication 

 between the bodily cause of sensation and the mind. 



But all these notions, whatever may be said for or against them in 

 general, are fundamentally defective, by reason of an oversight which 

 was inevitable, in the state of knowledge at the time in which they 

 were promulgated. What the older philosophers did not know, and 

 could not know, before the anatomist and physiologist had done his 

 work, is that, between the external object and that mind in which they 

 supposed the sensation to inhere, there lies a physical obstacle. The 

 sense-organ is not a mere passage by which the " tenuia simulacra re- 

 rum," or the "intentional species" cast off by objects, or the "forms" 

 of sensible things, pass straight to the mind ; on the contrary, it stands 

 as a firm and impervious barrier, through which no material particle of 

 the world without can make its way to the world within. 



Let us consider the olfactory sense-organ more nearly. Each of the 

 nostrils leads into a passage completely separated from the other by a 

 partition, and these two passages place the nostrils in free communica- 

 tion with the back of the throat, so that they freely transmit the air 

 passing to the lungs when the mouth is shut, as in ordinary breathing. 

 The floor of each passage is flat, but its roof is a high arch, the crown 

 of which is seated between the orbital cavities of the skull, which serve 

 for the lodgment and protection of the eyes ; and therefore lies behind 

 the apparent limits of that feature which in ordinary language is called 

 the nose. From the side walls of the upper and back part of these 



