CHAPTER IV 

 STIMULATION THE SENSES 



IN order that any organism may be able to make use of, or adapt 

 itself to occurrences, in the outer world, it must possess means 

 of obtaining knowledge of what is going on there. The various 

 things that happen must, in some way, produce changes in the 

 outer surface of the organism that is accessible to their influence. 

 In other words, there must be structures capable of being "stimu- 

 lated," or changes produced in them, by the forms of energy that 

 strike upon them. 



When this has taken place, the nerves connected with these 

 " receptors " or organs of sense, as we may now call them, convey 

 messages to the brain. They are then perceived in consciousness 

 in a manner at present inexplicable, and may, sooner or later, 

 result in muscular activity adapted to take advantage of the 

 information received. 



It will be seen that we cannot properly separate the discussion 

 of the senses from that of the nervous system, and we might have 

 taken the latter into consideration first. But whichever order is 

 chosen, it is impossible to treat either one without assuming 

 or forestalling what must necessarily be described later. Indeed, 

 although for convenience it is usual to subdivide physiological 

 phenomena into sections, they are, in reality, all parts of one system 

 acting as a whole. This will have been manifest to the reader 

 already, and for this reason no physiological text-book can be 

 understood by reading it through once. 



If we take a frog whose central nervous system consists of the 

 spinal cord only, a "spinal frog" as it is called, we shall find that 

 by stimulating the skin in a variety of ways we can produce 

 movements (E., p. 209). These are called " reflex," because the 

 message conveyed to the nerve centre is " reflected back " along 

 another set of nerves, and causes muscular contractions. The name 

 " reflex " is thus given to those movements which result from a 

 stimulus without necessarily involving conscious perception of the 

 stimulus. The phenomena of consciousness are only present when 

 the highest part of the brain, the cerebral hemispheres, are intact. 



