CHAPTER VII 



THE SPECIAL NERVOUS MECHANISMS 



The very general survey that we have just made of the rough 

 anatomy of the central nervous system will enable the reader to 

 study in greater detail the more special mechanisms into which 

 we, rather arbitrarily, decompose the whole. These mechanisms 

 are those of sensation, of motor control, and of co-ordination. 



The Sensory Mechanisms. 



" Sensation " involves the stimulation, by some physical 

 agency, of receptor organs distributed everywhere in the body. 

 We must suppose that all bodily tissues are irritable — that is, 

 that they react in some way to stimuli, which may be chemical 

 or physical. From what we know of the irritability of the sur- 

 face tissues of the lower organisms, we may also conclude that 

 this is a general irritabiUty, and that the same tissue is potentially 

 susceptible to light, electric, chemical, and mechanical stimuli. 

 But in the higher animals the general irritability of the tissues 

 of the lower organisms becomes modified by the evolution of the 

 special organs of sense. We must think of this generalised 

 susceptibility of the skin, say, as being restricted, so that any 

 one kind of stimulus must be intense enough to pass over a 

 " threshold " ; if it is too feeble, the receptor is not stimulated 

 at all. A special sense organ, therefore, is a part of the skin 

 or other tissue where the height of the threshold is reduced for 

 stimuli of some particular nature, and raised for all others. 

 Thus the retina is a highly specialised part of the embryonic 

 outer surface, which is extremely sensitive to the stimulus of 

 light, but is relatively insusceptible to changes of atmospheric 

 pressure, while its situation is such that it is not usually stimu- 

 lated chemically, electrically, or mechanically. Similarly, the 

 auditory hairs in the cochlear part of the internal ear are highly 

 susceptible to sound vibrations occurring in the atmosphere 

 outside, while they are so sequestered that they are not readily 

 exposed to stimuli of any other kind. The nerve terminations 

 of the olfactory and gustatory nerves in the mucous membranes 

 of the nose and mouth are so placed that they are very readily 



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