190 THE HUMAN BODY. 



is excited through special end organs which are attached to 

 some nerves. These stimuli are (1) Changes occurring in 

 central organs, of whose nature we know next to nothing, 

 but which excite the efferent nerve-fibres connected with 

 them. The remaining special stimuli act on afferent fibres 

 through the sense-organs. They are (2) Light, which by the 

 intervention of organs in the eye excites the optic nerve. (2) 

 Sound, which by the intervention of organs in the ear excites 

 the auditory nerve. (3) Heat, which through end organs in 

 the skin is able, by very slight changes, to excite certain 

 nerve-fibres: such slight changes of temperature being 

 efficient as would be quite incapable of acting as general 

 nerve stimuli without the proper end organs. (4) Chemical 

 agencies. These when extremely feeble and incapable of 

 acting as general stimuli, can act as special stimuli through 

 special end organs in the mouth and nose (as in taste and 

 smell) and probably in other parts of the alimentary tract, 

 where very feeble acids and alkalies seem able to excite 

 certain nerves, and reflexly through them excite movements 

 or stir up the cells concerned in making the digestive 

 liquids; for example the contraction of the gall-bladder 

 already referred to. (5) Mechanical stimuli when so feeble 

 as to be inefficient as general stimuli. Pressure on the 

 skin of the forehead or the back of the hand, equal to .002 

 gram (.03 grain) can be felt through the end organs of 

 the sensory fibres there, but would be quite incapable of 

 acting as a general stimulus in the absence of these. 



It will be noticed as regards the special stimuli of 

 afferent nerves that many of them are merely less degrees 

 of general stimuli; the end organs in skin, mouth,and nose 

 are in fact excited by the same things as nerve-fibres, but 

 they are far more irritable. In the case of the higher 

 senses, seeing and hearing, however, the end organs seem 

 to differ entirely in property from nerve-fibres, being 

 excited by sonorous and luminous vibrations which, so far 

 as we know, will in no degree of intensity directly excite 

 nerve-fibres. To make an end organ for recognizing very 

 slight pressures we may imagine all that would be needed 



