470 STUDIES IN ADVANCED PHYSIOLOGY. 



instance, a slight pressure on the finger gives an idea of 

 the nature of the object touched. A material increase of 

 such a pressure produces pain and no longer helps \\s in 

 understanding the environment. If in the case of the eye 

 a normal amount of light gives perceptions of sight, an ex- 

 cessively strong light blinds and hurts. It is probable that 

 many forms of pain, if not all of them, are excessive stim- 

 ulations of the nerves. On the other hand it is possible, 

 especially with the sense of touch to make the stimulation 

 of the nerves so slight as to render a distinct perception 

 difficult. The resulting sensations in such a case we com- 

 monly designate as those of a tickling or irritating nature. 



THE STRUCTURE OF AN ORGAN OF SPECIAL SENSE. 



The ordinary phenomena of the external world do not 

 as a rule affect nerves directly. In order to have such 

 phenomena produce a nervous impulse it is necessary to 

 provide the nerve with some form of specially adapted ap- 

 paratus which shall receive the external impressions and 

 translate or manipulate them in such a way as to give rise 

 to a nervous impulse. In fact the difference between nerves 

 of special sense and the ordinary nerves of the body lies in 

 this fact. Thus we have for an optic nerve the special ap- 

 paratus of the eye, the retina, for the auditory nerve the 

 labyrinth of the ear, and for the special sense of touch 

 peculiarly adapted corpuscles and end bulbs. The first 

 requisite, therefore, is a specially adapted end organ. 

 These end organs are in turn differentiated among them- 

 selves, one being adapted to one particular kind of external 

 impressions, such as light, say, another constructed on an 

 entirely different plan so as to catch the vibrations of sound. 

 A third so arranged as to be easily affected by changes in 

 pressure. 



But these end organs serve merely to start the nervous 

 impulses. They do not produce sensations of sight, hear- 

 ing or touch. In fact, the nervous impulses running along 

 the optic nerve, the auditory nerve or touch nerve are in all 



