262 PHYSIOLOGY OF MUSCLES AND NERVES. 



other motor nerves. But a difference is observable 

 even in the case of the secretory nerves or gland-nerves, 

 of which we have akeady had occasion to make mention. 

 When these nerves are irritated the appropriate nerves 

 beffin to secrete. The connection of these nerves with 

 the glands must from a physiological point of view be 

 entirely similar to that of the motor nerves with the 

 muscles. AYhen the latter are irritated the muscles 

 connected with them at once pass into a state of activity. 

 Just in the same way the gland-nerves, when they are 

 irritated, cause the glands connected with them to pass 

 into a state of activity. That this activity is quite 

 different from that of the muscles, is obviously due to 

 the entirely different structure of the glands and the 

 muscles. A glaud, unlike a muscle, cannot contract ; 

 when it becomes active, it secretes a liquid, this being 

 its activity. There is therefore no reason to assume 

 any difference in any of these nerves, the difference in 

 the terminal apparatus, in which the nerves end, being 

 sufficient fully to explain the difference in the pheno- 

 mena. 



But there are other nerves the action of which is 

 much harder to understand. Among these are the 

 sensory nerves. When these are irritated, they effect 

 sensations of different kinds, some being of light, others 

 of sound, and so on. Moreover these nerves are capable 

 of receiving irritation in a peculiar way, some by waves 

 of light, others by -sound vibrations, and others again 

 by heat-rays ; but in all cases, only when these influ- 

 ences act on the ends of the respective nerves. It is 

 not self-evident that these nerves are homofifeneous 

 in themselves or with the previously mentioned kinds. 

 Finally, it is yet harder to understand the action of 



