TRANSMISSION AND CONDUCTION 209 



conduction definite waves or impulses appear, alter- 

 nating with periods of rest. In one form or another 

 rhythmicity in excitation is of very wide occurrence in 

 organisms and ranges from such phenomena as the 

 rhythmic impulses to ciliary movement to the heart 

 beat. At this time it is impossible to do more than 

 note the occurrence of rhythmicity. 



While the development and evolution of the processes 

 of excitation and conduction involve changes in the 

 hereditary mechanism of protoplasm, there are certain 

 features of the conduction pattern which are clearly 

 related to the physiological gradients and certain others 

 which may be interpreted in terms of such relation. 

 Some of these relations have already been mentioned, 

 particularly in chapters iv and v, but certain facts and 

 questions remain to be considered. 



TRANSMISSION IN CERTAIN NON-NERVOUS OR 



NEUROID PATHS 



It is a familiar fact that in ciliated surfaces, whether 

 composed of single cells or of many cells, forming an 

 epithelium, the beat of successive cilia occurs in a 

 definite sequence of such a character as to give the 

 appearance of successive waves of bending and recovery 

 of the cilia, passing over the surface. Such ciliated 

 surfaces cover large parts or, in many cases, all of the 

 external surfaces of many of the lower aquatic inverte- 

 brates and of early developmental stages of numerous 

 animals, both invertebrates and vertebrates, and are 

 also found on many internal surfaces in various forms, 

 ranging from the sponges to man. Each beat of a 

 cilium is obviously due to an exciting impulse of some 



