828 



CENTRAL NERVOUS SYSTEM 



turbance to parts of the cell remote from the point of excitation; and (3) 

 response on the part of the protoplasm to which the disturbance has spread. 

 Thus when a paramecium swims against a hard object, an excitation is set 

 up by the contact at its anterior end ; the disturbance so produced spreads 

 to other parts of the cell and causes the reversal of the direction of the 

 stroke of the cilia in relatively remote regions, with the result that the 

 paramecium backs away from the obstacle, and then starts off in another 

 direction. The three fundamental processes of excitation, conduction, and 

 response occur whenever the neuromuscular system of an animal is brought 

 into play. The case of multicellular animals differs from that cited above 

 only in that the conduction is intercellular, so that the disturbance set up by 

 an excitation spreads from one cell to another. 



There exist in multicellular animals many examples of cells independent 

 of the influence of nervous tissue which respond directly to stimuli. 

 These are known as independent effectors. The most primitive muscle cells 

 known are those which close the terminal pores of sponges, thus regulating 

 the flow of water through these animals. Parker 1 has shown that these 

 muscles respond to a variety of stimuli in spite of the fact that no nerve 

 cells can be found in association with them. Ciliated epithelial cells, such 

 as occur in certain ducts and passageways in man are also independent ef- 

 fectors, acting independently of nervous control. The ameboid white blood 

 corpuscles also must be quite comparable in their mechanisms of response 

 to the protozoa. 



The forerunner of nervous conduction is seen in the activities of these 

 independent effectors. If a field of ciliated epithelium is examined micro- 

 scopically, it is found that the cilia are not beating in a disorderly way, but 

 that definite waves are passing across the field, each cilium beating a mo- 

 ment later than the preceding one. The appearance is quite like that pro- 

 duced by gusts of wind sweeping across a field of grain. In the sponges, 

 also, a stimulus applied several centimeters from the terminal pore will 

 cause its muscles to contract in spite of the absence of nervous tissue con- 

 necting the muscles with the point of stimulation. This type of intercel- 

 lular conduction by nonnervous tissue is called neuroid transmission. It 

 illustrates the significant fact that conductivity is not a specific property 

 of nervous tissue, but is displayed by many other tissues as well. 



Nervous tissue makes its first appearance in the Coelenterates (Fig. 

 203). A most primitive condition is found in the tentacles which sur- 

 round the mouth of the sea anemone. At the base of certain epithelial 

 cells fibrous processes are developed which connect with underlying mus- 



