1/8 HISTOLOGY 



rology as the periphery; and those other surfaces, which have retired 

 into the interior to form the ganglia and brains and receive, elaborate, 

 and send out the reports of the sensory nerve cells as motor commands, 

 are known as the central nervous system. This conception of the inde- 

 pendent and exact action and interaction of the neurons is known as 

 the "neuron theory," and is supposed to depend upon the absolute nerv- 

 ous separateness of each and every neuron, no matter how intimately 

 they may be united physically. In the light of recent research it is pos- 

 sible that the unit of nerve activity, while usually a neuron, is sometimes 

 a part of a neuron or even formed by two or more of them acting in 

 unison. 



Most nerve-paths have common meeting grounds with one or more 

 others for the exchange of the nerve impulses. Here are assembled 

 the perceptory and discharging organs of larger or smaller groups of 

 neurons together with the cell bodies of such as have the cell body near 

 either end -organ. Some of the neurons are confined entirely to this 

 region, and the whole mass together with certain connective tissue and 

 circulatory elements is known as a ganglion. Some ganglia are composed 

 principally of nerve cells whose perceptory and discharging end-organs 

 are one or both widely remote from the region, in other ganglia or at the 

 periphery. 



Some ganglia may be small and homogeneous as to the kind of cells 

 that are found in them, others larger or containing a greater variety of 

 cell elements. This condition is true of the greater number of animals. 

 In some higher forms numbers of ganglia of several different kinds are 

 collected into large central masses, which are closely assembled in some 

 central region to form the central nervous system, as the brain in the 

 mammals and man. The nerve cells and their products greatly outweigh 

 everything else in such centers, other elements being neuroglia, a little 

 connective tissue, and a considerable amount of circulatory medium. 

 The vertebrates and cephalopod mollusks possess well-developed exam- 

 ples of such brains. Some of the Arthropoda are only a step behind in 

 this respect. 



A classification of the nerve tissues and their cells according to their 

 use is the one we shall make use of here as far as possible. According to 

 the specialization of one or the other of the three fundamental cell-organs, 

 the cells (and the tissues) will be spoken of as perceptory, communicatory, 

 or motor. Of course all nerve cells can, as has been stated, perceive, and 

 it must be explained that, in this case, by perceptory cells are meant all 

 neurons that receive first hand a perception of exterior conditions through 

 a chemical or mechanical or physical stimulus. (The distinction 

 between physical and mechanical is here used for convenience.) These 

 three forms of cells will be considered as the tactile (including the static 



