514 MANUAL OF PHYSIOLOGY. 



Besides those acts of which we can recognize the cause, many 

 others occur in an amoeba which we are not able to trace to any 

 definite cause other than the energies derived from its special 

 powers of assimilation. We say, then, that not only can an 

 amo3ba feel local stimulation, transmit the impulse to remoter 

 parts of its body, and respond by movement to the stimulus, but, 

 moreover, as the result of intrinsic processes of a chemical nature, 

 it can initiate impulses which appear as motions, etc. We may 

 conclude from this fact alone that automatic action is one of the 

 vital properties of protoplasm. 



Now in the nerve centres we find, certainly in all the more 

 complex animals, that each of these kinds of action is commonly 

 distributed, so that different individual cells have each a different 

 act to perform, and thus an important division of labor takes 

 place. The first act is performed by a wonderfully elaborate set 

 of special organs adapted to the reception of the various extrinsic 

 impulses or sensations from without. The excitation is then sent 

 by nerve fibres to another great group of central nerve cells, 

 which are apparently employed solely in receiving the stimuli 

 from the peripheral organs, and then distributing the impulses to 

 their neighbors, which can direct, modify, analyze, classify, re- 

 distribute, or check the impulses, so that other nerve cells may 

 have the least possible amount of trouble, and at the same time 

 lose none of the advantage that is to be gained from the income 

 derived from stimulus coming from without. Connected with the 

 last group is another, the nerve cells, which lie out of the reach 

 of the ordinary peripheral impulses, but are capable of develop- 

 ing within themselves energies, and can initiate impulses with no 

 other aid than that of their nutrition and the chemical changes 

 resulting from their assimilation. 



These impulses are distributed to the peripheral active tissues, 

 muscles, glands, etc., probably through the medium of other sets 

 of cells analogous to the last group situated in the nerve centres 

 as well as to the local distributors which act as unions between the 

 other textures and the nerve fibres. 



The functions of these nerve cells which form centres of action 

 may be classified thus : 



