EXCITATION AND TRANSMISSION 73 



primary change in the membrane consequent upon 

 depolarization is this chemical reaction or a change in 

 colloid condition, or some other change is at present 

 uncertain, but since the whole series is indissociable 

 and more or less mutually related it makes little differ- 

 ence which we call primary and which secondary. For 

 present purposes the significant fact is that, according 

 to Lillie, excitation involves a chemical change, probably 

 oxidative hi nature, as well as other changes, in the sur- 

 face layer of protoplasm. 



x x Vv 



\ > *. 



a 



\ ~7 ~W ~7J~ 



FIG. 6. Diagram indicating the electric currents arising in relation 

 to an excitation gradient; the direction of migration of the positive ions 

 is indicated; for further explanation see text. 



The process of transmission of these changes from 

 point to point is accomplished by the electrical changes 

 which constitute one feature of the excitation. The 

 excited region (Fig. 6a) becomes electronegative (exter- 

 nally) to an unexcited region (b, c, etc.), in other words 

 a local electric circuit is established, the positive current 

 passing in the cell or tissue from the region of excitation 

 to unexcited regions with a " re turn' 1 current in the 

 external medium. In consequence of resistance the 

 strength of this current decreases with increasing dis- 

 tance from the region of excitation a, that is to say, a 

 gradient in electrical potential must arise in relation to 

 a region of excitation. Lillie points out that this local 



