CHAP, in] PBOPKRTIRS OF NERVOUS TISSUES. 105 



vital communication between a sensitive ectodermic cell exposed 

 to extrinsic accidents, and a muscular, highly contractile cell (or a 

 muscular process of the same cell) buried at some distance from the 

 surface of the body, and thus less susceptible to external influences. 

 (Fig. 16, A, B.) If in Hydra, we imagine the junction of the 

 ectodermic muscular process with the body of its cell to be drawn out 

 into a thin thread (as is said to be the case in some other Hydrozoa), 

 we should have just such a primary nerve. Since there would be 

 no need for such a means of communication to be contractile and 

 capable of itself changing in form, but on the other hand an ad- 

 vantage in its remaining immobile, and in its dimensions being 

 reduced as much as possible consistent with the maintenance of 

 irritability, the primary nerve would in the process of development 

 lose the property of contractility in proportion as it became more 

 irritable, i.e. more apt in the propagation of the waves of disturb- 

 ance arising in the ectodermic cell. 



We have already seen that automatism, i.e. the power of initiat- 

 ing disturbances or vital impulses, independent of any immediate 

 disturbing event or stimulus from without, is one of the fundamen- 

 tal properties of protoplasm. In simpler but less exact language, 

 such a mass of protoplasm as an amoeba, though susceptible in the 

 highest degree to influences from without, 'has a will of its own;' 

 it executes movements which cannot be explained by reference to 

 any changes in surrounding circumstances at the time being. A 

 hydra has also a will of its own ; and seeing that all the constituent 

 cells (beyond the distinction into ectoderm and endoderm) are alike, 

 we have no reason for thinking that the will resides in one cell 

 more than in another, but are led to infer that the protoplasm of 

 each of the cells (of the ectoderm at least) is automatic, the will 

 of the individual being the co-ordinated wills of the component 

 cells. In both Hydra and Amceba the processes concerned in 

 automatic or spontaneous impulses, though in origin independent 

 of, are subject to and largely modified by, influences proceeding 

 from without. Indeed the great value of automatic processes in a 

 living body depends on the automatism being affected by external 

 influences, and on the simple effects of stimulation being profoundly 

 modified by automatic action. 



The next step of development beyond Hydra, is evidently to 

 differentiate the single (ectodermic) cell into two cells, of which 

 one, by division of labour, confines itself chiefly to the simple de- 

 velopment of impulses as the result of stimulation, leaving to the 

 other the task of automatic action, and the more complex trans- 

 formation of the impulses generated in itself. The latter, which 

 we may call the eminently automatic cell (though much of the 

 work which it has to do is of the kind we shall presently speak of 

 as reflex action), will naturally be withdrawn from the surface of 

 the body, while the other, which we may call the eminently sensitive 

 cell, will still retain its superficial position, so that it may most 



