THE ACTIVE FORCES OF LIVING ORGANISMS 91 



the same as the overstretching of the elastic band, it 

 loses substance, and with it elasticity. In this way 

 the rhythm of the cell becomes like the state of the 

 elastic band, sometimes too contracted and sometimes 

 too relaxed. At the same time it will be understood 

 that, in speaking of the contraction and expansion of 

 the nerve-cells, only that degree is implied which one 

 would suppose to be necessary to produce a change of 

 rhythm going on amongst the molecules of the cells, 

 and without which, indeed, such a change is incon- 

 ceivable. The more highly pathological the nervous 

 rhythm is, the greater must be the degree of contrac- 

 tion or expansion in the tissue of the nerve-cell. 

 Though under normal conditions it is doubtless very 

 slight, yet it must always be an essential factor in 

 nervous action. 



It will be evident to everyone who reflects for a 

 moment that, if contraction and expansion are the 

 very essence of vaso-motor action — and hence of the 

 activity of the vaso-motor centres — any drug which 

 acts upon the latter must increase the tendency in 

 one direction or the other. It may cause increased 

 contraction for a time, and then give rise to a reaction 

 of increased expansion when it ceases to act ; but in 

 so doing it must diminish somewhat the equilibrium 

 between these opposite tendencies, and it cannot bring 

 about that alternate movement from one state to the 

 other which takes place with every beat of the heart, 

 and on which, as a great physician has said, the 

 health of the whole arterial system depends. Nor 



