STRUCTURE. 191 



has been observed that within a vegetal cell the strands of 

 protoplasm stretched in this or that direction contain moving 

 granules, showing that the strands carry currents. It has 

 also been observed that when the fission of a protozoon is so 

 nearly complete that its two halves remain connected only 

 by a thread, currents of protoplasm move through this 

 thread, now one way now the other. The inference fairly to 

 be drawn is that such currents pass also through the strands 

 which unite the protoplasts forming a tissue. What must 

 happen? So long as adjacent cells with their contents are 

 subject to equal pressures no tendency to redistribution of 

 the protoplasm exists, and there may then occur the action 

 sometimes observed inside the strands within a cell: cur- 

 rents with their contained granules moving in opposite direc- 

 tions. But if the cells forming a portion of tissue are subject 

 to greater pressure than the cells around, their contained 

 protoplasm must be forced through the connecting threads 

 into these surrounding cells. Every change of pressure at 

 every point must cause movements and counter-movements 

 of this kind. Xow in the Metazoa at large, or at least in all 

 exhibiting relative motions of parts, and especially in all 

 which are capable of rapid locomotion, such changes of 

 pressure are everywhere and always taking place. The 

 contraction of a muscle, besides compressing its components, 

 compresses neighbouring tissues; and every instant contrac- 

 tions and relaxations of muscles go on throughout the 

 limbs and body during active exertion. Moreover, each atti- 

 tude — standing, sitting, lying down, turning over — entails a 

 different set of pressures, both of the parts on one another 

 and on the ground; and those partial arrests of motion 

 which result from sitting down the feet alternately when 

 running, send jolts or waves of varying pressure through the 

 body. The vital actions, too, have kindred effects. An in- 

 spiration alters the stress on the tissues throughout a con- 

 siderable part of the trunk, and a heart-beat propels, down to 

 the smallest arteries, waves which slightly strain the tissues 



