38 PHYSIOLOGY 



physical and chemical factors which determine them. Finally, we have 

 to endeavour to form a complete conception of the chain of events 

 concerned in the discharge of each function and of their causal nexus. 

 We have compared the higher animal in the foregoing lines to a 

 colony of cells, and we often speak of an isolated cell of the body as if 

 it were an independent elementary organism. A better term for such 

 an aggregation of cells as presented by the higher animals is not 

 however ' cell colony,' but * cell state,' since, just as in the state 

 politic, no cell is independent of the activities of the others, but the 

 autonomy of each is merged into the life of the whole. With increasing 

 differentiation there is increasing division of function among the 

 various members of the state, and each therefore becomes less and 

 less fitted for an independent existence or for the discharge of all its 

 vital functions. The more highly civilised a man becomes and the 

 greater his specialisation in the work of the community, the smaller 

 chance would he have of existing on a desert island. Thus the life of 

 the organism is essentially composed of and determined by the recip- 

 rocal actions of the single elementary parts. It is evident that, if the 

 process of specialisation has gone far enough, a discussion whether 

 each unit has or has not an independent life is beside the mark, since 

 it cannot possibly exist apart from the activities of the other cells. 

 Of late years histologists have brought forward evidence which seems 

 to imply that an actual structural interaction exists, in addition to 

 the functional dependence which is a necessary resultant of specialisa- 

 tion. Even in the case of plant cells with their thick cellulose walls, 

 fine bridges of protoplasm can be made out passing from one cell to 

 another through pores in the cellulose wall. In animals protoplasmic 

 bridges are known to exist joining up adjacent cells in unstriated 

 muscle, epithelium and cartilage cells, and in some nerve-cells. The 

 conclusion has therefore been drawn that the morphological unit is 

 not the cell, but the whole organism, and that the division of the 

 common cytoplasm into cells is merely a question of size and con- 

 venience. There can be no doubt that the determining factor in the 

 division of cells is their growth ; the cell divides because it grows. 

 With increased mass of living substance it is necessary to provide for 

 increase of surface both of cytoplasm and of nucleus. Whether all the 

 tissues of the higher animals remain in structural continuity by proto- 

 plasmic bridges, &c., must be to us a matter of indifference, since all 

 that is necessary for the interdependent working of the different cells 

 of the body is a functional continuity, and this in the higher animals 

 is effected by the presence of a common circulating fluid and a reactive 

 nervous system connected by conducting strands with all the cells 

 of the body. 



