THE STRUCTURAL BASIS OF THE BODY 35 



cells. In the study of physiology our task consists, firstly, in the description 

 of the special part taken by each organ in the general functions of the body, 

 and, secondly, in the determination of the limiting conditions of such func- 

 tions and of the 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. 



In the foregoing lines we have compared the higher animal 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 func- 

 tion 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 essen- 

 tially composed of and determined by the reciprocal 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 specialisation. 

 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 convenience. 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 protoplasmic 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. 



