II] THE SIZE OF CELLS 37 



about as eight to one, that, in corresponding parts of the nervous 

 system of the two animals, there are more than 15,000 times as 

 many individual cells in one as in the other. In short we may 

 (with Enriques) lay it down as a general law that among animals, 

 whether large or small, the ganglion-cells vary in size within 

 narrow limits ; and that, amidst all the great variety of structural 

 type of ganglion observed in different classes of animals, it is 

 always found that the smaller species have simpler ganglia than 

 the larger, that is to say ganglia containing a smaller number 

 of cellular elements *. The bearing of such simple facts as this 

 upon the cell-theory in general is not to be disregarded ; and the 



Horse 



Rabbit 



Fig. 1. Motor ganglion-cells, from the cervical spinal cord. 

 (From Minot, after Irving Hardesty.) 



warning is especially clear against exaggerated attempts to 

 correlate physiological processes with the visible mechanism of 

 associated cells, rather than with the system of energies, or the 

 field of force, which is associated with them. For the life of 



* While the difference in cell-volume is vastly less than that between the 

 volumes, and very much less also than that between the surfaces, of the respective 

 animals, yet there is a certain diiJerenee ; and this it has been attempted to correlate 

 with the need for each cell in the many-celled ganglion of the larger animal to 

 possess a more complex "exchange-system" of branches, for intercommunication 

 with its more numerous neighbours. Another explanation is based on the fact 

 that, while such cells as continue to divide throughout life tend to uniformity of 

 size in all mammals, those which do not do so, and in particular the ganglion cells, 

 continue to grow, and their size becomes, therefore, a function of the duration of 

 life. Cf. G. Levi, Studii sulla grandezza delle cellule, Arch. Iktl. di Anat. e di 

 Emhryolog. V, p. 291, 1906. 



