THE DEEPENING OF PHYSIOLOGY. 305 



Kolliker was one of the most illustrious pioneers, and 

 even as veteran he has not ceased to lead. No small 

 part of the progress, however, has been due to the 

 discovery of new methods, which we especially associ- 

 ate with the names of the Italians, Golgi and Marchi, 

 and the Spaniard, Kamon y Cajal." * This method 

 of investigation has been aided by embryological 

 studies in which the development of the various parts 

 and elements has been worked out, and by compar- 

 ative anatomical studies which show the increasing 

 complexity of nervous structure as we ascend the 

 series. 



From very early stages it is evident that the central 

 nervous system consists of two classes of elements 

 (1) very numerous cells (spongioblasts) which serve 

 for the support (neuroglia) of the essential nervous 

 tissue, and (2) less numerous mother-cells of nerve- 

 cells or neuroblasts. 



Each neuroblast gives origin (in higher animals) 

 to an " axis-cylinder process " or nerve-fibre, and a 

 number of dendritic " protoplasmic processes." The 

 nerve-fibre passes from the cell in the central system 

 to its distribution, which may be in the nerve-cord 

 itself, or on muscle, or in peripheral sense-organs. 



Within what is called " the grey matter " of the 

 brain and spinal cord, these nerve-cells lie in a net- 

 work or feltwork of extraordinary complexity formed 

 by the branching of the processes of the cells and 

 fibres. Whether the fine twigs of the branches of 

 adjacent cells end freely, or are in contact or conti- 

 nuity with one another, or are in some cases inde- 

 pendent and in other cases inter-united, remains a 

 subject of discussion. But the majority of histolo- 

 gists have accepted the " Neuron-Theory " which 

 * Thomson, Science of Life, 1899, p. 62. 



