468 THE ORIGIN OF VERTEBRATES 



the secondary neurones show by their degenerative changes the 

 importance of their connection with the peripheral organs. 



In this way I can conceive the formation of a series of both 

 efferent and afferent relays in the nervous system by proliferation of 

 the original neural moiety of the neuro-epithelial elements, every 

 one of which is dependent upon its connection with the peripheral 

 epithelial elements for its due vitality, the whole system being a 

 scheme for co-ordination of a larger and larger number of peripheral 

 elements. Thus the cells of the vasomotor centre are in connection 

 with the whole system of segmental vaso-constrictor centres in the 

 lateral horns of the thoracic region of the cord, so that to cause 

 atrophy of these cells a very extensive removal of the vascular 

 system would be required. Each of the segmental centres in the 

 cord supplies a number of sympathetic segments, the connection 

 with all of which would have to be cut in order to ensure complete 

 removal of the connection of each of its cells with the periphery, and 

 finally each of the cells in the sympathetic ganglia supplies a number 

 of peripheral elements, all of which must be removed to ensure com- 

 plete severance. 



Thus, if we take any arbitrary number, such as 4, to represent 

 the number of peripheral organ-elements with which each terminal 

 neurone is connected, and suppose that each neurone has proliferated 

 into sets of 4, then a cell of the third order, such as a cell of the 

 vasomotor centre, would recpuire the removal of 64 peripheral elements 

 to cause its complete separation from the periphery, one of the 

 second order (a cell of the thoracic lateral horn) 16 elements, one of 

 the first order (a cell of a sympathetic ganglion) 4 elements. 



Such intimate inter-relationship between the neurones, both 

 afferent and efferent, and their corresponding peripheral organs does 

 not imply that all nerve-cells are necessarily as closely dependent 

 upon some connection with the periphery, for just as the proliferation 

 of epithelial or muscle-cells forms an epithelial or muscular sheet, 

 the elements of which are so loosely, if at all, connected together that' 

 their metabolism is in no way dependent upon such connection, so 

 also a similar proliferation of the neural elements may form con- 

 nections between nerve-cell and nerve-cell of a similarly loose 

 nature. 



It is this kind of proliferation which, in my opinion, would bind 

 together the separate relays of efferent and afferent neurones, and 



