Page:Popular Science Monthly Volume 75.djvu/267

Rh afford the material basis of the intellectual life. Thus in the vertebrates the primary sensory and motor neurones in number and importance are outstripped by the association neurones.

As thus sketched the development of the adjuster or central nervous element of the neuromuscular mechanism takes place in the region between the receptors and the effectors and in time after these two sets of organs have appeared. Its primary function is undoubtedly transmission involving the principle of the common path; secondarily it comes to be a repository of the effects of nervous stimulation whereby its principal function as a modifier of impulses is made possible.