2 THE ORIGIN OF VERTEBRATES 



I, a worker in the physiological laboratory at Cambridge ever since 

 Foster introduced experimental physiology into English-speaking 

 nations, should have devoted so much time to the promulgation of 

 a theory of the origin of vertebrates — a subject remote from phy- 

 siology, and one of the larger questions appertaining to comparative 

 anatomy. By what process of thought was I led to take up the 

 consideration of a subject apparently so remote from all my previous 

 work, and so foreign to the atmosphere of a physiological laboratory ? 



It may perhaps be instructive to my readers to see how one 

 investigation leads to another, until at last, nolens volens, the worker 

 finds himself in front of a possible solution to a problem far removed 

 from his original investigation, which by the very magnitude and 

 importance of it forces him to devote his whole energy and time to 

 seeing whether his theory is good. 



In the years 1880-1884 I was engaged in the investigation of 

 the action of the heart, and the nature of the nerves which regulate 

 that action. In the course of that investigation I was struck by the 

 ease with which it was possible to distinguish between the fibres of 

 the vagus and accelerator nerves on their way to the heart, owing to 

 the medullation of the former and the non-medullation of the latter. 

 This led me to an investigation of the accelerator fibres, to find out 

 how far they are non-medullated, and so to the discovery that the 

 rami commicnicantes connecting together the central nervous system 

 and the sympathetic are in reality single, not double, as had 

 hitherto been thought ; for the grey ramus communicans is in 

 reality a peripheral nerve which supplies the blood-vessels of the 

 spinal cord and its membranes, and is of the same nature as the 

 grey accelerators to the heart. 



This led to the conclusion that there is no give and take 

 between two independent nervous systems, the cerebro-spinal and 

 the sympathetic, as had been taught formerly, but only one nervous 

 system, the cerebro-spinal, which sends special medullated nerve- 

 fibres, characterized by their smallness, to the cells of the sympathetic 

 system, from which fibres pass to the periphery, usually non- 

 medullated. These fine medullated nerves form the system of 

 white rami communicantcs, and have since been called by Langley 

 the preganglionic nerves. Further investigation showed that such 

 white rami are not universally distributed, but are confined to the 

 thoracico-lumbar region, where their distribution is easily seen in 



