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 communicantes connecting together the central nervous system 
and the sympathetic are in reality single, not double, as had 
hitherto been thought; for the grey ramus comnwnicans 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 communicantes, 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 
