THE VAGUS NERVE OF THE SNAPPING TURTLE 
(CHELYDRA SERPENTINA) 
S. WALTER RANSON 
From the Anatomical Laboratory of Northwestern University Medical School 
NINE FIGURES 
In attempting to determine the physiological significance of 
the unmyelinated fibers,! which we have found in very large 
numbers in the cerebrospinal nerves of mammals, it is important 
to have some notion of their phylogenetic position. Do they 
represent a system of fibers acquired in the mammalian nervous 
system and, therefore, phylogenetically young and correspond- 
ingly incomplete in their ontogenetic development? Such an 
interpretation is suggested by the delayed and incomplete 
myelination of the pyramidal tract (Linowiecki ’14), a tract 
which is found only in the mammalian nervous system. The 
vagus nerve is well suited for a comparative study of the un- 
myelinated fibers of the cerebrospinal nerves, because, in the 
mammal, it has been shown to contain an unusually large num- 
ber of these fibers distributed in a characteristic way. The vagus 
of the snapping turtle is of special interest because its afferent 
ganglion cells are grouped differently from those of the mammal. 
On account of this arrangement of the ganglion cells it has been 
possible to settle certain questions which remained obscure 
after the study of the mammalian vagus (Chase and Ranson ’14). 
Large snapping turtles were used and the study was confined 
to the right vagus. Most of the material was fixed and stained 
1In a report before the Society of Normal and Pathological Physiology, Oct. 
27, 1914, Professor H. H. Donaldson advocated the use of the terms myelin, 
myelinated and unmyelinated in the place of medulla and its derivatives in the 
description of nerve fibers. This new usage has been adopted throughout this 
paper. 
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