THE FROG 53 
may be studied to much better advantage in an elasmobranch 
(skate or dog-fish). Only the spinal nerves will be considered 
here. 
The spinal nerves are segmentally arranged, and are given off 
in pairs on each side of the spinal cord. Each spinal nerve arises 
by two roots, a dorsal and a ventral. The dorsal root is known 
as the sensory root because it conducts nerve impulses from the 
sense organs to the cord (afferent impulses), and the ventral 
root is known as the motor root because it conducts nerve im- 
pulses outward (efferent impulses) to the muscles. Just outside 
the spinal canal the two roots unite to form a common trunk; a 
small spinal ganglion lying in the intervertebral foramen and 
surrounded by the white calciferous body or so-called peri- 
ganglionic gland is found on the dorsal root near its union with 
the ventral root, but as a rule the spinal ganglion is continued 
beyond the point of the union so that the two roots appear to 
meet in the ganglion. 
The distribution of the spinal nerves is best studied by examin- 
ing the inner surface of the dorsal body wall; on each side of the 
vertebral column the paired spinal nerves will be found exposed 
throughout a considerable part of their course. Before beginning 
the study of the cranial nerves, identify the sympathetic system 
(see below) in order to avoid injuring it. 
To expose the first and second spinal nerves, turn the roof of 
the oesophagus forward and cut it off close to the mouth cavity. 
(a) The first spinal or hypoglossal. This nerve leaves the 
spinal canal between the first and second vertebre; it passes 
around the side of the throat, turns forward below and behind 
the angle of the jaw and ends in muscles of the floor of the 
mouth. 
(b) The second spinal or brachial is the largest spinal nerve. 
It leaves the vertebral canal between the second and _ third 
vertebre, and passes outward to supply branches to the shoulder 
region and arm. Why is it so large? The second spinal is con- 
nected with the third by one or more delicate cross-branches ; 
usually it is also connected with the first by other very slender 
twigs. The nerves of this region where the cross-connections 
