THE FROG 75 



attending their dissection in the frog, the smaller cranial 

 nerves are omitted ; they may be studied to much better ad- 

 vantage 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 con- 

 ducts 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 impulses 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 sur- 

 rounded 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 ex- 

 amining 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 slender nerve 

 leaves the spinal canal between the first and second vertebrae; 



