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2. The lateral line branch, a large nerve which separates from 

 the trunk of the vagus just outside the cranium and runs backward 

 through the muscles, parallel to the vertebral column on a level 

 with the lateral line. It sends off numerous small twigs to the 

 sense organs of the lateral line canal. 



3. Four branchial nerves, which can be seen through the floor 

 of the anterior cardinal vein, leave the outer side of the vagus 

 trunk. Each divides into a pretrematic and posttrematic branch; a 

 pharyngeal branch, the last of which is the largest, arises from 

 each posttrematic. 



4. Beyond the branchial nerves the remainder of the vagus 

 passes backward as the intestinal or visceral trunk, to the end of 

 the pharynx, where it divides into a number of branches which are 

 distributed chiefly to the wall of the stomach. Near the point of 

 this last division the vagus is crossed by the hypobranchial nerve, 

 which should be noted and preserved. 



The occipital nerve penetrates the lateral wall of the cranium 

 close behind the root of the vagus and enters the canal of the vagus, 

 along which it passes. On emerging, it sends small branches to the 

 nearby muscle, while the principal portion runs on to join the 

 hypobranchial nerve. The occipital nerve will be found to arise 

 from the ventral surface of the medulla, below and behind the root 

 of the vagus, by two or more distinctly separated roots, which may 

 represent distinct nerves. 



SPINAL NERVES. The spinal nerves are those nerves which arise 

 from the sides of the spinal cord. They differ from the cranial 

 nerves not only in their origin outside the cranium, but also in 

 that each spinal nerve arises by two roots which spring from the 

 spinal cord near the dorsal and ventral surfaces. Each root passes 

 through a foramen in the cartilaginous wall of the neural canal, 

 the ventral a little anterior to the dorsal, after which they unite to 

 form the spinal nerve. Between the junction of the roots and its 

 foramen the dorsal root contains a mass of ganglion cells, which 

 cause an enlargement known as the dorsal root ganglion. The 

 typical course of a spinal nerve is around the body to the ventral 

 surface, giving off branches to the muscles and skin of its segment. 

 A short distance from the vertebral column the spinal nerves lie just 

 outside the peritoneum, through which many of them can be seen 

 and followed to about the level of the lateral vein. At this point 

 they pass outward into the muscles of the body wall. To dissect 

 any of the spinal nerves make a longitudinal incision along the 

 lateral line and separate the dorsal muscle mass from the lateral 

 muscles for some distance. The dorsal muscles can then be pressed 

 toward the vertebral column and dissected away from the perito- 

 neum. The spinal nerves, lying against the peritoneum, will be 

 exposed and can be followed easily, first to their roots, next ven- 

 trally. 



The hypobranchial nerve, to which attention was called at the 

 point where it crosses the vagus, is formed by the union of the 

 principal branches of the occipital and first two spinal nerves. The 

 third spinal nerve receives a branch from the second, and itself 

 accompanies the hypobranchial nerve closely without actually be- 

 coming a part of it. The union of nerves thus formed is known as 

 a plexus. After crossing the vagus the hypobranchial nerve forks, 

 one division passing, medial to, the other lateral to the anterior 

 cardinal vein; both run ventrally, following the last gill arch, and 

 reunite on the lateral wall of the pericardium, forming a trunk 



