NERVOUS SYSTEM. 215 



system of the vagus. It arises behind the optic lobes laterally from 

 the medulla oblongata by a series of roots, several of which admit 

 of being traced to behind the fourth ventricle upon the Rhornboidal 

 sinus. A large and small, or sensory and motor portion are found 

 to enter into the composition of the fifth pair ; several roots, c. g., 

 in the Sturgeon, the nervous fascicula of its third and fourth root do 

 not pass into the ganglion, which is formed by other roots, and prin- 

 cipally its first. Three main branches are to be distinguished, as 

 in Man, namely, the orbitar, supra, and infra-maxillary ; and besides 

 these, a large posterior branch (ramus opercularis), which proceeds 

 from the posterior root, and is principally distributed externally upon 

 the opercular apparatus of the gills ; it corresponds to the seventh 

 pair, which does not appear to be present as a separate n. facialis. 



The auditory nerve is always separate from the rest, and is far 

 more strongly developed in the Osseous than the Cartilaginous 

 Fishes ; it takes its origin quite close behind the fifth pair, with 

 which it is united by a branch of communication (communicans 

 faciei?). 



The ninth, tenth, and eleventh nerves, or the glasso-pharyngeal 

 vagus and accessory of Willis^ form a common system of nerves, with 

 many roots and united origins ; we must regard as its main branch 

 the branchial nerve (n. vagus'), with which the glosso-pharyngeal 

 nerve is frequently united, while, however, the nervus accesso- 

 rius is usually free and distinct from it. This vagus arises fre- 

 quently by special ganglia that are occasionally much developed, 

 forms also upon its roots ganglionic enlargements, gives three main 

 branches to the last three gills and to the pharyngeal maxillae, and 

 passes then to the stomach and swimming-bladder. The twelfth 

 cerebral nerve, or hypoglossal of Man and the higher Vertebrata, is 

 absent in Fishes, owing to the want of mobility in their tongue ; 

 still, however, it has been recently described as occurring in the 

 Sturgeon. 



From the vagus and accessory nerve proceed the large longitu- 

 dinal or lateral nerves of the Osseous Fishes, which run parallel 

 with the lateral lines, and straight within the muscles, to the tail, 

 and in their course enter into numerous communications with the 

 spinal nerves. Another longitudinal nerve is frequently present, 

 and is formed by two recurrent branches of the trigeminus and 

 vagus ; it passes through the supra-occipital bone, whence the nerve 

 of either side runs near the points of the spinous processes and be- 



