NERVES OF THE DOGFISH 355 



amphibians, where it divides into rami mentalis externus and 

 internus. Or if the condition in Chlamydoselachus, as described 

 by Hawkes, is primitive, the amphibian condition may have 

 been derived from that. 



Four varieties of lateral-line sense-organs occur in Squalus: 

 canal organs (neuromasts in canals), pit-organs (naked neuro- 

 masts in the skin), ampullae of Lorenzini, and certain peculiar 

 tubular organs (on anterior wall of spiracle, on lower jaw near 

 mandibular canal), probably modified ampullae. 



THE AUDITORY NERVE 



The root fibers of the auditory nerve pass from the ganglion 

 into the acusticum as a single large, broad root, slightly posterior 

 to the facialis roots (figs. 12 to 14, 31, 32, and 36, VHIr.). It 

 is well-nigh impossible from a study of cross-sections alone to 

 draw a sharp line of distinction between the auditory fibers and 

 the ventral lateral-line root. But in sagittal and horizontal 

 sections they are seen to be distinct (figs. 27 to 30, Vlllr., 

 rllv.). On entering the acusticum the auditory root is seen to 

 divide into anteriorly and posteriorly directed tracts, the poste- 

 rior tract being the larger (fig. 28, Vlllr.). 



The auditory ganglion at its anterior vestibular end is large 

 and globular, but posterior to the root fibers the ganglion cell 

 mass assumes the form of a vertical plate, though farther poste- 

 riorly it becomes a rounded column, the saccular ganglion (figs. 

 12, 31, and 32). The anterior end of the auditory ganglion is 

 wholly intracranial; but gradually passing out laterally and pos- 

 teriorly through the cranial wall, its posterior portion becomes 

 situated in a recess in the ventral part of the ear capsule. Near 

 the anterior end of the ganglion a large vestibular nerve passes 

 into the ear capsule, supplying the utriculus and the ampullae 

 of the anterior and horizontal semicircular canals. Succeeding 

 this large vestibular branch there follow numerous branches 

 dorsally and ventrally supplying the sacculus and finally the 

 ampulla of the posterior canal (figs. 12, 21, 31, and 32). 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 31, NO. 5 



