162 ME. F. J. COLE ON THE STKUCTUEE AND MORPHOLOGY OF 



above sense organ 11 of the line, which it supplies. The motor hyoideus {S*), the post- 

 spiracular nerve, passes downwards and becomes related to the anterior face of the hyoid 

 arch. With the distribution of this nerve we are not concerned. 



The anterior or larger division of the hyomandibular trunk (M^) consists, as already 

 described, of two parts : (a) the main trunk of the external mandibular lateral line nerve 

 {S^) ; and (b) the " internal mandibular" of Allis (i?^). I shall show later on that Allis 

 was wrong in his interpretation of this nerve, and that it really corresponds to the 

 modified pre-spiracular nerve, or chorda tympani, of the cartilaginous fishes, and as such 

 cannot be the internal mandibular, which is a post-spiracular nerve. The internal 

 mandibular or post-spiracular nerve is really the hyoideus. Sections show that this 

 trunk consists of two parts, an anterior chorda tympani and a posterior external man- 

 dibular. The chorda soon separates off, passes downwards, and becomes related to the 

 posterior face of the lower jaw. On leaving the hyomandibular bone the external man- 

 dibidar immediately passes straight downwards over the anterior downward process of 

 the hyomandibular, and accompanied by a blood-vessel which in spirit dissections looks 

 astonishingly like a nerve *. Shortly after emerging from the hyomandibular foramen a 

 long nerve is given off from the external mandibular (H"') which passes forwards through 

 the adductor mandibulse to innervate some pit organs in the neighbourhood of the infra- 

 orbital canal. It then courses downwards and forwards to supply certain of the pit 

 organs lying ventral to the hyomandibular canal. Before passing downwards, however, 

 it sends down a long fine nerve which innervates the jiosterior set of pit organs in 

 connection with the hyomandibular canal. These nerves are mentioned by Allis (1897, 

 p. 632). They also midoubtedly correspond to Allis's nerves mef.vl. and Tiief.mdl. 

 innervating the vertical cheek and mandibular lines of pit organs. 



Soon after leaving the canal in the hyomandibular a twig is given off, which, passing 

 downwards and backwards over the external face of the anterior process of the hyo- 

 mandibular, perforates the preoperculum ventral and somewhat internal to its anterior 

 upward process, and just dorsal to the point of the backward process of the quadrate. It 

 supplies sense organ 10 of the hyomandibular canal. After giving off the above twig 

 the external mandibular nerve passes downwards over the outer process of the hyo- 

 mandibular and begins to take a marked turn forwards. Leaving the hyomandibular it 

 passes over the symplectic, and whilst on this bone gives off the twig to sense orgau 9 of 

 the line. This twig jiasses forwards over the symplectic, perforates the preoperculum 

 internal to the facet for the quadrate and at about the anterior third of the facet, and so 

 reaches its sense organ. 



The external mandibular now begins to assume a horizontal position, and arriving at 

 about the middle of the posterior edge of the symplectic, where the body of the quadrate 

 overlaps it, passes almost horizontally, but still somewhat downwards, in a rough canal 



* Several of the more important nerves in the Cod are aeeompanied by blood-vessels, which often appear, as in the 

 case of the superficial ophthalmic trunk, to arise directly from the nerve. (They really perforate it.) In 

 specimens that have been along time in spirit these vessels are remarkably like nerves, ami Jiiive been taken for such 

 hi/ several natiiralists ! 



