THE CRANIAL NERVES AND LATERAL SENSE ORGANS OF ELSHES. 161 



anterior foramen in the lachrymal. It then courses forwards parallel and dorsal to the 

 ventral ramus, and, after giving off dorsally more twigs to the pit organs of the snout, 

 passes slightly downwards, perforates the lachrymal opposite the dorsal horder of the 

 infra-orbital canal and slightly posterior to sense organ 2, and JiuaUu supplies sense 

 oi'gan 1. 



(3) Kijomandibiihif or Operculo-mandibiilar Canal. 

 Hyomandibular Trunk. 



This large nerve (^), just at its origin from the trigemi no-facial ganglion, is divided liy 

 the orbital vein into two portions. The anterior ventral portion consists of two parts 

 representing the morphological palatine and pre- and post-spiracular nerves or the 

 "facial proper," whilst the posterior dorsal portion constitutes the "lateral line root of 

 the facial." These constituents run alongside one another external to the vein, but are 

 soon quite separable by the microscoj^e into an anterior bundle (iZ"'), composed of the 

 pre-spiracular and main trunk of the external mandibular branch, and a posterior bundle 

 (if"), which is mostly formed by the post-spiracular nerve, but also includes a branch of 

 the external mandibular. In dissections it was noticed that the pre-spiracular separated 

 off before the hyomandibular trunk divided into the two preceding bundles. 



External mandibular nerve. — The posterior branch above of this nerve arises as 

 already described, and then passes straiglit outwards, downwards, and backwards. It is 

 closely applied to the postero-external border of //\ and receives some lateral line fibres 

 from it ventral to the orbital vein. As the entire hyomandibular trunk passes down- 

 wards, on approaching the hyomandibular bone the two bundles E} and E- become 

 separated so as to form two distinct nerves. The posterior, or smaller of the two (i?^), 

 which is composed of a branch of the external mandibular -f the hyoide;in nerve, or post- 

 spiracular division of the facial proper — the latter being the larger bundle, passes 

 through the oblique backwardly-directed facial canal in the hyomandibular bone, whilst 

 the larger bundle (_£f') traverses a foramen in the hyomandibular larger, just ventral, 

 and somewhat anterior and internal to it. Distally the apertures are somewhat widely 

 separated owing to the backward direction of the facial canal. 



The hyoidean nerve + the posterior branch of the external mandibular, on emergino- 

 from the facial canal, immediately take a sharp turn downwards and slightly backwards, 

 coursing near and parallel to the posterior edge of the anterior downward process of the 

 hyomandibular. On leaving the facial canal the two parts of the nerve are seen to be 

 separated, in the sections the external mandibular being situated externally to the facial 

 proper component. The two nerves pass downwards and slightly backwards together, 

 and pass under the preoperculum at the base of the deep notch at its upper 

 extremity. The external mandibular fibres {W) leave the hyoideus near the ventral 

 extremity of the facial canal, and, passing outwards, do^vnwards, and backwards, soon 

 collect into two bundles. The upper passes downwards, perforates the preoperculum, 

 and supplies the 12th sense-organ of the hyomandibular line. The lower, after a some- 

 what long course downwards, obliquely tunnels the preoperculum some little distance 



