THE CKANIAL NERVES AND LATERAL SENSE OEGANS OF PISHES, 163 



formed by the superposition of the quadrate on the symplectic. Whilst passing between 

 these two bones it gives off the twig to tlie 8th sense organ, which, passing downwards 

 on the internal face of the quadrate, perforates the preoperculum near the anterior 

 extremity of its dorsal border (close to the quadrate facet) and thus reaches its 

 destination. 



Having passed between the symplectic and the quadrate, the external mandibular 

 courses downwards over the inner face of the latter internal to its head, and then by a 

 downward curve reaches the inner face of the articular, where it lies in a deep horizontal 

 groove situated at a level somewhat dorsal to this section of the hyomandibular canal 

 and opposite the head of the articular. Whilst on the head of the articular the twig to 

 sense organ 7 of the hyomandibular line is given off. This arises somewhat anterior to 

 the sense organ, passes backwards, enters the articular oi3posite and ventral to the 

 anterior edge of the facet for the quadrate, and passes transversely through it to its 

 sense oi-gan. At this point a branch of the inferior maxillary division of the trigeminus 

 comes down and runs alongside the external mandibular, lying immediately dorsal to it; 

 but although they may be very closely opposed there is no real connection, the ventral 

 branches of trigeminal branch curving round the inner and outer surfaces of the lateral 

 line nerve to reach their destination. 



The external mandibular nerve now commences to pass slightly downwards, and first 

 lies internal and somewhat above Meckel's cartilage, and afterwards, in front of the 6th 

 sense organ, is situated directly internal to it. The branch to sense organ 6 is separated 

 off somewhat in front of the twig to the 7th sense organ. It courses forwards and out- 

 wards, obliquely perforates the dentary, and after running alongside the sensory canal for 

 some little distance reaches and supplies its sense organ. The mandibularis externus is 

 now opposed to the inner face of Meckel's cartilage, and there gives off the twig to sense 

 organ 5. This proceeds downwards and forwards, and, obliquely piercing the dentary, 

 reaches its respective sense organ. At about this region a nerve is seen to pass down- 

 wards under Meckel's cartilage from its outer surface and then turn upwards to become 

 opposed to, but never connected with, the outer face of the external mandibular. It is at 

 first situated between the latter nerve and the cartilage, but finally comes to lie on the top 

 of the lateral line nerve at the place where the twig to the fourth sense organ perforates 

 the dentary. This nerve is undoubtedly Allis's branch " r.ghi.'' * springs from the inferior 

 maxillary division of the Vtli, and constitutes a part of the innervation of the geniohyoid 

 muscle. Of the t^\ags to sense organs 3 and 4^ nothing need be said further than that 

 they passed through separate perforations in the dentary, and that each gave off a very 

 small bundle of fibres which were traced ventral to the hyomandibular canal and then 

 lost. They doubtless innervated the pit organs of this region. The external mandibular 

 is still situated internal to Meckel's cartilage. 



Anterior to sense organ 4 the lateral line nerve commences to pass downwards so as to 

 occupy a position ventral and somewhat internal to Meckel's cartilage. Here also the 

 anterior termination of the main trunk of the inferior maxillary division of the trigeminus 



* Cp. AUis, 6, p. 639. 

 SECOND SEKIES. — ZOOLOGY, VOL. VII. 23 



