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Show 1895.] SENSOBY CANAL SYSTEM OF FISHES. 291 region, and some little distance in front of this a lateral branch passes off-the operculo-mandibular branch. The canal then makes a slight ventral curve, and posterior to the orbit divides into supra- and sub-orbital branches. The Supra-orbital Branch, like the main canal, is conducted in its course by a series of small drainpipe-like canal-bones. It passes forwards and inwards to a point directly above the orbit, and then makes a curve towards the nasal capsule, dorsal and anterior to which it terminates by a minute pore. The Sub-orbital Branch passes from the main canal on the lateral border of the frontal in a series of canal-bones forming the circum-orbital series. These conduct the branch around the inferior and posterior borders of the orbit, the branch terminating ventral and slightly posterior to the nasal capsule. The Operculo-mandibular Branch.-Leaving the main canal in the inferior and posterior portion of the pterotic, the operculo-mandibular branch passes into a small tube of bone which traverses the region between the pterotic and preoperculum, along the superior and posterior borders of the hyomandibular bone. Parker and Bettany regard this small tube-like bone as a supra-temporal. It cannot, I think, be regarded as other than one of the drainpipe- like canal-bones which conduct the main canal and branches over the greater part of the skull, and homologous with those figured and described in Amiurus catus (see p. 280). Entering the apex of the preoperculum, the canal traverses the anterior border, passing through the whole length of the bone. In its course it gives off fine branches, which are distributed over the inferior border of the bone; a number of finer branches pass off over the superior portion of the preoperculum. Leaving the preoperculum, a small dermal connection conducts the branch into the articular portion of the mandible, entering it on its lateral border. It passes downwards and forwards and into the dentary, traversing the ventral portion of the same. In the anterior portion of the dentary tbe branch divides in the substance of the bone, anastomosing previous to opening at a terminal pore. There is no connection between the two branches of either side. 3. The OCCIPITAL C O M M I S S U B E leaves the main canal on the inner border of the pterotic, and makes an anterior flexure over the region of the supra-occipital, and meets with the main canal of the opposite side. I have not figured or described the innervation, as the distribution of the cranial nerves is almost identical with that of Esox lucius. Variation. In the Salmon there seems to have once been a series of canals passing through certain bones of the head which have been replaced by the system borne by the series of drainpipe-like canal-bones already described. In young examples a series of canals are easily traced in the parietal, frontal, supraethmoid (Parker), and nasal bones. These canals vary a little in different specimens, but, generally speaking, 19* |
