THE CRANIAL NEKVES AND LATERAL SENSE ORGANS OE FISHES. 157 



divides into three twigs whilst still lying near the brain in the eye muscle canal and 

 opjjosite the edge of the inner ventral process of the frontal. The inner of the three 

 twigs passes inwards and upwards, perforates the frontal, and lies between the Irontal 

 and the sensory canal. In this position it passes forwards for some little distance and 

 innervates sense organ 4 of the supra-orbital line. The remaining two twigs doubtless 

 innei'vate pit organs in this region. 



Opposite the point where the twig to sense organ 4 perforates the frontal, the 

 ophthalmic trunk divides into two conspicuous nerves, a larger dorsal lateral portion, 

 and a small ventral trigeminal portion — as is shown by the nerves arising from them, 

 the lateral-line twigs arising from the dorsal nerve and the general cutaneous fibres from 

 the ventral iierve. As the trunk passes forwards the division becomes more mai'ked, 

 until two rounded nerves are distinctly differentiated. The lateral portion soon gives off 

 a twig which passes uj)wards and enters the frontal slightly anterior to the supra-oi'bital 

 commissure. It passes obliquely through the substance of the bone inwards and 

 forwards, perforates it dorsally, lying between it and the sujira-orbital canal, and finally 

 innervates sense organ 3 of the supra-orbital line. Anterior to the region where this 

 twig enters the frontal, the two divisions of the ophthalmic trunk begin to approximate 

 and once more continue their course together. At this region too, a small blind sac is 

 seen in the sections to open into the supra-orbital canal external and opposite to tlie 

 supra-orbital commissui-e and partly opposite the fourth sense-organ. It contains no sense 

 organs and no lateral line twigs could be traced to it, and seems to me to correspond 

 precisely to a much larger but otlierwise similar structure in the same position described 

 by Hyrtl in Lota. It therefore possibly represents a degenerate or modified dermal 

 tubule. 



In the region of sense organ 3 the supra-orbital trunk shows a tendency to split up 

 again, and can clearly be resolved into its two constituents with the higher power of the 

 microscope. This tendency, however, is soon lost and the nerves become inseparable as 

 before. It is here, moreover, tliat the ojilithalmic trunk leaves what I take to be the 

 eye muscle canal, and becomes for the first time perfectly round in transverse section. 

 It immediately enters the frontal at the junction of the frontal and pre- frontal (= lateral 

 ethmoid or parethmoid), and courses obliquely inwards and forwards in the spongy sub- 

 stance of tlie frontal. After leaving the latter bone it enters a large space bounded above 

 and internally by the frontal and below by the pre-frontal. Anterior to this sjmce the 

 trunk begins to pass ujiwards and inwards towards the Ligamentous portion of the supra- 

 orbital canal, which it accompanies, and an examination of it with the high power at 

 once reveals the distinctiveness of its lateral and trigeminal portions. These again 

 separate, and the ventral portion or trigeminal ophthalmic gives off a large nerve, which 

 could not be satisfactorily traced in the sections, bvit seemed to be a cutaneous sensory 

 nerve *. In front the two ophthalmics again more or less approximate. 



Before leaving the space in the frontal mentioned above, the lateral ophthalmic gives 

 off dorsally a twig {S.O.^) which passes inwards and forwards, curves upwards round the 



* The fibres of this nerve undoubted!}' came from the trigeminal ophthalmic. 



22* 



