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Show 38 PROF.T. J. PARKER ON CARCHARODON RONDELETII. [Jan. 18, anterior and ventral (fig. 23, va), formed of two distinct strands, a posterior and dorsal2 (vh), which arises mainly from the dorsal aspect of the metencephalon (fig. 20), immediately caudad of the corpus restiforme, but also receives a small bundle of fibres arising from the lateral region of the metencephalon, in common with the root of the eighth (viii.). The facial (vii.) has one main root formed of two strands, the ventral most of which is intimately united with the single root of the auditory nerve (viii.). Both dorsal and ventral roots of the fifth divide before leaving the skull, so that the nerve passes through the trigeminal foramen in four parts (fig. 20), each of which perforates separately the membrane of the foramen. The sixth nerve (vi.) arises by three distinct roots, the posterior of which is very slender and soon unites with the middle root. The vagus (x.) is an immense nerve arising by six lateral roots, of which the first four and the last two unite to form separate bundles, which leave the cranial cavity before joining into a common trunk. On the right side (fig. 20) the posterior root is double, and its hindmost factor arises at least 1 centim. caudad of the calamus scriptorius. A short distance cephalad of the origin of the posterior root of the vagus there arises from the ventral aspect of the metencephalon a distinct though small root (x1), formed by the union of several strands. This evidently corresponds with the nerve thus described by Balfour 3:-" The main stem of the vagus at a short distance from its central end receives a nerve which springs from the ventral side of the medulla, on about a level with the most posterior of the true roots of the vagus. This small nerve corresponds with the ventral or anterior roots of the vagus described by Gegenbaur, Jackson, and Clarke (though in the species investigated by the latter authors these roots did not join the vagus, but the anterior spinal nerves). Similar roots are also mentioned by Stannius, who found two of them in the Elasmobranchs dissected by him ; it is possible that a second may have been present in Scyllium, but have been overlooked by me, or perhaps may have been exceptionally absent in the example dissected." As the nerve-roots in Carcharodon were made out while the tough pia mater was quite intact, I feel satisfied that no other ventral root of the vagus was present in m y specimen. From the direction taken by the nerve it appears to join the vagus, not the spinal nerves ; but it was unfortunately severed, as shown in fig. 22, when the brain was removed. DESCRIPTION OF THE PLATES. P L A T E IV. Fig. 1. Cranium of Carcharodon rondeletii, dorsal aspect, X-j. 2. Cranium of Lamna cornubica, dorsal aspect, X f. 2 This root properly belongs to the seventh, as shown by Balfour and Marshall. 3 ' Elasmobranch Fishes,' p. 196 (Works, Memorial Edition, vol. i. p. 419). |