The Central Relations of the Cranial Nerves in Silurus Glanis and Mormyrus Caschive

THE CENTRAL RELATIONS OF THE CRANIAL NERVES IN SILURUS GLANIS AND MORMYRUS CASCHIVE H. BERKELBACH VAN DER SPRENKEL Assistant in the Anatomical Institute at Utrecht TWENTY-ONE FIGURES The following researches were started with the idea of study- ing the topography of the motor nuclei in the brain of Silurus glanis, since a description of these nuclei in the central nervous system of siluroid fishes has not yet been given and we may expect interesting' relations in this connection on account of the enormous development of special sensory systems and their secondwy connections.1 It is known that the siluroids2-including also the European representative of that order, Silurus glanis3-are distinguished by an extraordinary development of the sense of taste, the re- ceptive organs of which are distributed over the head and the body and are innervated by the ramus recurrens facialis, which is very large in these animals. This has been demonstrated, among others, by Herrick for Ameiurus melas, and our photo- graph (fig. lA), taken from an anatomical preparation made by Mr. Schepman, shows the same for Silurus glanis. It was to be expected that the preeminence of one set of sensory impressions should have a pronounced influence on the structure of the central nervous system, which would also appear 1 The olfactory and optic nerves are not discussed here because it was chiefly the medulla oblongata and basis mesencephali which interested me. 2 C. J. Herrick, The central gustat,orypaths in the brains of bony fishes. Jour. Comp. Neur., vol. 15, 1905, p. 375. The organ and sense of taste in fishes. Bul. U. S. Fish Commission for 1902, pp. 237-272, Washington, 1904. The cranial nerves and cutaneous sense organs of the North American siluroid fishes. Jour. Comp. Neur., vol. 11, 1901, p. 177. a F. Merkel, Ueber die Endigungen der sensiblen Nerven in der Haut der Wir- belthiere. Rostock, 1880. 5 6 H. BERKELBACH VAN DER SPRENKEL in the arrangement of the motor nuclei. The results justify this expectation, as appears from the topographic diagrams appended to this paper (fig. 21). The arrangement is characteristic of a ‘taste fish,’ and shows this character in a very pronounced way. I have thought it necessary to give also a description of the sensory roots and their connections, first because such a description, though given in Herrick’s paper for the North American forms, has not yet been given for the European representative of the siluroids, and second, because a knowledge of the sensory roots, centers and secondary paths is absolutely necessary for the understanding of the topography of the motor nuclei. Herrick’s excellent description of Ameiurus has been a valuable guide to me, the more so since I had a complete series of Ameiurus nebulosus at my disposal. After having finished my work on Silurus, it seemed interesting to me to examine another fish, whose physiological char- acteristics were very different from those of Silurus. I chose Mormyrus caschive, whose motor nuclei have not yet been de- scribed and which is, moreover, an interesting object on account of the enormous development of the lateralis nerves and valvula cerebelli. The study of the latter has also enabled me to correct some statements occurring in the literature concerning this peculiar brain. The Central Institute for Brain Research in Amsterdam has put at my disposal series of sections through the brains of Silurus glanis and Mormyrus caschive. The objects, embedded in cel- loidin, were cut into sections of 25 micra and alternately stained after van Gieson and Weigert-Pal. The Weigert-Pal series was counterstained with paracarmine. My projections have been made after the van Gieson series. In order to get the most exact reconstruction of the topographic positions of the motor nuclei and roots, I projected them on the sagittal plane lying medially in the raphe in the following way. In each section the distance from the ventral border of the bulb to the floor of the fourth ventricle was measured with an ocular CRANIAL NERVES OF SILURUS AND MORMYRUS 7 micrometer. If the most ventral part of the bulb was not in the region of the raph6, it was projected on the sagittal plane of the raph6. The micrometer being put parallel with the raph6, a movem.ent of the object table perpendicularly to the micrometer was sufficient to project the point in question on the medial plane. At the dorsal limit of the bulb the first curve toward the horizontal in the lining of the ventricle was registered. In the same way the ventral and dorsal limits of each nucleus were registered and projected on the medial plane. In my projections 6ach section was counted as 2 mm. Since each section was 25 micra thick and the series contained only alternating sections, these 2 mm. represent 50 micra, thus giving an enlargement of 40 diameters. The same magnification was applied to the dorso-ventral dimensions, which likewise gave the natural relations forty times enlarged. Since my projections have been four times reduced for repro- duction, the figures represent the natural relations ten times magnified and projected on the sagittal plane through the raph6. The ventral border of the medulla has been taken as a horizontal line. It is actually slightly curved, but the curve of the oblongata in teleosts is generally so insignificant that the error hereby introduced has only a slight influence on the exact relations. Figure 2 shows the projected points connected by lines. The dotted line represents the floor of the ventricle, or rather, since the ventral border of the bulb is drawn straight, it represents the algebraic addition of the curves occurring in the floor of the ventricle and those in the ventral border of the bulb. The limits of the nuclei are indicated by full lines, in doubtful cases by dashes alternating with dots. Local interruptions in the nuclei are indicated by curved lines. Beneath the base line, representing the ventral border of the bulb reduced to a horizontal line, the levels of entrance of the motor roots are registered, thus enabling us to see the distances between the roots and the spaces occcupied by their entrance. Figure 3 is based on figure 2 and is designed to give a simpler view of the relations, the curves being smoothed by the omission 8 H. BERKELBACH VAN DER SPRENKEL of small irregularities. In order to indicate to which nuclei the roots correspond, the same markings are used as in Kappers’ diagrams. In figure 4 the exact place of entrance-not merely the level- of each root is projected, together with its central course. Of the latter, only the most important points were projected, these being connected by rather smooth lines. These three projections are intended to show the natural forms and positions of the nuclei and roots in the medulla and basis mesencephali in the most painstaking way. The differences between these projections and Kappers’ diagrams lie chiefly in the fact that Kappers draws each nucleus as it appears when projected on its own radial, the center of the radius being the middle of the bottom of the IYth ventricle, whereas I projected all the nuclei on the medial plane of the raph6. From this it results that the place of the V and VII nuclei is less ventral in my projection than in Kappers’ diagram. Moreover, Kappers does not aim to show the exact form of the nuclei, but only the spaces within which they are found. SILURUS GLANIS The eye-muscle nerves in Silurus are poorly developed, in conformity with the life habits of this adimal which belongs to the bottom-feeders, living in the mud and searching for their food chiefly by exploring the bottom with their taste organs. This is in contrast to the plankton feeders, which swim around near the surface and seek their food chiefly with their visual apparatus. The ocuZomotor nerve is, consequently, relatively thin. Ap- proaching the mesencephalon in the cleft between the inferior lobes and the base of the midbrain, it pierces through the base in a caudo-frontal direction (fig. 4). As figure 2 shows, the nucleus lies several sections in front of the entrance of the root, its caud.al part being the larger. The cells are located near the raph6, but it is not an unpaired nucleus in the strict sense, since there is clearly a small region devoid of cells between the left and the right nucleus. The dorso-ventral and transverse diameters of CRANIAL NERVES OF SILURUS AND MORMYRUS 9 the nucleus are approximately the same, but in the caudal part the transverse dimension may be a trifle smaller. Directly behind the third nerve a considerable decussation of cerebellar fibers is found, as is the case in most teleosts. The trochlearis enters the brain near the transition from the tectum opticum to the cerebellum. Its fibers are not as thick as those of the I11 nerve. After having entered the brain, they first run in a medio-dorsal direction, bending slightly caudad. It is extremely difficult to trace the IV root in the intricate net-work of fiber systems found in this level of the brain and it may be that its course as indicated in my projection (fig. 4) needs correction. Its entrance, however, as well as the location of the nucleus, are not in doubt. The IV nucleus has a relatively lateral position under the floor of the ventricle and it is not connected with the I11 nucleus. This separation of the I11 and IV nuclei occurs often in teleosts, but is not a constant feature.

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