Fig. 216.—Vertical longitudinal section of the brain of a Sturgeon (Acipenser ruthenus.) c.p, Posterior commissure; c.r, cranial roof; mc, mesocoele; op.ch, optic chiasma; p.ch.p, posterior choroid plexus; v.c, valvula cerebelli; v.t, velum transversum; v.iii, v.iv., the third and fourth ventricles; other lettering as in Fig. 210. (From Goronowitsch.)
The most obvious feature in the brain of the Dipnoi is the great development of the cerebral hemispheres. In this respect these Fishes approach the Amphibia, but in other features of brain-structure they present points of agreement with most other groups of Fishes without being closely related to any one of them. In Protopterus[[446]] (Fig. 217) the hemispheres are quite distinct except behind, and the walls of their spacious lateral ventricles are entirely nervous. Olfactory lobes are sessile on their anterior extremities, and behind and below they enlarge into ventral lobes which probably represent the hippocampal lobes of the higher Vertebrates. A vesicular pineal body at the end of a slender stalk overlies a singular conical projection from the roof of the thalamencephalon or "pineal pillow."
Fig. 217.—Dorsal (A), ventral (B), and lateral (C) views of the brain of Protopterus annectens. C, Cerebellum; C.H, cerebral hemisphere; D.S.E, branches of the sinus endolymphaticus; In, infundibulum; L.I,, lobi inferiores; M.O, medulla oblongata; O.L, olfactory lobe; Op.L, optic lobe; P, pituitary body; P.B, "pineal pillow"; S.E, sinus endolymphaticus; Sp.c, spinal cord; Sp.n, spinal nerve; Vel, velum transversum; Z, pineal body; IV.V., fourth ventricle; ii., iii., iv., v., vi., vii., viii.1, viii.2, viii.3.4, ix., and x., roots of the cranial nerves. (From Burckhardt.)
The optic lobes form a single oval body, and, as in Petromyzon and the Amphibia, the cerebellum is very small. A posterior choroid plexus covers the roof of the fourth ventricle, and an anterior plexus in connexion with the roof of the thalamencephalon projects downwards into the third ventricle, and is also prolonged forwards into each lateral ventricle. In Neoceratodus[[447]] the brain is certainly more primitive and distinctly less Amphibian. As compared with Protopterus the olfactory lobes and the cerebellum are larger, and the optic lobes are paired. The smaller hemispheres are non-nervous dorsally and medianly, the roof and inner wall of each being formed by an extension of the thick, glandular choroid plexus which forms the roof of the thalamencephalon.
The Spinal Nerves.—The spinal nerves of Cyclostomes (e.g. Petromyzon) consist of a series of dorsal nerves arising on each side from the dorsal surface of the spinal cord, and of a similar double series arising from the ventral surface, the dorsal nerves regularly alternating with the ventral nerves. Each myotome is supplied by a dorsal and a ventral nerve which pass separately to their peripheral distribution in the skin and muscles. In Fishes, as in the higher Vertebrates, each dorsal nerve, now termed a dorsal root, enlarges into a ganglion and then unites, either before or directly after issuing from the neural canal, with the next ventral nerve or ventral root in front to form a main spinal nerve. At the same time the spinal nerves of opposite sides tend to form pairs in the same transverse plane. After the union of the two roots the spinal nerve divides into three typical branches: a dorsal nerve (ramus dorsalis), and a ventral nerve (ramus ventralis), both of which include somatic sensory or afferent fibres, and somatic motor or efferent fibres, for the innervation of the skin and muscles of the dorsal and lateral portions of a myotome; and a visceral branch (ramus visceralis), composed of afferent and efferent visceral fibres, which supplies the adjacent viscera (alimentary canal and its glands and blood-vessels), and helps to form the sympathetic nervous system.[[448]] The somatic afferent and the visceral afferent fibres enter the spinal cord by the dorsal roots, the somatic efferent leaving the cord through the ventral roots, although the visceral efferent fibres traverse both roots. In the region of the paired fins more or fewer of the rami ventrales unite to form a plexus, the brachial or the pelvic plexus, from which the nerves to the fins take their origin.
The Cranial Nerves.—It is usual to describe the cranial nerves of Cyclostomes and Fishes as consisting of ten serially disposed pairs, viz.: the olfactory (i.), optic (ii.), oculomotor (iii.), trochlear (iv.), trigeminal (v.), abducens (vi.), facial (vii.), auditory (viii.), glossopharyngeal (ix.), and the vagus (x.) Like the spinal nerves, the cranial nerves collectively include somatic sensory (general cutaneous) and motor fibres, and also visceral sensory and motor fibres, all of which have their own special centres in the brain, but the proportions of these nerve components differ greatly in different nerves. Certain preoral nerves (iii., iv., and vi.) are exclusively somatic motor; others (i. and ii.) are special sensory nerves for the olfactory and visual organs; but most of the other cranial nerves include several components, and are therefore "mixed" nerves. Besides these components some cranial nerves include also a quasi-independent system of nerve-fibres, which converge from certain cutaneous sense-organs to an independent centre in the medulla oblongata, the tuber acusticum,[[449]] and is probably derived from the general cutaneous system of nerve components. Such nerve fibres, including also the auditory nerve, which has its origin from the same centre, constitute the lateralis system. Perhaps the most striking feature in the postoral cranial nerves is the predominance of the visceralis or sympathetic system over the somatic. Omitting the lateralis fibres and a relatively few somatic sensory fibres, visceral fibres, sensory and motor, are the principal components of all these nerves, including v. but excluding viii. The reason for this is to be found in the fact that splanchnic or visceral muscles in relation with the jaws and branchial arches have usurped the place of somatic muscles in the muscular system of the head. For developmental and other reasons the olfactory and optic nerves stand in a category of their own, and the same may be said of the third, fourth, and sixth nerves, which innervate the muscles of the eyeball. The remaining nerves, all of which have their origin in the medulla oblongata, possess certain features in common, and as they are related to the gill-clefts in such a way that each forks over a cleft, they may be conveniently distinguished as "branchial" or "branchiomeric nerves." A typical branchial nerve consists of (1) a principal ganglion near the origin of the nerve from the brain; (2) a main trunk which gives off (3) a somatic sensory branch or dorsal nerve to the skin; (4) a palatine nerve (visceral sensory) to the oral or pharyngeal mucous membrane; (5) an epibranchial ganglion which is associated with a transitory embryonic epibranchial sensory organ at the dorsal border of a branchial cleft; (6) a pre-branchial nerve (visceral sensory), skirting the anterior margin of a cleft in its ventral course; and (7) a post-branchial branch (visceral motor) similarly related to the hinder margin.
Fig. 218.—Diagram showing the principal branches of the cranial nerves in a Fish, mk.c, Meckel's cartilage; ol.o, olfactory organ; p.q, palato-quadrate; s, spiracle; i-v, branchial clefts; i, ii, iii, iv, vi, the first, second, third, fourth, and sixth cranial nerves. The remaining nerves are differently shaded. Black.—The trigeminal nerve: g.g, Gasserian ganglion; md, mandibularis; mx, maxillaris; op.p, ophthalmicus profundus. Oblique shading.—The lateralis system and its centre (t.a), the tuber acusticum: bucc.vii, buccalis branch of vii; md.ex, external mandibular branches of vii; l.n.x, lateralis nerve, with its supra-temporal branch (s.t), and its commissural connexion (c) with op.s.vii, the ophthalmicus superficialis of vii; viii, auditory nerve. Dotted.—The facialis proper, including c.t, chorda tympani; gn.g, geniculate ganglion; hy, hyomandibularis, with its motor branches m, m, m; p.n.vii., palatine. Dark grey.—The glossopharyngeal (ix), with its pre- and post-branchial branches and its palatine nerve, p.n.ix; anastomosing with the palatine branch of vii (Jacobson's anastomosis). White.—The vagus: x1-4, the branchial nerves, ganglionated and forking over clefts ii-v; v.n.x, visceral nerve; oc, occipito-spinal nerves; d.r and v.r, the dorsal and ventral roots of the first two spinal nerves. (Slightly modified after Wiedersheim.)