Fig. 210.—Diagram of the general structure of the brain in Craniates. A, vertical longitudinal section; B, dorsal view showing the brain cavities on the right side. c, Cerebellum; c.c, central canal of the spinal cord; c.h, cerebral hemispheres; c.s, corpus striatum; F.B, fore-brain; f.m, foramen of Munro; H.B, hind-brain; in, infundibulum; l.v, lateral ventricle; m, mesocoele; M.B, mid-brain; m.o, medulla oblongata; o.l, olfactory lobe; op.l, optic lobe; op.t, optic thalamus; p, paraphysis; pc, prosocoele; pn.o, pineal organ; p.o, parietal organ; pr, prosencephalon; pt, pituitary body; rh, rhinocoele; sp.c, spinal cord; s.v, saccus vasculosus; th, thalamencephalon; iii, iv, third and fourth ventricles. (After Parker and Haswell.)
Scarcely less complicated, and perhaps even more interesting from a morphological standpoint, are the structures arising out of the thalamencephalon. By thickenings of its lateral walls two large ganglia, the optic thalami, are formed, and on the inner or dorsal aspect of each of these a ganglion habenulae is developed. From the sides of the thalamencephalon the primary optic vesicles are derived, which later become transformed into the retinal parts of the paired eyes and the optic nerves. Besides the optic vesicles there is a second pair of embryonic outgrowths which arise from the roof of the thalamencephalon. These outgrowths form stalked vesicles and represent a pair of degenerate visual organs. Usually they become so displaced that the left one lies in front of the right, and they appear as if median. The subsequent fate of the vesicles differs greatly in different Craniates. Both persist in the Lamprey, the right vesicle to some extent retaining its primitive visual function as a parietal eye and directly overlying the left or pineal vesicle. In Elasmobranchs the two unite to form a glandular organ, the so-called pineal body of the adult, and in Teleosts the left vesicle disappears, leaving the right as a pineal body.[[441]] There is also an embryonic median outgrowth from the roof of the prosencephalon, the paraphysis, which soon disappears and whose significance is not known. A median hollow downgrowth from the floor of the thalamencephalon forms the infundibulum, which becomes attached to a caecal diverticulum from the roof of the mouth. With rare exceptions the diverticulum loses all connexion with the mouth, and, as the pituitary body or hypophysis, it appears as an appendage to the extremity of the infundibulum. In the Crossopterygii the connexion is retained even in the adult by means of a slender canal extending from the pituitary body and opening into the oral cavity. Laterally, the base of the infundibulum grows out into a pair of rounded lobes, the lobi inferiores, and distally into a thin-walled glandular sac, the saccus vasculosus, which lies just behind the pituitary body. The cavity of the thalamencephalon persists as the third ventricle or diacoele. The parts of the brain developed from the mid-brain and the hind-brain are much less complicated, and, except for variations in size, they present a fairly uniform character in most Fishes.
In the mid-brain the roof bulges out into a pair of optic lobes, and by the growth of lateral thickenings in its floor two thick strands of longitudinally disposed nerve fibres, the crura cerebri, are formed. The cavity of the mid-brain remains as the mesocoele, and from it an extension may be prolonged into each optic lobe.
From the hind-brain are formed the cerebellum or epencephalon and the medulla oblongata or metencephalon, the former as a dorsal bulging, the latter as a ventral thickening. Except where the cerebellum is developed the dorsal wall remains epithelial, and forms the roof of the persistent cavity of the hind-brain, the fourth ventricle or metacoele, which retains its primitive continuity with the central canal of the spinal cord. Lateral lobe-like outgrowths from the dorsal columns of the medulla are conspicuous structures in some Fishes, and are known as corpora restiformia. The paired portions of the brain are connected across the middle line by a series of transverse commissures. The more important modifications of the brain in Cyclostomes and Fishes will now be briefly dealt with.
Fig. 211.—Dorsal (A) and ventral (B) views of the brain of Petromyzon marinus. ch.pl.1, Anterior choroid plexus forming the roof of the prosencephalon and thalamencephalon; ch.pl.2, aperture in the roof of the mid-brain exposed by the removal of the middle choroid plexus; ch.pl.3, the fourth ventricle exposed by the removal of the posterior plexus; cr.crb, crura cerebri; crb, cerebellum; crb.h, cerebral hemispheres; dien, thalamencephalon; inf, infundibulum; l.gn.hb, left ganglion habenulae; med.obl, medulla oblongata; nv.1, olfactory; nv.2, optic; nv.3, oculomotor; nv.5, trigeminal; and nv.8, auditory nerves; olf.l, olfactory lobes; opt.l, optic lobes; pn, pineal organ; r.gn.hb, right ganglion habenulae. (From Parker and Haswell, after Ahlborn.)
Fig. 212.—Dorsal view of the brain of Myxine. c.r, Corpora restiformia; m.o, medulla oblongata; n.p, naso-pituitary canal; ol.o, olfactory organ enclosed in its fenestrated cartilaginous capsule; op.l, optic lobes; pr, prosencephalon; s, s, dorsal roots of spinal nerves; sp.c, spinal cord; th, thalamencephalon. (From Wiedersheim, after Retzius.)
In the Cyclostome Petromyzon there is a small prosencephalon with an undivided prosocoele, and on each side of it a small cerebral hemisphere which appears as a mere appendage to the much larger olfactory lobe (Fig. 211). The prosocoele divides in front into two outwardly directed branches, and of the two diverticula into which each branch divides one extends as a lateral ventricle into the hemisphere of its side, and the other as a rhinocoele into the corresponding olfactory lobe. The ganglia habenulae are unusually large, the right one being larger than the left. The optic lobes are large, but not obviously double. So small is the cerebellum that it seems to be little more than a narrow transverse band crossing the fore-part of the fourth ventricle. The roof of the brain is largely epithelial, especially in the prosencephalon, the thalamencephalon, and the hind-brain. Over these epithelial areas the pia mater is unusually vascular and forms a series of "choroid plexuses." The ventricular system is complete and continuous. By contrast with the Lamprey the brain of Myxine[[442]] is very primitive, more so perhaps than in any other Craniate (Fig. 212). In a dorsal view the brain is divided into four pairs of laterally expanded and longitudinally compressed lobes by a median longitudinal fissure and three transverse fissures. The two anterior lobes are little more than the thickened anterior wall of the thalamencephalon, although, judging from their histological structure, they represent a very imperfectly differentiated prosencephalon and olfactory lobes. The second and largest pair constitute the thalamencephalon. The last two pairs of lobes represent a transversely divided pair of optic lobes, or "corpora quadrigemina." There is a large medulla oblongata with a pair of corpora restiformia, but the cerebellum is entirely absent. The ventricles are subject to some individual variation. Third and fourth ventricles are generally recognisable, either as isolated cavities or connected by a remnant of the mesocoele. In the feeble development of the prosencephalon, in the striking preponderance of the mid-brain over the rest of the brain, and in the absence of a cerebellum, Myxine is unique amongst Craniates.