Fig. 213.—The brain of a Dog-Fish (Scyllium canicula). A, dorsal view; B, ventral view. The choroid plexuses covering the roof of the third and fourth ventricles have been removed. b.o, Olfactory lobe; ep, origin of the stalk of the pineal body; f.b (in A), prosencephalon; f.b (in B), cerebral hemispheres; fr, fourth ventricle; h.b, cerebellum; h.p, pituitary body; i.f, lobi inferiores; m.b, optic lobes; m.d, medulla oblongata; sc, saccus vasculosus; th, thalamencephalon; t.o (i) olfactory peduncle; i.-x. cranial nerves. (From Wiedersheim.)
In Elasmobranchs among Fishes the brain attains a much higher grade of structure. In Scyllium (Fig. 213) there is a large prosencephalon, and directly in front of it a pair of imperfectly differentiated cerebral hemispheres, while from its antero-lateral regions the large olfactory lobes arise. The prosocoele divides in front into four diverticula, of which the two inner ones extend into the hemispheres as lateral ventricles, and the two outer as rhinocoeles into the olfactory lobes (Fig. 214). In connexion with the infundibulum there is a pair of sacci vasculosi, consisting mainly of gland-tubules, opening into the infundibular cavity.[[443]] The cerebellum is exceptionally large, but it does not form a "valvula cerebelli." Large ear-like corpora restiformia are present. The third and fourth ventricles alone retain an epithelial roof in relation with choroid plexuses.
In all essentials the brain of the Holocephali is a repetition of the Elasmobranch type, more especially of the elongated form seen in Notidanus. Indications of a higher grade of structure are, however, to be seen in the reduction of the prosencephalon which, with its prosocoele, is now scarcely distinguishable from the thalamencephalon and its ventricle; and in the more complete differentiation of the cerebral hemispheres from one another and from the rest of the brain. Large frilled corpora restiformia are conspicuous structures on each side of the medulla oblongata. Besides the usual intra-cranial pituitary body, there is also a separate extra-cranial portion lodged in a pit on the ventral surface of the basis cranii: in the embryo the two are continuous.
Fig. 214.—Horizontal longitudinal section of brain of Chiloscyllium, to show the ventricles; semi-diagrammatic. cer, Origin of cerebellar ventricle or epicoele; dia, third ventricle; iter, mesocoele; meta, fourth ventricle; opt, optocoele, or cavity of an optic lobe; para, lateral ventricles; pros, prosocoele; rh, rhinocoele. (From Parker and Haswell.)
In the Teleostomi the brain is distinctly of a more primitive type than in any other Fishes (Fig. 215).[[444]] The most striking feature is the absence of cerebral hemispheres, the evolution of the primary fore-brain proceeding no farther than the formation of an undivided prosencephalon with a non-nervous roof, and a prosocoele which forms a continuous cavity with the third ventricle, or at the most is only separated from it by an infolding of the epithelial roof or velum transversum.
Fig. 215.—A, dorsal view of the brain of a Trout (Salmo fario); B, a vertical longitudinal section. c.il, Commissura interlobularis; g.h, ganglion habenulae; h.c, habenular commissure; i.c, inferior commissure; l.i, lobus inferior; myc, myelocoele; p.c, posterior commissure; v.o, valvula cerebelli; v.t, velum transversum; ii., optic nerve; v.iii., v.iv., third and fourth ventricles; v, vii, viii, ix, x, fifth, seventh, eighth, ninth, and tenth cranial nerves; remaining reference letters as in Fig. 210. (A, From Wiedersheim; B, after Haller.)
Amongst other diagnostic characters may be mentioned the predominance of the mid-brain over the other divisions, the anterior extension of the large cerebellum into the mesocoele as a "valvula cerebelli," and the absence of corpora restiformia. This type of brain is most strongly marked in the Teleostei, but in other Teleostomes some, like Acipenser,[[445]] are typically Teleostean in this respect (Fig. 216), while others, such as Lepidosteus, have small cerebral hemispheres with lateral ventricles as well as a prosencephalon.