. The Cambridge natural history. Zoology. 37: FISHES. 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 rhino- coele into the corresponding olfactory lobe. The ganglia habenulae are unusually large, the right one
. The Cambridge natural history. Zoology. 37: FISHES. 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 rhino- coele 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 prosen- cephalon, the thalamencephalon, and Dorsal view of the brain the hind-brain. Over these epi- thelial areas the pia mater is un- usually vascular and forms a series of " choroid plexuses.'' The ventri- cular system is complete and con- tinuous. By contrast with the th ftaiameneephainn (From Lamprey the brain of Mvxine 1 is Wiederslieini, after Eetzms.) . 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 longi- tudinally 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 1 Holm, M
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
