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, Morph. Jahrh. x.xix. 1901, p. 365. 



Fia. 212. 



of 2[[iXLiie. c.Ty Corpora resti- 

 formia ; m.o, medulla oblongata ; 

 n.p, naso- pituitary canal ; ol.o^ 

 olfactory organ enclosed in its 

 fenestrated cartilaginous capsule ; 

 op. I, optic lobes ; pr, prosen- 

 cephalon ; s, s, dorsal roots of 

 spinal nerves ; sp.c, spinal cord 



