H2 COMPARATIVE ANATOMY. 



mid-brain, and thus may be easily overlooked (Figs'. 11-i and 

 115, Zll 



The prob .Cation <>t' tin- ecivUdlum into tin- ventricle of tlie mid-brain, 

 seen only exceptionally in Kla-niol.ram-lis, is jnesmi a.- a rule in Td.-M>UMns, 

 IMI! the extent of its development varies much. Tin- pineal gland does not 

 dili'er essentially from iliai of Elasmobrancha ami (Jam ads, tliou-h it never 

 extends int.. tin- roof of tin- skull, ami ii.-ually remains within tin- brain- 

 membranes. 



A,- in Elasmobranchs, lol.i inferiores and a saccus vasculosus are 

 pre>ent in connection \\itli tin- infumlibnlmu. Tin- saccus vasculosus is both 

 L'lamlular ami vascular in structure, and its duct passes into the infundibulum ; 

 hence ii is sometimes called tin- " infundibular ^laud." 



Ganoidei, Dipnoi, and Amphibia. Even apart from the 

 brains of Lepidosteus and Amia, which are formed on the 

 Teleostean type, a common ground-plan cannot be laid down for the 

 brains of other Ganoids, Dipnoaus, and Urodcles; in a certain tnea- 

 suiv they may be said to form one group, but in many points they 

 resemble thr"lrain of Petromyzon. They are all distinguished by 



Jl JWeCs Jfif Jf 



FIG. 116. BIIAIN OF Polypteras biehir. Si>K' vii-\\. 



/, olfactory nerve ; II, optic nerve ; Lol, olfactory lobe ; VII, prosencephalon, with 

 a lateral cleft at N, and the cerebral peduncles (Pcdc) at its base, wliich I'.uliate 

 into the hemispheres at ''.> ; Zll, thalamencephalon, at the base of which is the 

 iufuiidibulnni (In/) with the pituitary body (//) ; ME, mid-brain : IIH, cere- 

 bellum ; NH, medulla oblongata ; I!, spinal cord. 



a marked development uf the ci.'i-eb]-um, while the cei'ebellum is 

 only represented hy~i~sinall "Transversi 1'nld uf nervous matter 

 on the anterior end of tin- medulla oblongata (Figs. 11G and 

 1 I 7, HH}. 



Tin- mid-brain is always paired; it encloses the narrow 

 aimeduct of Sylvius, and extends laterally outwards into the 

 optic tract. The extremity of the epiphysis sometimes extends 

 into the roof of the skull (Acipenser, (Jeratodus), and the thalam- 

 encephalon is not visible to any great extent on the dorsal side, 

 though much more of it i< exposed in Vmdela limn in Gvm- 

 nophioiia and Anura, in which the individual regions, (specially 

 the largely devcloj^ed hemispheres and the broadened mid-brain 

 (Fig. 1 IN, I'//, Mil], are much more closely approximated than in 

 Urodeles and Ganoids.' 



1 As in Elasmobranchs, the epiphysis of Aimra is produced into a luni; tube, the 

 distnl end of \vliieli not, only passes into the roofuf I lie skull, hut (in tin? embryo) 

 extends even to the skin. Jt becomes reduced later on. 



