142 COMPARATIVE ANATOMY. 



mid-brain, and thus may be easily overlooked (Figs. 114 and 

 115, ZH). 



The prolongation of the cerebellum into the ventricle of the mid-brain, 

 seen only exceptionally in Elasniobranchs, is present as a rule in Teleosteans, 

 but the extent' of its development varies much. The pineal gland does not 

 differ essentially from that of Elasmobranchs and Ganoids, though it never 

 extends into the roof of the skull, and usually remains within the brain- 

 membranes. 



As in Elasmobranchs, lobi inferiores and a saccus vasculosus are 

 present in connection with the infundibulum. The saccus vasculosus is both 

 glandular and vascular in structure, and its duct passes into the infundibulum ; 

 hence it is sometimes called the " infundibular gland." 



Ganoidei, Dipnoi,, and Amphibia. Even apart from the 

 brains of Lep.idosteus and Amia, which are formed on the 

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

 brains of other Ganoids, Dipnoans, and Urodeles; in a certain mea- 

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

 resemble the brain of Petromyzon. They are all distinguished by 



I T,ol VKS 1H NJL Jfff 



JL P#fcCs Inf M 



FIG. 116. BRAIN OF Polypterus bichir. (Side view.) 



/, olfactory nerve ; //, optic nerve ; Lol, olfactory lobe ; VH, prosencephalon, with 

 a lateral cleft at S, and the cerebral peduncles (Pedc) at its base, which radiate 

 into the hemispheres at Cs ; ZH, thalamencephalon, at the base of which is the 

 infundibulnm (Inf) with the pituitary body (H) ME, mid-brain ; HH, cere- 

 bellum ; NH, medulla oblongata ; R, spinal cord. 



a marked development of the cerebrum, while the cerebellum is 

 only represented by a small transverse fold of nervous matter 

 on the anterior end of the medulla oblongata (Figs. 116 and 

 117, HH). 



The mid-brain is always paired ; it encloses the narrow' 

 aqueduct of Sylvius, and extends laterally outwards into the 

 optic tract. The extremity of the epiphysis sometimes extends 

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

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

 though much more of it is exposed in Urodela than in Gym- 

 nophiona and Anura, in which the individual regions, especially 

 the largely developed hemispheres and the broadened mid-brain 

 (Fig. 118, VH, MH\ are much more closely approximated than in 

 Urodeles and Ganoids. 1 



1 As in Elasmobranchs, the epiphysis of Anura is produced into a long tube, the 

 distal end of which not only passes into the roof of the skull, but (in the embryo) 

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



