THE BRAIN IN THE SEPARATE CLASSES 



167 



The brain of Polypterus differs from that of other ganoids in several respects. 

 There is no differentiation of cerebral hemispheres; the optic lobes and the cere- 

 bellum are moderate, the latter being thin in the median line and the valvula 

 smaller. The medulla oblongata has thin walls and the ventricle is large. The 

 brain has a primitive appearance, but it shows little resemblance to those of the 

 amphibia or of the dipnoi. 



DIPNOI. — The brains of Lepidosiren and Protopterus differ considerably 

 from that of Ceratodus. In all the cerebrum is larger than the optic lobes and 

 the olfactory bulb is separated from the cerebrum by a long olfactory tract. In 

 Ceratodus the hemispheres are united above by a part of the chorioid plexus, 

 while internally they are separated from the diencephalon by a well-marked ve- 

 lum. The pinealis is long and rests upon a large ' zirbelpolster, ' developed as 

 an outgrowth of the roof of the third ventricle in front of the superior commis- 

 sure. The optic lobes are separated into two hemispheres, while the cerebellum 

 is scarcely more than a transverse plate and is, together with the fossa rhom- 

 boidea, covered with a complicated chorioid plexus. In Protopterus (fig. 175) 



Fig. 175. — Brain of Protopterus, after Burckhardt. ch, cerebellum; e, epiphysia 

 structures; h, hypophysis; i, infundibulum; m, mid-brain; se, saccus endolymphaticus 

 sp, spinal nerves; /, cerebrum; 1-12, cranial nerves. 



the elongate hemispheres are parallel, the pinealis and its 'polster' are smaller 

 and the mid-brain has but a single rounded lobe. 



AMPHIBIA. — The brain of the amphibia, which shows considerable resem- 

 blance to that of the dipnoi, has the parts more distinct than is usual in the 

 vertebrates, and, except in the gymnophiones, the flexures have disappeared in 

 the adult. The hemispheres are separated by a deep longitudinal fissure, but 

 in the anura the two halves of the cerebrum are secondarily connected by a 

 transverse band, just behind the olfactory lobes, leaving a gap farther back. 

 The telencephalon is relatively larger than in fishes, the increase being due in 

 part to the invasion of the pallium by nervous matter, the medial wall being 

 the thicker, while the corpus striatum is in the outer wall. The pallium has the 

 nerve cells in the inner side, the outer being composed of fibres, and there is no 

 trace of a cortex (fig. 176). The diencephalon, broad in the anura, narrower 

 in the urodeles (fig. 176) and csecilians, is visible from above. Infundibulum 

 and hypophysis are well developed, but the saccus vasculosus and inferior lobes 

 are smaller than in fishes. In the gymnophiones, owing to the pontal flexure 

 (fig. 160) the hypophysis is brought beneath the medulla oblongata. Both 

 paraphysis and pinealis are present, the latter not reaching the cranial roof excep- 

 in the anura, the conditions in this group having already been mentioned (p. 156). 

 The optic lobes are large and in the anura and to a less degree in the gymnot 



