BRAIN. 155 



vasculosus and inferior lobes. On its roof is a large pinealis which reaches the skull in 

 a few ganoids. The parietal organ appears in the embryo and soon degenerates; the 

 paraphysis is usually well developed. The optic lobes are large and are usually 

 divided into two hemispheres by a median groove, but this occasionally is scarcely 

 noticeable. The cerebellum is large, much larger than appears from the surface, 

 since a considerable part, the valvula, projects into the ventricle of the mid-brain. 

 In the cerebellar region there is sometimes an enormous development of the lobes 

 of the vagus (fig. 155). 



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 cerebel- 

 lum 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 



FIG. 157. Brain of Protopterus, after Burckhardt. cb, cerebellum; e, epiphysial 

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

 spinal nerves; t, cerebrum; 1-12, cranial nerves. 



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

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

 internally they are separated from the diencephalon by a well marked velum. 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 commissure. The optic 

 lobes are separated into two hemispheres, while the cerebellum is scarcely more than 

 a transverse plate and is, together with the fossa rhomboidea, covered with a com- 

 plicated chorioid plexus. In Protopterus (fig. 157) the elongate hemispheres are 

 parallel, the pinealis and its 'polster' are smaller and the mid-brain has but a 

 single rounded lobe. 



AMPHIBIA. The parts of the amphibian brain are more distinct from each 

 other than is usual in vertebrates, and, except in the gymnophiones, the flexures 

 have largely disappeared in the adult. There is a deep intercerebral fissure 

 between the hemispheres, but in the anura the two halves of the cerebrum are 

 connected by a transverse band just behind the olfactory lobes. The telencephalon 

 is relatively larger than in fishes, the increase being due to the invasion of the pal- 

 lium by nervous matter, while the corpora striata are relatively smaller than in other 

 ichthyopsida. In the pallium the inner part is largely composed of nerve cells, 

 the outer layer consisting of nerve fibres. 



The diencephalon, broad in the anura, narrower in the urodeles and caecilians, 



