BRAIN 



215 



\ 



is relatively much smaller than in Elasmobranchs and Teleosts : it 

 gives rise to a valvula cerebelli, and a complicated choroid plexus 

 roofs over the fourth ventricle. 



Amphibians. The cerebral hemispheres of the Amphibia are 

 distinguished from those of the Dipnoi by a higher development 

 of the pallium, which, however, is differentiated even in the latter 

 group into an external layer of nerve fibres and an internal cellular 

 layer (central gray matter). The basal 

 ganglia (corpora striata) are less marked, 

 except in the Gymnophiona, and merely 

 form a more or less prominent thickening 

 of the wall of each hemisphere projecting 

 into the lateral ventricle. A hippocampal 

 lobe is not distinctly developed, but a 

 hippocampus is represented by elevations 

 of the central gray matter, which are con- 

 nected right and left by a small anterior 

 pallial commissure just above the anterior 

 commissure (Fig. 164, D) 



The Amphibian brain does not, how- 

 ever, lead directly towards that of Reptiles. 

 Although the telencephalon is more highly 

 differentiated than in lower forms, the di- 

 encephalon and mesencephalon are sim- 

 pler than in Fishes ; and, on the whole, 

 the brain of Amphibians is less com- 

 plicated than that of any other Verte- 

 brates, except Lampreys. 



In Urodeles the individual parts are 

 more elongated and separated from one 

 another than in Anurans, and the dien- 

 cephalon is therefore more freely exposed. 

 The hemispheres are almost cylindrical, 

 and the olfactory lobes are distinct from 

 one another, while in the Anura they are 

 fused for a short distance anteriorly 

 (Fig. 164). The diencephalon and optic 

 lobes are much broader in Anurans than 

 in Urodeles. The cerebellum consists 

 simply of a small transverse fold, and is especially rudimentary 

 in Urodeles. 



The infundibulum and hypophysis are well developed : a saccus 

 vasculosus (infundibular gland) is no longer distinct as in Fishes, 

 but is represented by the so-called infundibular process. The 

 epiphysis does not extend beyond the skull in Urodeles, but in 

 Anuran larvae it reaches the integument, undergoing reduction 

 later, when the bony skull-roof is formed; indications of its 

 extracranial portion can, however, be more or less distinctly 



Fio. 163. BRAIN OF Cera- 

 todus fofiteri. Dorsal 

 view. (From Parker 

 and Has well's Zoology.) 



aud, auditory nerve ; cbf, 

 cerebellum ; fac, facial 

 nerve ; gl, glossopharyn- 

 geal ; med, medulla ob- 

 longata ; mes, mesen- 

 cephalon ; oc, oculomotor 

 nerve ; opt, optic nerve ; 

 pros, cerebral hemis- 

 pheres ; rh, olfactory 

 lobes ; ?</, vagus nerve. 



