LUNG-FISH 



387 



of smell, the hemispheres (A) are of great size — ^relatively larger than in 

 any other group of vertebrates except the higher mammals such as 

 Man (Fig. 168, c.H). Not only are the hemispheres of large size but 

 they show an advance in complexity which is of much interest from its 

 foreshadowing conditions characteristic of the higher vertebrates. One 

 of the chief of these characteristics is that the ganglion-cells in the wall 

 of the hemisphere become arranged in definite layers forming what is 

 known as a cortex. Now in Lepidosiren the examination of transverse 

 sections through the wall of the hemispheres shows that there already 

 exists a layer of ganglion-cells foreshadowing the cortex of the higher 

 vertebrate (Elliot Smith). 



A feature of this archaic tjpt of brain of importance to the student 



tx>. 



Fig. 168. 

 Brain of Lepidosiren as seen in longitudinal vertical sections. a.c, Anterior commissure ; 

 c.H, liemisphere ; cer, cerebellum ; ch, optic chiasma ; h.c, habenular commissure ; h.g, habenular 

 ganglion ; i.g, infundibular gland ; l.p, lateral plexus ; p.c, posterior commissure ; par, paraphysis ; 

 pin, pineal body ; t.o, roof of mesencephalon. 



is that it shows with peculiar distinctness the main brain - regions 

 which in the higher vertebrates tend to become difficult to recognize 

 but are here spread out diagrammatically in a longitudinal series one 

 behind the other as is shown in Fig. 167. Study of a sagittal section 

 through such a brain (Fig. 168) is peculiarly instructive, showing as 

 it does the main constituent regions of the brain in their primitive 

 relations to one another and at the same time displaying in clear detail 

 features that form important landmarks in brain topography. 



In such a section we see at its hinder end the medulla oblongata 

 with its thick floor and membranous roof, the latter becoming thickened 

 anteriorly to form the cerebellum (cer). In front of this in turn the 

 brain-roof bulges upwards to form the roof of the mesencephalon (t.o) 

 its anterior limit indicated by an important landmark — the posterior 

 commissure (p.c). In front of this lies the roof of the thalamencephalon, 



