30 NATURAL HISTORY OF VERTEBRATES. 



mirable introduction to the study of this complicated organ. In a horizontal section 

 lilve that shown in Fig. 33 B, the various coelise are seen communicating with each 

 other, and with the central canal of the cord ; only the prosocoele (1) appears in this 

 plane, shut off and divided into two separate cavities, the walls of which are the 

 ' cerebral hemispheres.' It will be observed, however, from Figs. 33 A, and 35, 2, that 

 the jsrosocoele really does communicate with the thalamocoele behind, and that the 

 thin double septum which dips down into it is only partial. It is further instructive 

 to notice that only the lateral walls and the floor of the cavity are thick ; both roof 

 and septum ai-e very thin, and between the folds of the latter a thin sheet dips down 

 from the vascular membrane {pia inciter) which everywhere closely surrounds the 

 brain, and transforms the septum into a nutritive organ for this part of the brain, 

 known as a choroid ple.vus. Such thin places in the walls of the cceliw are of great in- 

 terest and importance, because, although thin in one animal, they may t)e extremely 

 thick in another. Thus the roof of the prosocoele in man forms the greater part of 

 the bulk of his brain. From Fig. 33 A it will be noticed that the ))roseiicej)halon gives 

 off anteriorly two lobes, the olfactory lobes or rhinencejihala, which are almost solid 

 (Fig. 35), e.vc.ept for two cavities, the rhinocoeles, which open into the prosocoele 

 behind. Occasionally the rhinencephala are separated from the prosencephalon by long 

 olfactory tracts, but generally they lie quite close to the bi'ain. 



From a study of these horizontal sections one would conclude that the walls of the 

 thalamoccele are almost entirely thin, but that such is not the case an examination of 

 Fig. 35, 4, and Fig. 34, will demonstrate. The former figure passes through the origin 

 of the optic nerve from the thick walls of the thalamocoele (the optic thalami), and 

 also through two thick cushions in the roof (Jiabenular ganf/liii) which are situated on 

 either side of a tubular process which here projects upwards from the cavitj'. The 

 latter, the ' epiphysis,' or pineal body, can be best seen in the mesal sagittal section 

 (Fig. 34), which is further well calculated to show that the planes separating the 

 regions of the brain need by no means be frontal. Those between the thalamenceph- 

 alon here and the regions in front and behind it are directed from above and in front, 

 downwards and backwards, so that sections 3 and 5 (Fig. 35), cut through parts of the 

 thalamencephalon as well as the pros- and mes-encej)halon respectively. This over- 

 lapping is perha|)s more marked ventrally, where the thalamoccele runs back into a 

 long ' inf undibulura,' connected with the ventral wall of which is the ' hypophysis ' 

 or pituitary body. Both epiphysis and hypophysis appear to be functionless organs 

 in adult vertebrates ; their morphological significance we shall return to afterwards. 



It was noted above that the roof of the mesocoele (3) becomes considerably devel- 

 oped owing to the cephalic flexure. This development jiersists even after the straight- 

 ening out of the flexure, so that very little of the mesencei)halon shows upon the base 

 of the brain. The roof, however, may overlaj) the region in front and behind (Fig. 

 35, 6), giving rise to the prominences known as optic lobes (corpora bi- or quadri-gemina) 

 throughout the vertebrate series. 



Behind the mesencephalon the floor of the brain is remarkably uniform, and gives 

 origin to most of the cranial nerves, but the roof is thickened in front into the ' cere- 

 bellum ' (Fig. 35, 7), while it is thin further back (Fig. 35, 8). The difference in the roof 

 renders it desirable to distinguish between an anterior epencephalon and a ]iosterior 

 metencephalon ; the cavities of which epi- and meta-coeles, 4 and 5, freely communicate 

 with each other. In mammals the cerebellum develops important lateral lobes which are 

 connected with each other by a bridge-like commissure of nerve-fibres (the pons Varo- 



