132 COMPARATIVE ANATOMY. 



canal of the spinal cord. The primary fore-brain and hind-brain 

 c.Mch become differentiated into two parts, and thus five divisions 

 of the brain may be distinguished. Counted from before backwards 

 these are Prosencephalon (secondary fore-brain), Thalamen- 

 cephalon (primary fore-brain), Mesencephalon (mid-brain), 

 Metencephalon (secondary hind-brain), and Myelencephalon 

 (primary hind-brain). The prosencephalon is also spoken of later 



FIG. 105. DIAGRAM OF THE EMBRYONIC CONDITION OF THE CENTRAL NERVOUS 



SYSTEM. 



G, brain, with its three primary vesicles, /, II, III ; li, spinal cord. 



as the cerebrum, the mid-brain as the optic lobes, or corpora 

 bigemina, 1 the metencephalon as the cerebellum, and the 

 myelencephalon as the medulla oblongata. 



The olfactory lobes arise from the secondary fore-brain, which 

 becomes divided into two cerebral hemispheres by a longi- 

 tudinal fold, the basal portion of the vesicle becoming thickened 

 to form a great mass of nerve-centres; this may be distinguished 

 from the remaining peripheral part of the vesicle, or pallium, 

 as the central portion (Fig 106, Cs). 



Throughout the animal kingdom the prosencephalon plays a 

 most important part, for the intellectual condition of the animal 

 depends upon the extent of its development. It consequently 

 attains the greatest perfection in Mammals, and above all, in 

 Man. While in the lower Vertebrates the central portion of the 

 fore-brain is provided with only three small commissures, con- 

 necting its two halves, in Mammals the two hemispheres become 

 fused together along one portion of their inner surfaces, and thus 

 give rise to the great commissures spoken of as the corpus 

 callosum and the fornix. 2 While the outer surface of the 

 hemispheres in all Vertebrates below the Mammalia is more or less 

 smooth, in the latter fissures (sulc-i) and convolutions (gyri) 

 may be present. These consist of folds of the gray cortical sub- 

 stance, which cause a greater or less increase of the superficial 

 area. 



The following structures arise from the thalamencephalon : 

 the optic thalami, formed as thickenings of its lateral walls; 

 the primary optic vesicles, arising as paired basal and lateral 

 outgrowths, from which the optic nerves and retina are derived 

 later (Fig. 106, Tho, Opt] ; the pineal gland or cpiphysis (Z) t 

 developed as a tube-like outgrowth of the roof ; and finally, the 



1 In Mammals, each optic lobe becomes divided into two parts, and indications of 

 a similar division are seen in some Lizards (see Fig. 119. A). 



* Traces of a fornix are seen in certain Reptiles (e.g. Psammosaurns). 



