204 



COMPARATIVE ANATOMY 



inftmdibulum) by means of an opening, the foramen of Monro, and 

 may be continued into the corresponding olfactory lobe as an 

 olfactory ventricle or rhinoccele. Each optic lobe also usually con- 

 tains an optic ventricle, or optoccele, communicating with the 

 mesocoele (iter or aqueduct of Sylvius). There 

 may be a distinct metacoele in the cere- 

 bellum opening into the myelocoele or fourth 

 ventricle.^ 



All five cerebral vesicles lie at first in the 

 same horizontal plane, but in the course of 

 development a cerebral flexure takes place, 

 the axis of the vesicles becoming bent down- 

 wards, so that at a certain stage the mesen- 

 cephalon forms the apparent apex of the 

 brain. In Mammals, the parts of the brain 

 become still further folded on one another, 

 so that a parietal, a Varolian, and a cervical 

 bend may be distinguished (Fig. 153) : this 

 process is connected with the further develop- 

 ment of the skull and the rapid longitudinal 

 growth of the brain. 

 FIG. 152. DIAGRAM OF In Fishes and Amphibians the cerebral 

 THE VENTRICLES OF THE fl exure l a t er becomes practically obliterated, 

 VERTEBRATE BRAIN. , , ., , r -, , ,, ,, 



but it persists more or less markedly in the 



Cc, central canal of the higher types, more particularly in Mammals. 



5*, ( J&.3 In , the latter Class, moreover, the original 



brain, which encloses relation of the parts becomes still further 



the iter (.4g), communi- complicated by the large development of the 



VSFjftSk v cerebral hemispheres, which grow backwards, 



tricles; NH, medulla and thus gradually overlie all the other 



oblongata, with the parts of the brain : this condition of things 



rI th e V e e rel le ffi! attains its S reatest Potion in Man. 



spheres, with the lateral 



ventricles (8V); ZH, Amphioxus. The conical and enlarged 



aSTwMkTtmh anterior end f the s P inal cord of the Lancelet 



each lateral ventricle contains a widened portion of the central 

 communicates with the canal which must be looked upon as a 

 **^\ This P ens f^ely to the exterior 

 men of Monro (FM). dorsally by a neuropore, which represents 

 the last indication of the primitive connec- 

 tion of the central nervous system with the outer skin. It is 

 impossible to say with any degree of certainty to what extent 

 this " brain " of Amphioxus corresponds to parts of the Craniate 

 brain. 



Cyclostomes. The brain of these forms remains in many 



1 A so-called " fifth '' ventricle, lying between the corpus callosum and fornix, 

 is found in Mammals, but morphologically it has nothing to do with the ventricles 

 jroper, and simply represents a space between the thin internal walls (np.j>ftint 

 ucidum) of the two hemispheres. 



i: 



