134 COMPARATIVE ANATOMY. 



A series of unpaired ventricles, lying in the longitudinal axis 

 of the brain, as well as paired ventricles can always be distinguished. 

 The principal paired cavities lie within the cerebral hemispheres, 

 and are known as the lateral ventricles (Ventviculus 1 and 2) 

 (Fig. 107, SV) ; each of these communicates with the unpaired sys- 

 tem by means of an opening, the foramen of Monro (Fig. 107, 

 FM}. In Teleostei, certain Amphibia, and Sauropsida, each optic 

 lobe also contains an optic ventricle, communicating with the 

 unpaired system of ventricles. The latter consists of a third 

 (within the thalamencephalon) and a fourth ventricle (in the 

 medulla oblongata), as well as of the aqueduct of Sylvius, 



FIG. 107. DIAGRAM OF THE VENTRICLES OF THE VERTEBRATE BRAIN. 



Vff, cerebral hemispheres, containing the lateral ventricles (SF) ; ZH, thalamen- 

 cephalon, with the third ventricle (HI) ; in Mammals the paired septum lucidum, 

 lying anteriorly to the thalamencephalon, encloses the "fifth ventricle " ; each 

 lateral ventricle comrrmnicates with the third ventricle by a small aperture, the 

 foramen of Monro (FM) ; MH, mid-brain, which encloses the aqueduct of Sylvius 

 (Aq), communicating between the third and fourth ventricles ; HH, cerebellum ; 

 NH, medulla oblongata, enclosing the fourth ventricle (IV) ; Cc, central canal 

 of the spinal cord (R). 



which passes through the mid-brain and connects these two. For 

 further details, such as the relations of the different ventricles to 

 particular parts of the brain, compare Figs. 106 and 107. A so- 

 called fifth ventricle, lying between corpus callosum and fornix, 

 is found in Mammals, but morphologically it is quite different 

 from the others. 



All five cerebral vesicles lie at first, in the same horizontal 

 plane, but in the course of development the axis of the vesicles be- 

 comes bent downwards, so that at a certain stage the mesencephalon 

 forms the apparent apex of the brain (Fig. 108, SB}. In Mammals, 

 the parts of the brain become still further folded on one another, so 



