THE COMMON FROG. [chap. 



vesicle {iter a tertio ad quartum ventricuhnii) is becoming reduced in relative 

 size. 4. The cerebrum is here enlarged, and the inequality in thickness of the wall 

 of the primitive vesicle is increased. The thickened upper part of the wall of 

 the cerebrum is the fornix (/). 5. This figure shows the cerebrum still more 

 enlarged, and with a triradiate cavity (/, i, 2. 3). The fornix has now come to 

 look slightly downwards ; dotted Imes indicate the downward extension of its 

 anterior part, into the corpora mammillaria. 6. Here the cerebrum is still more 

 enlarged and backwardly extended. The fornix is shown bordering the descending 

 cornu and extending into the temporal lobe {tt) of the cerebrum, which lobe is 

 destined to descend (when the brain is fully developed) so much more that it 

 comes to advance forwards. The fornix borders the margin of the very thin outer 

 wall of the descending cornu, which when torn forms the fissure of Bichat. The 

 bending back of the cerebrum has now almost enclosed (between the fornix and 

 the velum) the space {x) which in Fig. 4 is widely open, making what is morpho- 

 logically called the outside of the brain come practically to be in its very centre. 

 a, fore-brain ; h, mid-brain ; c, hind-brain ; cb, cerebellum ; cr, cerebrum ; d, cavity 

 of the medulla ;/, fornix ; I, lateral ventricle ; m, medulla oblongata ; ma, corpora 

 mammillaria ; o, olfactory lobe ; /, pons Varolii ; //, pineal gland ; pt, pituitary 

 body ; q, corpora quadrigemina ; r, crura cerebri ; t, lamina terminalis ; tl, tem- 

 poral lobe of the ctrebruin ; x, space, enclosed by'the extension backwards of the 

 cerebrum ; i, anterior cornu of lateral ventricle ; 2, its middle or descending 

 cornu ; 3, its posterior cornu. 



may, however, be more insignificant than in the frog, 

 as, for example, in the lamprey, where they are 

 actually smaller than the olfactory lobes. In that 

 the cerebral lobes of the frog each contain a cavity 

 (the lateral ventricles) they have a character which 

 is constant in all animals above fishes, they open by a 

 common aperture (foramen of Monro) into the cavity 

 of the next brain segment behind. (Fig. 74, <^.) 



3. This third segment retains a great relative mag- 

 nitude compared with that of man. (Fig. 74, d) 



4. The fourth segment, however, consisting of the 

 optic lobes, attains a still further relative development, 

 though consisting only of two bodies instead of four, 

 but these contain a cavity not found in the corpoi'a 

 quad^^igeiniiia of the human brain. (Fig. 74, g) 



5. The fifth segment, the cerebellum, is very srnall, 

 and smaller than the same part in animals both higher 

 and lower in the scale ; indeed, in the frog class, this 

 organ may be said to be at its minimum. When cut 

 it exhibits no trace of an arbor vitce. (Fig. 74, k) 



