IV. VERTEBRATA 



469 



condition with five vesicles followed upon this with three, the mid brain 



remaining undivided, while the hind Ijrain divides into ccrcbeUinn {melen- 



ccphalon, cb) and medulla oblongata {niyelencephalon, m) ; the fore brain 



into cerebrum and 'hvixt brain. This is not exact so far as the hind 



brain is concerned, for cerebellum and medulla are roof and floor of the 



same cavity (fig. 526). The fore brain becomes divided, by lateral 



outgrowths on either side in front, into three 



parts, an unpaired portion, the 'twixt brain, 



and paired anterior parts, the cerebrum. 



Introducing the terms of human anatomy 



for the separate parts of the brain, the first 



vesicle consists of the two cerebral hemispheres 



{telencephalon) whose dorsal and lateral walls 

 ^j 



rn 



v-sr 



Fig. 525. Fig. 526. 



Fig. 525. — Diagram of a vertebrate brain (from Wiedersiieim). Aq, aqueduct; 

 Cc, central canal; Fil, foramen of ifonro (connection of lateral ventricles with each 

 other and with the third) ; -ffi?, cerebellum; IIH, corpora bigemina (optic lobes) ; NH, 

 medulla oblongata; R, spinal cord; SV , lateral ventricles; VH.., cerebrum; ZH, optic 

 thalami ('twixt brain); ///, IV, third and fourth ventricles. 



P'lG. 526. — Scheme of brain in sagittal section, c, cerebrum; ch, cerebellum; cc^ 

 canal of spinal cord; di, notochord; cs, corpus striatum; /^, hypophysis; i, infundibulum; 

 m, medullary region; o, optic chiasma; 0/, olfactory lobe; ol, optic lobes; p, parietal eye. 



are called the pallium, while in the floor of each hemisphere is an enlarge- 

 ment, the corpus striatum (cs). The spaces in the hemispheres are the 

 first and second ventricles (5F). From the front of each hemisphere arises 

 a distinct region, the olfactory lobe or rhinencephalon {of), which gives 

 origin to the olfactory nerve. Sincfe the organ of smell is frequently at 

 some distance from the brain, the olfactory nerve must elongate, as in 

 Amphibia (fig. 568), or the olfactory lobe must lengthen, as in many 

 Elasmobranchs (fig. 549). In the latter case the swollen end of the 

 lobe, called the hulbus olfactorius, is close to the olfactory epithelium and 

 is connected with the brain by a long stalk, the tractus. Both, as parts 

 of the brain, must be distinguished from the olfactory nerve. 



Only the lateral walls of the second vesicle become thickened, produc- 

 ing the optic thalami or diencephalon, directly adjoining the corpora 



