152 



HIGH SCHOOL ZOOLOGY. 



have already seen in the cerebelhim of the bird a similar ]ilan 

 for accommodating a large surface of grey matter in a compara- 

 tively small space, and the cat's cerebellum also exhibits the 

 characteristic " arbor vitsQ " arrangement. The cerebral hemi- 

 spheres are not inde- 

 pendent of each other, 

 ^,y^_^v» ;^^^^^__, — .^'W^^'^iJ-';^ for, apart from certain 



-Cir transverse bundles of 

 fibres present in the 

 lower classes, there is 

 also formed between 

 ,. J. , .^ J- , ^- c^ ■ t , those parts of the hemi- 



Fijr. 103.— Median lon^tudinal section of brain of cat i . 



(Modified after Wilder.) Sphei^CS wlucll project 



.9 Spinal cord with contained central canal mo me- j^^ck OVer the true rOof 

 diiUa oblon^'ata ; p, pons varolii ; cb, cerebellum, form- 

 in ir the floor and roof of the 4tii ventricle //i^'.^ optic ^f the brain (Fig, 103) 

 lobes or ooiTJoraquadrigemina, the roof of the mesocoDle \ o / 



or " iter," the floor is formed by the crura in front of the j^^-^ jniportant transverse 

 jions. In front of the mirt-brain is the thalamic region, ' ^ 



the cavity of which (3rd \entricle) is traversed by acorn- commissure the COrpUS 

 iiiissure, c', uniting the side walls or thalanii ; the ' . 



roof is partly formed by the epiiihysis (above men), and callOSUni, whicll Serve.S 

 the floor by the hypophysis, h ; but the chiasma. ch. of 



the optic nerves, 11, also forms part of the floor, while to effect COmmunica- 

 the anterior thin wall (terminal lamina) is above the 



chi.osma. The cerebral hemispheres, cr, are riijht and tions between tho twO 

 left of the lamina, and connected by cc, the conjusoallo- . ^• ^ 



simi ; ol, the olfactory lobe * Sides. In some higher 



mammals the cerebral hemispheres project so far forward as 



to conceal the olfactory lobes from above, but in the cat these 



are well marked off from the rest of the brain. 



9. As far as the sense-organs are concerned, we shall find that 

 the nose and ear ai-e those which exhibit conspicuous ad van 

 tages in structure over the lower classes. The olfactory cavity 

 in the fowl is not entirely smooth, but in the cat the olfactory 

 mucous membrane covers an extremely complicated surface, 

 fuinished chiefly by the ethmoid bone, but also supported by 

 independent turbinal bones. The olf^xctory lobes rest upon 

 the cranial surface of the mesethmoid, sepai*ated by a median 

 crest in that bone, and sending the olfactoiy nerve-fibres down- 



