AMPHIBIA. 211 



very clearly marked off. They are closely united in the middle 

 line, and each contains an olfactory ventricle continuous with the 

 lateral ventricle of its side. 



The mid-brain is a small axial part lying immediately behind 

 the thalamencephalon, with which it is directly continuous. It 

 contains a narrow canal, the Sylvian aqueduct, opening in front 

 into the third ventricle, and with roof and side-walls swollen into 

 two large ovoid optic lobes, each of which contains an optic ventricle 

 opening into the aqueduct, while its thickened floor is formed by 

 the crura cerebri, two masses of longitudinal fibres. 



The hind-brain is mainly constituted by the bulb (medulla 

 oblongata) which is continuous in front with the mid-brain, and 

 behind with the spinal cord, which it much resembles in structure. 

 The bulb contains a relatively large fourth ventricle, connected by 

 the aqueduct with the third ventricle. Its roof is shaped like a 

 triangle with forwardly-directed base, and is constituted by a 

 vascular membrane. On the under side of the thickened floor 

 is a median groove, the ventral fissure. The cerebellum is a small, 

 solid, transverse ridge, just behind the optic lobes, which arises 

 as a dorsal outgrowth from the posterior vesicle, and together 

 with the bulb makes up the hind-brain. 



The spinal cord is a thick-walled tube, somewhat flattened 

 from above downwards, which is contained in the spinal canal. 

 It merges into the bulb in front, and from this point gradually 

 tapers backwards (dilating, however, where the limb-nerves come 

 off), and ends in a filament, the filum terminate, which occupies 

 the canal of the urostyle. The small central canal of the spinal 

 cord opens into the fourth ventricle. Dorsally, the cord exhibits 

 a deep cleft, the dorsal fissure, and there is a similar ventral fissure 

 which passes on to the base of the hind-brain. 



(2) There are twenty pairs of cranio-spinal nerves ten cranial 

 belonging to the brain, and ten spinal belonging to the spinal 

 cord. 



Cranial Nerves. The two first pairs belong to the fore-brain. 

 I. The olfactory arise from the outer and front sides of the 

 olfactory lobes, pass through the foramina in the transverse 

 septum of the sphenethmoid, and supply the olfactory epithelium 

 of the nasal sacs. II. The optic nerves. On the ventral side of 

 the thalamencephalon there is an X-shaped structure, the optic 

 chiasma. The posterior thickened limbs of this, the optic tracts, 



