302 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



Each contains a small olfactory ventricle continuous with the 

 -corresponding anterior cornu. 



The mid-brain is traversed by the Sylvian aqueduct. Above it 

 presents the optic lobes (corpora quadrigemina) which are over- 

 lapped by the cerebral hemispheres. Each of them is subdivided 

 into two, so that there are four elevations, two anterior and larger 

 nates, and two posterior testes. There are no optic ventricles. 

 Two very distinct longitudinal masses of fibres, the crura cerebri, 

 form the floor of the mid-brain. 



Hind-brain. The bulb (medulla oblongata) presents well- 

 marked dorsal and ventral fissures bounded by narrow bands, the 

 dorsal and ventral pyramids. The dorsal pyramids diverge in 

 front and sweep round the sides of the thin roof of the fourth 

 ventricle, on each side of which is a projection, the corpus resti- 

 forme. On the under surface the ventral fissure and pyramids 

 .are interrupted by a broad band of transverse fibres, the pons 

 Varolii, just behind the crura cerebri. Immediately behind this 

 there is a rectangular area, the corpus trapezoideum, on each side 

 of the ventral pyramid, occupied by transverse fibres. 



The cerebellum is very large and made up of a median vermis 

 marked by deep transverse furrows, and a pair of much convoluted 

 lateral lobes, to each of which a small rounded flocculus similarly 

 furrowed, is attached. The cerebellum is united to the rest of 

 the brain by three pairs of commissures or peduncles. The 

 anterior peduncles connect it with the testes, the middle pedun- 

 cles with the pons Varolii, and the posterior peduncles with the 

 corpora restiformia. 



The spinal cord is cylindrical, and possesses dorsal and ventral 

 fissures. There is no sinus rhomboidalis. 



(2) Cranio- Spinal Nerves. There are twelve pairs of cranial 

 nerves, as in the pigeon, which take origin and are distributed 

 in a similar manner. The olfactory lobes send numerous fibres 

 through the pores in the cribriform plate to the olfactory mucous 

 membrane. The optic nerves form a chiasma, but the optic 

 tracts are relatively narrow. The trigeminal nerves arise from 

 the sides of the pons, and the facial and auditory from the sides 

 of the corpora trapezoidea. The abducent nerves come off from 

 the extreme front of the ventral pyramids. The exits of the 

 cranial nerves have already been described (p. 274). Outside 

 the skull it may be noted that the mandibular ramus of the fifth 



