THE FROG 25 



Remove the brain from the cranium and study its lateral and 

 ventral aspects. 



In the side view recognize all the regions seen from above, 

 noting the prominent way in which the metencephalon projects 

 above the myelencephalon. Recognize also the broad fibre 

 tract the optic tract, part of which has been seen in the dorsal 

 view extending downward over the side of the diencephalon. 

 Examine carefully the side of the olfactory lobe and see if you 

 can recognize two roots to the olfactory nerve. Find the roots 

 of nerves V, VI, IX-X already seen and look for the root of III 

 near the ventral surface, immediately below the middle of the 

 optic lobe. Examine carefully the contour of the ventral surface. 



Draw a side view of the brain, X4. 



Also make a similar drawing of the ventral surface. Note 

 the distance to which the inferior roots of the olfactory nerves 

 extend backwards. Follow the optic tracts to the union (in 

 reality a crossing chiasma) on the ventral surface and note the 

 origin of the optic nerves from this chiasma. Immediately 

 behind the chiasma is a median swelling, indistinctly two-lobed, the 

 infundibulum, with the roots of the III nerve projecting between 

 its sides and the main part of the brain. Behind the infundib- 

 ulum is a three-lobed projection, the hypophysis, lying on a bed 

 of cross-fibres, the lemnisci. In the ventral surface behind the 

 hypophysis see the median groove, the ventral fissure, marking 

 off the two sides of the brain and extending back on the cord. 

 The two regions thus differentiated have the special name of 

 pedunculi or crura cerebri from the mesencephalic region forward, 

 beneath the infundibulum and chiasma, to the telencephalon. 



With a sharp scalpel carefully slice off the roof of the brain 

 until the cavities are exposed in its whole extent, from the front 

 of the cerebral hemispheres back to the limits of the fossa rhom- 

 boidalis. Of these make out a pair of cavities (ventricles) lying 

 in the telencephalon (ventricles 1 and 2). Do they extend into 

 the olfactory lobes? Also an unpaired cavity (third ventricle) in 

 the diencephalon, connecting in front with the first and second 

 by transverse passages (foramina of Monro; interventricular 

 foramina) . The wall between the anterior end of the third ventri- 

 cle and the sagittal fissure (p. 22) is the lamina terminalis. In 

 the optic lobes are a pair of mesencephalic ventricles, which, since 

 they do not occur in mammals, are not numbered. The median 



