THE DOGFISH 21 



meninx primitiva, which should not be removed until later. 

 Extend the opening so that all parts can be seen, and continue 

 the opening back into the anterior part of the spinal column. 



In the brain, as thus exposed, make out the following regions. 

 ,In front (1) a paired cerebrum (telencephalon) composed of two 

 indistinctly separated cerebral hemispheres, each prolonged 

 laterally into a large olfactory lobe (rhinencephalon). Behind 

 the telencephalon is (3) the nearly equally large mesencephalon, 

 consisting of a pair of optic lobes. Gently separate cerebrum 

 and optic lobes and see (2) the diencephalon (thalamencephalon) 

 or 'twixt-brain connecting the two but lying at a lower level. The 

 cerebellum or metencephalon (4) is an unpaired oval body 

 overlapping a part of the mesencephalon in front, while behind 

 it overhangs a part of (5) the myelencephalon or medulla oblon- 

 gata, a triangular region of the brain which tapers off into the 

 spinal cord (medulla spinalis) behind. Note the position of a 

 slender nerve (trochlearis, IV) which emerges from the dorsal 

 surface of the brain between mes- and metencephalon. Follow 

 it to its passage through the cranial wall. On the roof of the 

 diencephalon look for the epiphysial structures. These are a 

 slender stalk extending dorsally to the cranial wall, the epiphysis; 

 and in front of this a flattened outgrowth, the paraphysis, which 

 leans against the meninx on the posterior side of the telencephalon. 



Now carefully remove the meninx from the top of the brain. 

 See the entrance of the choroid artery into the cerebrum in the 

 middle line in front. Are there any signs of division longitudinal 

 or transverse in the cerebellum? In front and to the .sides of 

 the metencephalon the lateral portion of each side of the myelen- 

 cephalon is folded into a projecting structure, the corpus restiforme. 

 The removal of the envelopes from the dorsal surface of the my- 

 elencephalon exposes a triangular opening in the roof, the fossa 

 rhomboidalis, leading into the cavity of this region, the fourth 

 ventricle of the brain. In the floor of the ventricle, on either 

 side of the middle line, are a pair of longitudinal ridges, the fasciculi 

 longitudinales mediales. A little lateral to each of these is a 

 second lobulated ridge, the lobus visceralis, while more anterior 

 and in the upper lateral wall, extending forward under the cere- 

 bellum, is a third elevation, the tuberculum acusticum. Note 

 also that there is a dorsal fissure in the spinal cord, a continuation 

 of the fossa rhomboidalis. 



