14 GUIDES FOR VERTEBRATE DISSECTION 



of the brain is exposed. In this make out the following 

 parts: 



In front a pair of elongate cerebral hemispheres, telencephalon 

 or prcsencephalon, each prolonged in front as a narrower olfactory 

 nerve.* Between and behind the hemispheres is the choroid 

 plexus, which is followed by the 'twixt-brain (di- or thalamen- 

 cephalon) and midbrain (mesencephalon), which are separated 

 externally by a slight furrow. On the roof of the diencephalon 

 is a flattened vesicle, the pinealis. The cerebellum (meten- 

 cephalon), a slight transverse fold, lies behind the mesencephalon, 

 and bounds anteriorly the fossa rhomboidalis, a triangular opening 

 into the large posterior region of the brain, the medulla oblongata 

 (myelencephalon) . (If very carefully opened it is possible to 

 preserve the roof of the third ventricle in the diencephalon and 

 the large choroid plexus covering the fossa rhomboidalis.) 



Draw the brain from above X6, and then insert the nerves 

 arising from it, as follows: 



I. The olfactory nerve, arising from the anterior end (olfactory 

 lobe) of the telencephalon. II. The optic nerve, passing out from 

 the floor of the diencephalon and appearing at the sides, beneath 

 the hinder angles of the cerebrum. III. The oculomotor nerve, 

 appearing in the angle between mes- and myelencephalon. IV- 

 The trochlearis nerve, not readily found except by use of the 

 microscope. It arises from the roof between mes- and meten- 

 cephalon. V. The trigeminal nerve, arising as a strong trunk 

 from the anterolateral portion of the myelencephalon and passing 

 obliquely outwards and forwards. Follow it "hrough the cranial 

 wall to its enlargement, the Gasserian ganglion. From this 

 ganglion three branches are easily followed (a) an ophthalmicus 

 profundus, passing straight forward; (6) a maxillaris superior, 

 following more closely the margin of the upper jaw; and (c) the 

 mandibularis, going to the lower jaw. VII-VIII. The facialis 

 and auditory nerves arise just behind the trigeminal by several 

 superimposed roots. The dorsal root goes straight to meet the 

 Gasserian ganglion, while the othv3r roots fuse and send laterally a 

 strong facialis proper and, behind, branches to the inner ear. (The 

 sixth nerve is closely associated with the facial so that it is 



* The division of what arc apparently cerebral hemispheres into hemi- 

 spheres proper and olfactory lobes is not very evident, being marked by a 

 very slight constriction. 



