22 GUIDES FOR VERTEBRATE DISSECTION 



In front an elongate telencephalon composed of right and left 

 halves (cerebral hemispheres, separated by the sagittal fissure) . 

 Anteriorly the two hemispheres are separated by a slight con- 

 struction from an olfactory lobe (rhinencephalon). Are the two 

 olfactory lobes distinct from each other? 



Behind and between the diverging posterior ends of the 

 telencephalon and lying at a somewhat lower level is the dien- 

 cephalon or 'twixt-brain, which in turn is bounded behind by 

 a pair of prominent rounded optic lobes, the mesencephalon. 



Immediately behind the mesencephalon and lying at a lower 

 level is a transverse band, the greatly reduced metencephalon 

 or cerebellum. The lateral ends of this rest upon the myelenceph- 

 alon or medulla oblongata, a large region which extends back to 

 the posterior end of the cranium. 



Study these parts more in detail. Make out in the roof of the 

 diencephalon the position of the paraphysis already noted, and 

 immediately behind it a short longitudinal stalk (pedicle of the 

 epiphysis) which reaches back to its origin from about the middle 

 of the 'twixt-brain. Pull the optic lobes slightly toward the 

 posterior end and see, at the hinder end of the diencephalon, a 

 narrow optic tract extending outwards and forwards between the 

 optic lobe and 'twixt-brain. Note the direction of the major 

 axis of the mesencephalic hemispheres. 



If carefully opened the dorsal surface of the myelencephalon 

 will be found to be covered with a loose tissue, the taenia or roof 

 of the fourth ventricle. Removal of this exposes a triangular 

 opening (fossa rhomboidalis) into the cavity (fourth ventricle). 

 The lateral walls of this fossa form the eminentia acustica. Is 

 the fossa rhomboidalis continued backwards as a groove upon the 

 dorsal surface of the spinal cord? 



Draw the brain, X4, from the dorsal surface on the drawing 

 of the spinal cord, leaving space on one side of the sheet to insert 

 the cranial nerves, to be studied next. 



The Cranial Nerves.* The frog, like other Ichthyopsida, 

 has but ten pairs of nerves given off from the brain inside the 



* The study of the cranial nerves is facilitated by allowing the head to 

 remain from two to four weeks in the nitric alcohol, after which the muscles, 

 etc., are readily removed with the forceps. This, however, renders the brain 

 very tender and interferes with its subsequent removal from the head and 

 its dissection. The complete dissection of the nerves takes more time than 



