DISSECTION OF A RAT. 95 



dogfish? Tip the cerebellum forward, and see the large 

 triangular opening in the roof of the medulla. 



On the lower surface of the brain see the cut optic 

 nerves. From which division of the brain do they arise ? 

 Behind the optic nerve find a median lobe, the hypophysis. 



With a sharp scalpel make a series of cross sections through 

 the cerebrum. Are the two halves completely separate ? 

 In each half find a cavity (ventricle), and above it in the 

 solid tissue a transverse lighter band (corpus callosum). 

 Draw the section. Make similar sections through the 'twixt- 

 brain and the optic lobes. How many cavities do you find 

 here ? Draw each section. 



Cut a longitudinal vertical section through the cerebel- 

 lum to left of median line, and notice the way in which the 

 cerebellum is folded. The somewhat bush-like structure is 

 known as the arbor vitae. Make a sketch of it. Cut 

 transversely through the rest of cerebellum and medulla, 

 and in the section see the folds of cerebellum cut in the 

 opposite direction; and, below, the thick floor (pons varolii) 

 of the medulla. Cut through the medulla farther back. 

 Do you find a central canal in the section ? 



On the ventral surface of the neck, just outside the caro- 

 tids, dissect away carefully, keeping the fore legs stretched 

 out, until you find nerves (white cords) going from the ver- 

 tebral region to the sides of neck. Can you make out two 

 roots to each nerve ? Just in front of the ribs notice that 

 the nerves are larger, and that they go to the fore limb just 

 in front of subclavian artery and vein. How many of 

 these nerves as they arise from the neck interlace to form 

 the brachial plexus (the network from which the limb 

 nerves arise). Trace them into the limb. Sketch the 

 plexus. 



Separate the muscles in the bend of the knee, exposing 



