248 THE BIOLOGY OF THE ANIMAL 



beneath it find a cavity, the third ventricle,* in 

 the thalatnencephalon. Notice the thicliened sides 

 of the thalamencephalon which form the optic 

 thalami. Posterior to the thalamencephalon find 

 the two optic lobes. How do they compare in 

 size with the cerebral hemispheres ? With the 

 olfactory lobes? What is their shape ? Behind the 

 optic lobes find a transverse band, the cerebel- 

 lum. What is its shape ? Back of the cerebel- 

 lum comes the medulla oblongata, which grad- 

 ually tapers into the spinal cord. What is the 

 shape of the medulla ? Can you distinguish a di- 

 viding line between the medulla and the cord ? 

 In the medulla find a cavity, the fourth ventri- 

 cle. Is the ventricle continued under the cere- 

 bellum ? Does it extend back into the spinal cord ? 

 Draw the dorsal surface of the brain. 



With a sharp scalpel or scissors cut off horizon- 

 tally the upper surface of the brain, thus exposing 

 the cavities or ventricles. ISTotice that the brain 

 is hollow, with its walls variously bent and folded. 

 Notice also that some of the cavities lie in the 

 medimi or axial line, and that the others are 

 paired lateral outgrowths of the former. Do the 

 walls of the parts of the brain vary in thickness 

 in different regions ? Begin at the fourth ventri- 

 cle and trace forward the system of cavities. 

 What parts of the brain form the walls of this 

 ventricle ? Find the duct, the iter a tertio ad 

 quartum ventriculum or the aqueduct of 

 Sylvius, which leads from the fourth ventricle 

 forward to the third ventricle. What is the rela- 

 tion of this duct to the optic lobes ? Are the latter 

 hollow ? If so, what is the shape of the cavities ? 



