44 ELEMENTS OF COMPARATIVE ZOOLOGY. 



nerves. Trace the posterior ones back to their union 

 (plexus) to form the sciatic nerve going to the hind limb. 



In the brain, between the eyes, are the cerebral hemi- 

 spheres. Are they separate ? In front are the olfactory 

 lobes. Are they separate ? Behind the cerebrum, and at a 

 lower level, is the 'twixt-brain. Next come the optic lobes, 

 and behind them the medulla. What has become of the 

 cerebellum (p. 15) ? 



Sketch the brain and spinal cord from above, inserting 

 all the nerves seen, and making the sketch twice the size of 

 nature. 



Cut across the olfactory nerves and turn the brain back- 

 wards. This will show the optic nerves. Cut these as far 

 as possible from the brain, and do the same with other 

 nerves farther back, at last removing the brain from the 

 skull. 



On its under surface trace the optic nerves back to the 

 brain. Does the right nerve connect with the right optic 

 lobe ? Behind the optic nerves is a small projection, the 

 pituitary body. How many nerves can you find arising 

 from the side of the medulla ? 



"With a sharp scalpel split the brain horizontally and ex- 

 amine the cavities found. Are they all connected ? The 

 larger cavities are called ventricles. Those in the hemi- 

 spheres are the first and second, that in the 'twixt-brain the 

 third, and that in the medulla the fourth. Are there ven- 

 tricles in the optic lobes ? Draw the brain, showing all cavi- 

 ties and connections found. 



From another frog make a skeleton by removing as much 

 of the flesh as possible with scissors and scalpel, then boil it 

 with a little soap in the water, and pick away as much 

 more as you can, taking care not to separate the joints.* 



* Much better skeletons can be made by cleaning off the flesh and 



