36 LABORATORY WORK. 



here, with a strong pair of scissors cut away the roof of the 

 skull bit by bit, taking great care not to injure the brain 

 This is easiest done on a decalcified specimen. Then in 

 the same way cut away the neural arches of the vertebrae. 

 This wil 1 expose the brain and spinal cord. The later 

 work will be more easily followed if the animal be put for 

 a day or more in 70% alcohol. 



In the spinal cord notice the spinal nerves given off at 

 regular intervals on either side. How many are there? 

 What relationship do they bear to the bodies of the verte- 

 brae? Examine these spinal nerves more closely, and see 

 if each is double (has dorsal and ventral roots). Follow one 

 out by carefully cutting away the bone, and see where the 

 roots unite. Has either root an enlargement (ganglion)? 

 Look in the dorsal part of the body-cavity for these spinal 

 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 paired 

 optic lobes, and behind them the medulla. What has 

 become of the cerebellum (pp. 20, 29)? 



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 arising from the 

 region of the 'twixt-brain. 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 



