THE FROG 171 



their position from the first, which is called the hypoglossal, 

 in order down to the tenth. What nerves form the brachial 

 plexus? Find the fourth, fifth and sixth. Of what nerves 

 is the sciatic plexus made ? Note the enlargements on the 

 nerves near the spinal column. Nerves coming from the 

 spinal cord are called spinal nerves. Turn the animal on 

 the dorsal side and with the scalpel lay bare the spinal cord 

 and the nerves just observed leading out from it. Do the 

 nerves come out by one root? Do you find a swelling on 

 the nerve just as it leaves the spinal cord? Draw the spinal 

 system of nerves, showing what you saw on both sides. 

 The brain may be exposed by cutting into the soft bone 

 covering it. Remove the bony covering from the cervical 

 vertebra to the front of the skull. Find the tough mem- 

 brane surrounding the brain just under the skull, the dura 

 mater. Remove this and find another softer membrane 

 closely enveloping the skull, the pia mater. Do you find 

 these coverings on the spinal cord also ? 



(2) The lobes of the brain are now exposed. Compare 

 the different parts with those of the fish, in location, name, 

 and size. Beginning with the anterior portion find the 

 olfactory lobes, almost hidden by the central hemispheres. 

 Behind the cerebrum find the pineal body, which in some 

 reptiles supplies a nerve to the pineal eye before noted. 

 What does this fact suggest to you? Find next the optic 

 lobes and trace the nerves to the eyes. What are the 

 nerves called? 



(3) Find next the cerebellum and the medulla oblongata. 

 Note the division of the brain into halves. What is such 

 symmetry called ? Why ? How do the different parts com- 

 pare with those of the fish? Draw the dorsal view of the 



