12 ELEMENTARY PHYSIOLOGY. [l. 



5. In the dog 



a. The dura mater is much thicker and the pia 

 mater more obvious. 



b. The cerebral hemispheres have deep fissures. 



c. The pia mater dips down into the fissures, 

 above the pia mater and bridging over the 

 fissures may be observed the thin transparent 

 arachnoid membrane, also distinctly visible as a 

 covering to the pia mater at the base of the 

 brain. In the space between the arachnoid and 

 pia mater is contained the clear watery sub- 

 arachnoid (or cerebro-spinal) fluid. A smaller 

 quantity of fluid also exists between the 

 arachnoid and dura mater. 



d. Compare the exposed surface with the sketch 

 made of the surface of the brain of the rabbit, 

 noting the relative sizes of the cerebrum and 

 cerebellum in each. 



6. With a scalpel divide the front of the cerebral 

 hemispheres from the olfactory lobes. Lift up 

 with the handle of a scalpel the extreme front of 

 the cerebrum, and turning it backwards bring 

 into view the optic nerves. Cut these through 

 with a sharp pair of scissors close to the skull. 

 Still turning the brain back cut through succes- 

 sively all the other cranial nerves. A little 

 behind the optic nerve is the small but evident 

 third nerve (motor oculi), close behind this the 

 considerably smaller fourth nerve (trochlear), 

 farther back in the hollow behind the attach- 

 ment of the tentorium lies the thick fifth nerve, 

 to the median side of which the small sixth 



