STUDY OF BRAIN OF SHEEP. 91 



If the work has been carefully done, when all the nerves are cut, 

 the uninjured brain comes out of the skull completely enclosed in 

 its membrane of dura mater. This connective tissue-sac should 

 then be cut along the mid-line of the dorsal surface, removed from 

 the brain, and placed in 4.% formalin solution. The brains, as fast 

 as they are removed, should be put in a shallow dish, the bottom 

 of which is covered with a soft layer of cotton, and over them should 

 be poured a 4 solution of formalin. When the brains are ' once 

 hardened in this way they can be kept in alcohol and used year after 

 year. 



To show the internal structure of the brain a horizontal cut should 

 be made about half an inch from the lower surface of the brain 

 through one of the cerebral hemispheres and through half of the 

 cerebeflum. By making a vertical cut also, along the ventral sur- 

 face, at one side of the median line the lower third of one-half of the 

 brain is removed, and the disposition of the gray and white matter 

 within the brain is shown. 



Each two pupils should be supplied with a hardened brain pre- 

 pared as described, a piece of, or, if possible, a whole dura mater, a 

 rectangular piece of bone from the top of the skull with skin and 

 wool attached, a dissecting needle. 



A. Protection of brain. 



1 . What kind of outer covering has the sheep ? 



How does it protect the brain? 



2. Examine the piece of bone which was re- 



moved from the top of the skull. What 

 layers can you distinguish? Why is this 

 arrangement of bone-tissue of great ad- 

 vantage as a means of protection? 



3. What are the characteristics of the dura 



mater (the sac covering the brain) ? Try 

 to tear it. 



4. By means of a dissecting needle lift from 



the surface of a portion of the brain a 

 thin covering called the pia mater. De- 

 scribe it. Does it cover all parts of the 



