THE PERCH 45 



The Brain. If the brain has been kept in strong formalin, as 

 directed, it will have been hardened and will be in good condi- 

 tion for dissection ; if, however, its condition is for any reason no 

 longer suitable, a fresh animal must be taken. 



Remove all the skin and the thick muscles from the head and 

 the high-arched dorsal portion of the trunk just back of it. With 

 a strong scalpel or scissors cut away the roof of the skull and the 

 dorsal wall of the neural canal of the spinal column. The brain 

 does not nearly fill the cavity of the skull but is surrounded by the 

 granular, fatty tissue already mentioned; carefully remove this 

 substance and expose the brain. 



Study its dorsal surface. It is made up of five divisions. The 

 first and anterior division is the cerebrum ; it consists of a pair of 

 hemispheres, at the anterior ends of which project the olfactory 

 lobes. The third division, or midbrain, consists of the paired optic 

 lobes, the largest part of the brain, between which and the cere- 

 brum appears a small, median, sunken area,— the diencephalon, 

 the second division. Projecting dorsally from this division is the 

 long and slender pineal body, or epiphysis, which is the rudiment 

 of a third optic nerve ; it may have been removed in exposing the 

 brain. Back of the optic lobes are the fourth division, the cerebel- 

 lum, and the fifth division, the medulla oblongata, which is con- 

 tinuous with the spinal cord. Note the longitudinal median groove 

 in the medulla and spinal cord, and the paired lateral swellings, 

 the restiform bodies, at the anterior end of the former. 



Exercise 13. Draw a dorsal view of the brain on a scale of 3 or 4. 



Carefully cut away one side of the skull and expose the side 

 of the brain. Find the ten cranial nerves. The first nerve is the 

 long olfactory, which passes forward from the olfactory lobe to 

 the nasal capsule. The second is the. optic nerve, which arises just 

 in front of the optic lobe from the ventral surface of the dien- 

 cephalon; the two optic nerves cross each other immediately, 

 forming the optic chiasma, and pass each to the eye on the oppo- 

 site side of the body. By carefully pressing the floor of the brain 

 case away from the brain the root of the optic nerve will be seen ; 

 the nerve may then be traced to the optic foramen where it enters 



