i 6 VERTEBRATE ZOOLOGY 



the roof of the skull and expose the dorsal surface of the brain ; 

 do not disturb, however, any part of the orbits of the eyes or the 

 nerves in them. 



The brain, as it appears in a dorsal view, is made up of five 

 divisions,— the cerebrum, diencephalon, optic lobes, cerebellum, 

 and medulla oblongata, of which the cerebrum and cerebellum 

 are much more prominent than the other three divisions. The 

 cerebrum is the anterior division and is composed of a pair of 

 rather indistinctly marked hemispheres, from each of which a large 

 olfactory lobe projects forward to the nasal capsule. 



The second division, the diencephalon, is a small area just back 

 of the cerebrum. Its dorsal wall is thin and supports the pineal 

 body, a very slender projection which extends forward and dorsally 

 to the skull ; it may have been removed with it. The third division, 

 the optic lobes or midbrain, consists of a pair of rounded bodies 

 back of the diencephalon. The fourth division is the cerebellum, 

 a prominent, elongated body which extends forward over the pos- 

 terior portion of the optic lobes and backward over the fifth divi- 

 sion; it is divided into several lobes. The fifth division is the 

 medulla oblongata ; it forms the hinder portion of the brain and 

 is continuous with the spinal cord ; in its delicate dorsal wall is a 

 large depression called the fossa rhomboidalis, at the sides of which 

 is a pair of lateral projections, the restiform bodies. 



Exercise 17. Draw the dorsal aspect of the brain on a scale of 2 or 3 

 and carefully label its parts. 



The Cranial Nerves. Ten pairs of cranial nerves are present, 

 which, with their most important branches, are the following : 



1. The olfactory nerves form a large group of minute fibers which 

 extend from the anterior end of each olfactory lobe into the nasal 

 capsules. Look for them on the right side of the head ; they will 

 be seen by pressing the anterior end of the olfactory lobe back 

 from the wall of the nasal capsule. 



2. The optic nerves arise on the ventral surface of the dien- 

 cephalon, where they will be seen when the ventral surface of the 

 brain is studied ; each passes through a foramen in the orbit to 

 the eye. Press the skull from the brain and find them. 



