THE RABBIT 307 



of the metencephalon, the fourth subdivision of the vertebrate 

 brain. The dorsal portion of the third subdivision, the mesen- 

 cephalon, or midbrain, is concealed beneath the hinder end of the 

 hemispheres and may be brought into view by bending them for- 

 ward. It consists of four slight elevations called the corpora 

 quadrigemina. 



Back of the cerebellum and covered by it is the medulla ob- 

 longata, which is the dorsal portion of the myelencephalon, the 

 fifth subdivision of the vertebrate brain; gently lift up the 

 cerebellum and note the dark-colored posterior choroid plexus, 

 which forms the roof of the fourth brain ventricle, that in the 

 medulla. The spinal cord is the continuation of the medulla 

 oblongata. 



Exercise 30. Make a semidiagrammatic drawing of the dorsal aspect 

 of the brain. 



The Lateral Aspect of the Brain. Chip away the side of the 

 skull and expose the entire lateral surface of the brain and the 

 roots of the cranial nerves. Of these nerves there are twelve pairs. 

 The first (anterior) pair, the olfactory nerves, pass, in the form 

 of many small fibers, from the olfactory lobes to the nasal cap- 

 sules. Note the connection between the olfactory lobe and the 

 pyriform lobe, which is the posteroventral portion of the hemi- 

 sphere. Tilt the brain to one side and observe the large optic 

 nerves, the second pair of cranial nerves ; also the optic chiasma, 

 from which they spring. 



Back of the hemispheres is the broad pons with its transverse 

 fibers ; it is the ventral portion of the metencephalon, being the 

 bridge which joins the two sides of the cerebellum. Just in front 

 of it and springing from near the midventral line may be seen, by 

 tilting the brain to one side, the two small oculomotor nerves, the 

 third pair of cranial nerves, which innervate the following muscles 

 of the eyeball : the superior rectus, internal rectus, retractor, in- 

 ferior rectus, and inferior oblique. From the lateral side of the 

 brain, between the cerebellum and the hemisphere on each side, 

 springs the small trochlear nerve, the fourth cranial nerve; it 

 innervates the superior oblique muscle of the eyeball. 



