THE CAT 249 



Sylvian fissure. Between the Sylvian and the ectosylvian gyri is 

 the ectosylvian sulcus, which is not complete but is interrupted 

 dorsally. On the ventrolateral surface, beneath the Sylvian fissure, 

 is the pyriform lobe, which is continuous with the olfactory lobe. 

 Just back of and above the olfactory lobe, at the anterior end of 

 the hemisphere, is the orbital gyrus, and behind that the sigmoid 

 gyrus, with the presylvian and cruciate sulci between them, the 

 former being ventral to the latter. Along the hinder margin of 

 the hemisphere is the posterior gyrus. 



Behind the cerebrum in the dorsal aspect is the cerebellum. Its 

 surface is also marked by the presence of gyri and sulci, which 

 have, however, a different appearance from those of the cerebrum. 

 Three general regions are distinguishable, the median vermis and 

 the lateral hemispheres. The cerebellum forms the dorsal portion 

 of the metencephalon, the fourth subdivision of which the verte- 

 brate brain is composed. The dorsal portion of the third sub- 

 division—the mesencephalon, or midbrain— is concealed beneath 

 the hinder end of the hemispheres, and may be brought into view 

 by bending them forward. 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 cere- 

 bellum 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 continuous with the medulla oblongata. 



Exercise 38. Make a semidiagrammatic drawing of the dorsal aspect 

 of the brain. 



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

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

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

 pairs. The first pair, the olfactory nerves, pass, in the form 

 of many small fibers, from the olfactory lobes to the nose. Tilt 

 the brain to one side and observe the large optic nerves, the sec- 

 ond pair of cranial nerves; also the optic chiasma, from which 

 they spring. 



