COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 333 



hypophysis was torn from it. At the sides of the infundibulum are the roots of 

 the oculomotor nerves. Posterior to the diencephalon is the depressed medulla. 

 Between the medulla and the optic lobe is the slender root of the trochlear nerve. 

 On the ventral surface of the medulla are the roots of the abducens nerves; they 

 should also be sought in the floor of the cranial cavity. On the sides of 

 the medulla look for the roots of the fifth, seventh, eighth, ninth, and tenth 

 nerves, situated in a row. The twelfth nerve arises from the ventral surface of 

 the medulla about on the same level as the ninth and tenth roots. The eleventh 

 nerve arises from the spinal cord by several roots and ascends to a position 

 immediately behind the tenth root. 



Draw the ventral view of the brain. 



6. Sagittal section. — Make a median sagittal section of the brain and study 

 the cut surface. In the medulla note the fourth ventricle overhung by the cere- 

 bellum. Note the thick ventral wall of the medulla and the pontal flexure caus- 

 ing a ventrally directed bend in the medulla. In the cerebellum observe the 

 small cerebellar ventricle and the arrangement of the gray and white matter 

 resulting in section in a treelike appearance, called the arbor vitae. Each fold 

 of the cerebellum consists of a central plate of white matter surrounded by a 

 thick covering of gray matter. Anterior to the cerebellum is a region consisting 

 dorsally of the mesencephalon and ventrally of the diencephalon. The optic 

 lobes do not appear in the section, but the median part of the midbrain forms 

 the dorsal part of the section. A narrow cavity, the third ventricle, is present in 

 the diencephalon and extends into the infundibulum. In front of the latter 

 appears the optic chiasma. Note how the cerebral hemisphere arches back over 

 the diencephalon and midbrain, and note the strong connection of the diencepha- 

 lon with the hemisphere. The cavity of the cerebral hemisphere is not visible 

 in the median section. The medial wall of the hemisphere is called the septum, 

 its dorsal wall the pallium. Cut into the latter, noting its thinness, and find 

 inside the cavity or lateral ventricle of the hemisphere and the great mass, the 

 corpus striatum, bulging from the floor. The function of the corpus striatum is 

 not definitely known, but it seems to have a steadying effect on voluntary move- 

 ments, and the delicacy and precision of movement necessary in flight may 

 account for the relatively enormous size of the corpus striatum in birds. 



F. THE NERVOUS SYSTEM AND SENSE ORGANS OF THE MAMMAL 



For the complete dissection of the nervous system a new specimen is neces- 

 sary, but the'greater part of this system can be worked out on the same specimen 

 as used for preceding systems. If a new animal is provided, open it by a longi- 

 tudinal cut from the perineum through the anterior end of the sternum. If the 

 old specimen is used it will not be possible to see the branches of the sympathetic 

 system and the vagus to the viscera or the peripheral distribution of some of 

 the cranial nerves. In working on the nerves all structures other than nerves 



