COMPARATIVE ANATOMY 79 



Ventral Surface of the Brain. — Extending posteriorly 

 from the olfactory bulbs to the middle of the cerebral hemi- 

 spheres are white olfactory tracts. These end in the pyriform 

 lobes of the cerebral hemispheres. 



In the mid-ventral line is the optic chiasma where the optic 

 nerves cross. The optic tracts extend back from the chiasma. 



The region behind the optic chiasma is the diencephalon. 

 The median rounded elevation is the tuber cinereum, to which 

 is attached the pituitary body. If a small aperature shows 

 in the tuber cinereum, the pituitary is not present; it was 

 probably torn off in removing the brain. Two small mammillary 

 bodies, not distinctly separated from the tuber cinereum, lie 

 behind the pituitary. Just posterior to these, near the median 

 line, are the roots of the third or oculomotor nerve. Lateral 

 to the third nerve roots and the mammillary bodies, and partly 

 covered by the pyriform lobes, are the cerebral peduncles. These 

 are bundles of fibers connecting the cerebral hemispheres 

 with the rest of the brain and the spinal cord. The fourth or 

 trochlear nerves extend as slender threads across the peduncles. 



The transverse band of fibers posterior to the peduncles is 

 the pons Varolii. At the posterior borders of the pons arise the 

 fifth or trigeminal nerves. 



The medulla oblongata occupies the region posterior to the 

 pons. A median ventral fissure extends its entire length and 

 continues down the spinal cord. The longitudinal bands of 

 fibers on either side of the ventral fissure are the pyramids. 

 Several cranial nerves arise from the medulla. The sixth or 

 abducens arises from the pyramids near the median line. The 

 seventh or facial nerve roots are just posterior to the fifth. The 

 roots of the eighth or auditory nerve are posterior to the seventh. 

 The ninth or glossopharyngeal and the tenth or vagus arise 

 near together by several rootlets. The numerous roots of the 

 eleventh or spinal accessory arise in a line posterior to the vagus. 

 The roots of the twelfth or hypoglossal emerge from the lateral 

 borders of the pyramids. 



Brain Bisected. — Cut along the longitudinal fissure be- 

 tween the cerebral hemispheres so as to divide the brain into 

 right and left halves. Examine the median surface of one half. 



The Commissures. — These are bundles of fibers connecting 

 the two sides of the brain. Find again the corpus callosum. 



