354 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



posterior pointed end of the fourth ventricle is a tract terminating in a club-shaped 

 area, the clava. Lateral to each clava is another elongated area, the tuberculum 

 cuneatum. These two belong to the somatic sensory column. Anterior to them 

 a white bundle passes toward the cerebellum, disappearing ventral to an elevation 

 which lies just ventral to the hemisphere of the cerebellum. The bundle is the 

 restiform body or posterior peduncle of the cerebellum which conveys impulses 

 from the medulla and spinal cord to the cerebellum. The elevation over the 

 restiform body is the area acustica or primary auditory center. The general 

 resemblance of these structures to those found in the dogfish should be evident. 



Draw the dorsal view of the brain. 



c} The ventral aspect of the brain: Note the basilar artery (continuation of 

 the two vertebral arteries) running in the midventral line and forming a circle 

 around some structures in the center of the ventral surface. This circle, the 

 circle of Willis, is joined on each side by the internal carotid artery. Note the 

 arteries arising from the basilar and circle of Willis and distributed over the brain, 

 coursing in the pia mater. The arteries should be removed. 



At the anterior end of the ventral surface are the two olfactory bulbs. From 

 each one a definite white tract, the olfactory tract, extends obliquely caudad and 

 terminates posteriorly in a lobe, the pyriform lobe, which forms the posteroventral 

 part of the cerebral hemispheres. The fissure or sulcus which separates the pyri- 

 form lobe from the rest of the cerebral hemisphere is called the rhinal fissure. 

 Inclosed between the two pyriform lobes is the ventral side of the diencephalon, 

 or thalamencephalon. At the anterior end of this is the optic chiasma from which 

 the optic nerves project. The region between the optic chiasma and the olfactory 

 tracts is called the anterior perforated substance. Behind the optic chiasma is a 

 slight rounded elevation, the tuber cinereum, from which the pituitary body or 

 hypophysis depends by a stalk. In case the pituitary body was torn off in 

 removing the brain, a slitlike aperture will be noticed in the center of the tuber 

 cinereum marking the place of attachment of the pituitary body. Immediately 

 posterior to the attachment of the pituitary body is an area, the mammillary 

 body, not distinctly marked off from the tuber cinereum. Posterior to this is a 

 depressed area, the posterior perforated substance, from which arise the two third, 

 or oculomotor, nerves. From beneath (dorsal to) the pyriform lobes a thick white 

 bundle will be seen passing obliquely backward on each side of the posterior 

 perforated substance. These bundles are the cerebral peduncles, belonging to 

 the midbrain. The fourth or trochlear nerves arise on the side of the brain 

 between the cerebellum and the inferior colliculi, and pass ventrally over the 

 outer surface of the peduncles. 



The remainder of the ventral surface of the brain belongs to the hindbrain, 

 and consists of the pons and the medulla oblongata. The pons is the heavy band 

 of fibers which crosses the ventral surface of the hindbrain immediately behind 

 the posterior perforated substance. By following it around to the sides of the 



