COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 317 



rior end of the brain. You will next see. the two optic nerves entering the 

 ventral surface of the diencephalon. Cut through them and lift the brain farther. 

 Next, pare away the wall- of the orbit on one side. It will then be seen that 

 certain structures attached to the ventral surface of the diencephalon extend 

 ventrally into a deep pit (sella turcica) in the floor of the cranial cavity. Take 

 especial care to lift these out intact. Then cut through the remainder of the 

 cranial nerves and cut across the spinal cord and lift the brain out of the cranial 

 cavity. 



Examine the ventral surface of the brain. Note the forward continuation 

 of the internal carotid artery on the midventral line of the brain. It forks around 

 the ventral part of the diencephalon (farther posteriorly in the skate) and passes 

 forward to the telencephalon, distributing many branches to all parts of the 

 brain as well as to the orbit. The ventral surface of the brain presents nothing 

 new except as regards the diencephalon, where several additional structures are 

 visible. The two optic nerves are seen attached to the anterior end of the 

 ventral surface of the diencephalon; as they enter the latter they cross, the 

 crossed region being named the optic chiasma. From the chiasma a broad band, 

 the optic tract, extends dorsad and caudad into the dorsal part of the diencephalon 

 and into the optic lobes. It is readily seen, especially in the spiny dogfish, by 

 scraping off the primitive meninx at this place. It is thus evident that the visual 

 impulses pass into these two portions of the brain. Posterior to the optic chiasma 

 the floor of the diencephalon bulges ventrally and posteriorly as the infundibu- 

 lum, consisting in large part of two rounded lobes, the inferior lobes. These are 

 devoted mainly to smell and taste. From between the two inferior lobes a 

 stalk projects caudad and widens into a soft sac. This whole structure is the 

 hypophysis or pituitary body. The dorsal part of the sac has thinner walls, 

 generally discolored because of contained blood; it is called the vascular sac or 

 saccus vasculosus. The hypophysis is generally more or less torn in removing 

 the brain. It is known to be a gland of internal secretion in mammals and 

 probably has the same function in other vertebrates. Dorsal to the vascular 

 sac the roots of the oculomotor nerves will be found springing from the floor of 

 the midbrain. The ventral surface of the remainder of the brain presents nothing 

 new. The roots of the cranial nerves should be identified on the medulla. On 

 the ventral surface of the medulla will be found the roots of the sixth nerves. 

 If not identifiable on the brain, they will usually be found adhering to the floor 

 of the cavity from which the brain was removed. 



Draw a profile view of the- brain. 



7. The sagittal section and the ventricles of the brain.— The brain like the 

 remainder of the central nervous system is hollow. Its cavities are known as 

 ventricles and are continuous with each other by means of narrow passages. 

 The fourth or last ventricle of the brain has already been identified as the cavity 

 within the medulla oblongata. The ventral portion of this ventricle is named 



