28 NERVOUS SYSTEM OF VERTEBRATES. 



diencephalon and marks the cephalic border of the inferior lobes. 

 These are not true nerves but central brain tracts, and will here- 

 after be called the optic tracts. In all vertebrates except bony 

 fishes the two tracts as they enter the diencephalon form a deeus- 

 sation which is more or less completely hidden in the wall of the 

 brain. This is known as the optic chiasma. In bony fishes the 

 chiasma is carried out from the brain wall and in many cases 

 the two tracts cross one another at a considerable distance from 

 the brain wall, on their way to the eyes. 



The telencephalon or forebrain in the different classes of verte- 

 brates presents great differences in both size and structure. The 

 forebrain of some primitive selachians (e.g. Heptanchus, Fig. 2) 

 consists clearly of paired lateral lobes which are somewhat elong- 

 ated. At its anterior end each lobe is produced forward and 

 laterally as a slender cylinder which becomes enlarged as the 

 olfactory bulb. This lies in contact with the inner surface of the 

 olfactory sac. From the epithelium of the olfactory sac, nerve 

 fibers pass through the wall and directly into the olfactory bulb. 

 These fibers constitute the olfactory or first cranial nerve. The 

 slender portion connecting the bulb with the forebrain proper is 

 the olfactory tract. Dorsally a membranous roof (tela chorioidea) 

 connects and covers the lateral lobes, enclosing the forebrain 

 ventricle. The ventricle in its caudal part is common to both 

 lobes and in its cephalic part divides in Y-shape and continues 

 through the olfactory tracts to the olfactory bulbs. The common 

 cavity together with that of the diencephalon is usually known 

 as the third ventricle of the brain. The lateral or olfactory por- 

 tions are to be compared broadly with the lateral ventricles of 

 the mammalian brain (see Chapter XVII) and the point of their 

 separation from the third ventricle is the foramen of Monro. 

 It will appear later that the third ventricle extends a short distance 

 forward beyond the opening into the lateral ventricles and these 

 openings are therefore true lateral structures. It is necessary, 

 therefore, to speak of paired foramina of Monro. The caudal 

 border of the roof of the forebrain is marked by the velum trans- 

 versum. In front of this the roof is produced dorsally into a tube 

 or sac which varies in size and is more or less complexly branched 



