304 NERVOUS SYSTEM OF VERTEBRATES. 



fishes and from the whole olfactory lobe and nucleus praeopticus 

 of ganoids arise the fibers of the tractus olfacto-habenularis which 

 decussate in part in the habenular commissure and end in the 

 nucleus habenularis. The epistriatum in the sturgeon receives 

 a large tract of ascending fibers from the hypothalamus, the 

 tractus lobo-epistriaticus, which decussates in the anterior com- 

 missure. This tract may bring gustatory impulses to the epi- 

 striatum. The neurites of the epistriatum cells in the sturgeon 

 find endings in the striatum; in bony fishes they are said to go in 

 the tractus strio-thalamicus to the thalamus. The anterior com- 

 missure contains fibers from. the lateral olfactory nucleus to the 

 epistriatum of the other side. From the nucleus praeopticus a 

 tract goes back over the optic chiasma to the hypothalamus. 



The scheme of fiber tracts given in Figure 148 shows that in 

 essentials they agree with those of selachians. The olfactory lobe 

 is smaller and does not extend up into the roof of the ventricle. 

 The greater extent of the choroid plexus is due to the receding of 

 the olfactory centers which in selachians help to form the roof. 

 On this account there is no dorsal commissure but tertiary olfac- 

 tory fibers cross in the anterior commissure and end in the epistri- 

 atum as in selachians. The primitive crossing of olfactory tract 

 fibers which has not been seen in selachians is present in ganoids 

 and bony fishes. The tracts to the hypothalamus and nucleus 

 habenularis are essentially alike in all groups thus far described. 

 The epistriatum of selachians is described as the place of ending 

 of tertiary olfactory fibers only, while in ganoids it receives both 

 secondary and tertiary olfactory tracts. Further investigation 

 will probably show that the secondary olfactory fibers to the 

 epistriatum are not absent in selachians. A prolonged discussion 

 as to the nature of the pallium in bony fishes has been due espe- 

 cially to the elevation of the so-called pallium of selachians to the 

 dignity of a true cortical center. Now that it is known that the 

 pallium of selachians is a part of the olfactory lobe and that there 

 is a close correspondence between the fiber tracts in selachians and 

 bony fishes, it requires only the recognition of the slight develop- 

 ment of the olfactory apparatus in bony fishes to explain the 

 condition of the pallium. A greater or less part of the proper 



