THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 301 



The commissures in the forebrain of selachians differ in some 

 respects from those in other vertebrates. In the floor (lamina 

 terminalis) in front of the recessus praeopticus is the anterior 

 commissure which contains two sets of fibers : one coming from the 

 tractus strio-thalamicus and the other from the lateral olfactory- 

 nucleus and both ending in the deeper parts of the lateral wall, 

 the epistriatum. In the roof is a large commissure which contains 

 two kinds of fibers. The greater part of it consists of a decussation 

 which is said to be composed of fibers from the dorsal part of the 

 mesial olfactory nucleus and not of olfactory tract fibers. The 

 fibers of this decussation end in the regio uncinata and in the 

 epistriatum. In addition to this a direct tract arises in the mesial 

 nucleus and ends in the epistriatum of the same side. These 

 tracts in the selachian brain are new as compared with the brain 

 of cyclostomes. It must be held clearly in mind that these fibers 

 form the third link in a chain of which the olfactory nerve and 

 the olfactory tract form the first two links. These fibers therefore 

 constitute a tractus oljacto-corticalis. The nucleus which receives 

 these fibers is a center of the same grade as the olfactory cortex of 

 higher vertebrates. That part of the epistriatum which forms 

 this nucleus must be called the olfactory cortex or archi- pallium. 

 The fibers which go out from this center are not well understood, 

 but it is known that the neurites of cells in the epistriatum connect 

 it with the striatum (tr. cortico-medialis). From this the tractus 

 strio-thalamicus carries impulses to the thalamus. In the roof 

 also is another fiber crossing which is said to be composed of true 

 commissural fibers between the lateral olfactory nuclei. This 

 commissure has no counterpart in other vertebrates, but an 

 analogous commissure is seen in the medulla oblongata of some 

 fishes. In some bony fishes the facial lobes are so largely developed 

 as to fuse together over the ventricle and in the lobus impar so 

 formed are found commissural fibers. So here, the lateral olfac- 

 tory nuclei have arched over the ventricle and a large commissure 

 is formed where they have fused together. 



The scheme of the fiber tracts given in Figure 147 will make 

 clear the general relations of the central olfactory apparatus. 

 It should be noticed particularly that the tractus olfacto- 



