l86 NERVOUS SYSTEM OF VERTEBRATES. 



olfactory commissure has been seen in selachians but a large tract 

 arises from the dorsal olfactory nucleus on either side and passes 

 to the opposite side in the roof of the forebrain to end in the epi- 

 striatum. The decussation of these tracts seems to take the place 

 of the larger part of the anterior commissure of other fishes, since 

 the anterior commissure proper in selachians is very small. It 

 should be noticed that tliis is a tertiary olfactory tract and that 

 it forms a decussation and not a true commissure. In some other 

 fishes similar fibers are found which run from the olfactory nuclei 

 on one side through the anterior commissure to the epistriatum 

 of the other side. The pecuharities in selachians are the large 

 size of the olfactory nuclei and of the tertiary tract and the conse- 

 quent shifting forward of this tract (cf. further Chap. XVIII). 



The olfactory apparatus of fish-hke vertebrates may be sum- 

 marized as follows. The fibers of the olfactor}'^ nerve arise from 

 the sense cells and end in the glomeruli of the olfactory bulb. 

 The fibers from the cells of the bulb constitute the (secondary) 

 olfactory tract which is distributed to the olfactory' lobe and the 

 epistriatum. A part of the fibers of the olfactory tract, including 

 probably all those to the epistriatum, cross to the opposite side 

 of the brain in a special part of the anterior commissure. From 

 the nuclei of the lobe tertiary tracts go to the epistriatum through 

 the anterior commissure, to the nuclei habenulae through the 

 habenular commissure and to the hypothalamus for the most part 

 without crossing. The epistriatum gives its fibers to the striatum 

 which in turn sends a tract to the thalamus. From the thala- 

 mus and hypothalamus tracts are sent to the medulla oblongata 

 and cerebellum where they make direct and indirect connections 

 with the motor nuclei. From the nuclei habenulae a tract goes 

 to the corpus interpedunculare and adjacent nuclei in the base 

 of the mesencephalon, from which further connections with 

 motor nuclei are-made. See Figure loi and the figures in Chapter 

 XVIII. 



In amphibia, reptiles, birds and mammals the olfactory appa- 

 ratus dift'ers from that described for fishes chiefly in the greater 

 development of the tertiary centers within the forebrain which 

 constitute the olfactory cortex of the cerebral hemispheres. The 



