THE OLFACTORY APPARATUS. 183 



The olfactory tract is the secondary central tract of the olfactory 

 apparatus; the nuclei of the olfactory lobe are the secondary 

 nuclei. 



These secondary nuclei form a larger or smaller part of the 

 forebrain in different vertebrates according to the importance 

 of the olfactory organ. In cyclostomes the lateral lobes of the 

 forebrain are divided by a vertical groove into anterior and pos- 

 terior halves. The anterior half is the olfactory bulb ; the posterior 

 half is mostly occupied by the lobe, which is not divided into nuclei 

 but is a continuous mass. In selachians the bulb is distinct 

 from the lobe and a longer or shorter olfactory tract is present. 

 The olfactory lobes occupy the anterior, the lateral and a larger 

 or smaller part of the dorsal wall of the forebrain. The olfactory 

 organ is very important in selachians and the large development 

 of the olfactory centers accounts for the relatively great size of the 

 forebrain in those fishes. In bony fishes the olfactory organ 

 is much less important and the centers are correspondingly small, 

 being confined to the lateral and ventral part of the forebrain. 

 The caudo-lateral part becomes especially differentiated and is 

 known as the nucleus thaeniae. In amphibia and reptiles the 

 olfactory lobe has a similar extent. In mammals, owing to the 

 great development of the cerebral hemispheres, the olfactory lobe 

 is relatively very small. It includes the tuberculum olfactorium 

 and praecommissural body occupying a small area on the ventral 

 and mesial aspect of the hemisphere in front of the commissures, 

 and the pyriform lobe which extends along the ventral surface 

 of trie corpus striatum at the junction with the lateral cortex. 



The olfactory lobe may be divided into cephalic and lateral 

 portions. The cephalic portion occupies the front wall of the 

 forebrain, in front of the commissures. In lower vertebrates 

 it includes two or more collections of cells which have been called 

 the (median, lateral and dorsal) postolfactory nuclei. The 

 lateral portion is an extensive nucleus covering the outer surface 

 of the corpus striatum, a part of which is the nucleus thaeniae. 



The cells of all of these nuclei are irregular stellate or spindle 

 cells which show no definite orientation. The fibers which end 

 in these nuclei are all olfactory tract fibers which come from the 



