THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 297 



tory tract fibers do not pass back on the same side but cross to the 

 opposite side in the upper part of the lamina terminahs, forming 

 the olfactory decussation. These fibers then enter the dorsal part 

 of the wall of the median ventricle and do not go to the lateral 

 lobes. This lateral wall of the ventricle is directly continuous 

 caudally with the lateral wall of the diencephalon and through the 

 foramen of Monro with the caudal wall of the olfactory lobe. The 

 nucleus which occupies this area is the epistr latum (Fig. 146). 

 The olfactory tract, then, goes in greater part to the olfactory lobe 

 of the same side and in lesser part through a decussation in the 

 lamina terminalis to the epistriatum of the other side. 



From the olfactory lobe two tracts arise. One passes internal 

 to the optic tracts and enters the inferior lobes of the diencephalon, 

 the tr actus oljado-hypothalamkus. The other goes upward and 

 backward through the epistriatum to the nucleus habenulae, 

 where the larger part of its fibers cross to the opposite nucleus, 

 forming the commissura habenularis. This is the tr actus oljacto- 

 hahenularis. In addition to fibers of the olfactory tract which 

 end in the epistriatum an equally large tract comes to it from the 

 hypothalamus by way of a decussation behind the chiasma. This 

 is an important tract the equivalent of which is probably found in 

 all vertebrates, usually in close relation with the tractus strio- 

 thalamicus. It should be recognized as an independent tract 

 under the name of the tractus loho-epistriaticus. The epistriatum 

 is characterized by pyramidal cells whose dendrites are studded 

 with Httle knobbed spines. The cell-body Hes next the ventricle 

 and the neurite arises from the basal part of a dendrite and pro- 

 ceeds away from the ventricle. The neurites descend through the 

 lateral wall of the median ventricle and end in the ventral half 

 of the same wall, which constitutes the corpus striatum. The 

 cells of the striatum are irregularly arranged bipolar or stellate 

 cells whose neurites form the tractus strio-thalamicus, which 

 ends in the central gray of the thalamus. Surrounding the 

 preoptic recess is a layer of cells of very primitive character 

 forming the nucleus praeoptkus. The neurites from this nucleus 

 in part run dorsad to enter the tractus olfacto-habenularis and 

 in part run back over the chiasma into the hypothalamus. The 



