CORRELATING CENTERS IN THE DIENCEPHALON. 281 



overtopped by the visceral lobe and the nucleus habenulae respect- 

 ively. The position of the nucleus habenulae is in everyway 

 analogous to that of the vagal or facial lobe. In both cases a 

 more median and ventral body has been hypertrophied and has 

 pushed up until it has overtopped a more dorsal and lateral body. 

 The nucleus habenulae has overtopped the geniculatum just as 

 the vagal lobe or the facial lobe in bony fishes has overtopped 

 the acusticum. 



In mammals the nucleus habenulae is composed of a mesial 

 portion similar in structure to the nucleus in fishes, and of a 

 lateral portion which has a very different structure (Fig. 142). 

 Two similar portions are to be recognized in the nucleus habenulae 

 of some bony fishes also. Several fiber tracts are said to be con- 

 nected with the nucleus habenulae in addition to those mentioned 

 above but it is not known how the two portions of the nucleus are 

 related to these tracts. 



5. THE NUCLEUS OF THE TRACTUS STRIO-THALAMICUS AND THE 

 SUBSTANTIA RETICULARIS THALAMI. 



The large nucleus of the tractus strio-thalamicus, as already 

 stated, lies in the stem region of the thalamus. In bony fishes 

 it is divided into the two large nuclei dor salts and v entrails (nucleus 

 rotundus Fritsch). From these nuclei arise the large tractus 

 thalamo-bulbares et spinales which constitute an important part 

 of the fundamental bundles in the medulla oblongata. It is to 

 be noticed that the striatum is not a secondary olfactory nucleus, 

 but is a correlation center for olfactory, gustatory and perhaps 

 other impulses, and that the tractus strio-thalamicus is a descending 

 tract or motor tract. The pathway is broken once in the thalamus 

 and the fibers arising here probably make connection with widely 

 separated motor nuclei in the brain and spinal cord. These 

 functional relations emphasize the statement made above that these 

 tracts represent the stem or base of the interbrain and forebrain. 

 Essentially the same motor conduction path is found in all verte- 

 brates, although its functional relations may be somewhat modi- 

 fied in mammals on account of the cerebral cortex. 



In all vertebrates there remains surrounding the ventricle a 



