THE DIENCEPHALON AND THE OPTIC NERVE 219 



such as occur in lower vertebrates that lack the cerebral cortex (Herrick, 1917) 

 Both receive fibers from the olfactory centers and both send fibers to the corpus 

 striatum, but none to the cerebral cortex (Sachs, 1909). There is some evidence 

 of a clinical nature to show that the activity of these centers may be accompanied 

 by a crude form of consciousness (Head and Holmes, 1911; Head, 1918). Pa- 

 tients in whom the paths from the thalamus to the cortex have been interrupted 

 are aware of many sensations, but cannot discriminate among them. The 

 thalamus seems to be the chief center for the perception of pain and the affec- 

 tive qualities of other sensations, and in this respect it plays an important 

 role in consciousness independently of the cerebral cortex. 



The more lateral group of centers, which includes the lateral nucleus of the 

 thalamus, the pulvinar, and the geniculate bodies, is of more recent origin and 

 has been called the neothalamus. They serve as relay stations on the somatic 

 sensory paths to the cerebral cortex. The medial lemniscus and spinothalamic 

 tracts terminate in the ventral subdivision of the lateral nucleus. In the pul- 

 vinar and lateral geniculate body terminate fibers from the optic tracts, while 

 the lateral lemniscus ends in the medial geniculate body. From these nuclei 

 sensory fibers of the third order run to the cerebral cortex. The lateral nucleus, 

 exclusive of the pulvinar, is therefore a relay station on the paths of cutaneous 

 and deep sensibility, and it is connected with the parietal and frontal cortex 

 through the thalamic radiation. The pulvinar and lateral geniculate body are 

 stations on the optic pathway, and the medial geniculate body on that for hearing. 



The thalamic radiation can best be considered in detail after we have ac- 

 quired some familiarity with the structure of the cerebral hemisphere (p. 263). 



The fiber tract connections, established by the various nuclear masses composing the 

 thalamus, among themselves and with other parts of the brain, are not as yet well known. 

 This is particularly true of the descending tracts. It is known that from the region of the 

 thalamus a large bundle, the thalamo-olivary tract, descends to the inferior olivary nucleus. 

 Some authors also describe a thalamospinal tract which arises in the thalamus and is closely 

 associated with the rubrospinal tract. 



It is fairly well established that each of the ascending sensory tracts of the tegmentum 

 has its own particular field of distribution within the ventral nucleus of the thalamus; and it 

 is, therefore, probable that there are corresponding functional differences in the various 

 subdivisions of this nucleus. Beginning at the lateral side and passing medialward, the 

 terminals of these various tracts are as follows: The spinothalamic tract ends in the most 

 lateral part of the ventral nucleus. Next comes the field, within which terminate the fibers 

 of the central tract of the trigeminal nerve, and which includes the nucleus arcuatus and 

 nucleus centralis. The medial lemniscus ends in the most medial part of the inferior nucleus, 

 including the nucleus centralis. This corresponds to the relative position which these tracts 

 occupy in the tegmentum of the mesencephalon, where the spinothalamic tract is the most 

 lateral of the three. 



