THE NERVOUS SYSTEM 52!) 



the ventral expansion of the interbrain, such as the corpora mamil- 

 laria, tuber cinereum, infundibulum and posterior lobe of the hy- 

 pophysis. The epithalamus and hypothalamus are principally con- 

 nected with olfactory paths (see p. 536 and Fig. 359). Certain 

 extensions forward of the tegmentum are also termed subthalamic 

 {e.g., corpus subthalamicum or corpus Luysii). 



The thalamus comprises the great bulk of the interbrain. It con- 

 sists of a number of nuclei forming links in afferent and efferent 

 pallial paths and of other nuclei connected with the corpora striata. 

 There is much difference of opinion both as to the number of the nuclei 

 and their connections. According to some authorities the thalamus 

 may be regarded as divided into internal and external segments 

 (usually separated by the lamina meduUaris medialis). The in- 

 ternal segment consists of an anterior nucleus, median nucleus, the 

 "median center" or nucleus of Luys, and a nucleus arcuatus. The 

 external segment consists of a dorso-lateral, an external ventro- 

 lateral, an internal ventro-lateral, and a ventral nucleus. To the 

 external segment should be added the pulvinar and lateral and 

 medial geniculate bodies (metathalamus) . The various nuclei of 

 this external segment receive the fibres of the aferent pallial paths 

 and complete the paths by sending fibres to the cortex pallii. These 

 paths are (i) the medial lemniscus, spino-thalamic, and secondary 

 trigeminal tracts (general sensory from body and face) to the ventro- 

 lateral nuclei and thence to the cortex of the central region of the 

 pallium ; (2) the lateral fillet or brachium of inferior colliculus (hearing) 

 to the medial geniculate body, and thence to the temporal region of 

 the pallium; (3) the optic tract to the lateral geniculate body and 

 thence to the occipital pallial cortex; (4) part of the superior cere- 

 bellar peduncle (also said to be distributed to nuclei of inner segment) . 

 The visceral (including gustatory) and vestibular paths to the pallium 

 are not definitely known. (See also p. 481.) As the olfactory nerve 

 belongs to the endbrain, its path to the pallium does not traverse the 

 thalamus. Besides giving rise to the above thalamo-cortical fibres, 

 the external thalamic segment in all probability receives many de- 

 scending fibres from the cortex palHi. The various fibres connecting 

 thalamus and cortex constitute the thalamic radiations. In general 

 the anterior parts of the cortex are connected with the anterior part 

 of the external thalamic segment, the middle with the middle, and 

 the posterior with the posterior. It is also probable that the thalamo- 

 cortical fibres from the various lateral nuclei are arranged dorso- 

 34 



