286 NERVOUS SYSTEM OF VERTEBRATES. 



interbrain may now be reviewed in a few words. The motor 

 columns are represented only by the nucleus of the III nerve and 

 the thalamic nucleus of the fasciculus longitudinalis median's, 

 which belong to the somatic motor division. The presence of 

 efferent sympathetic fibers in the III nerVe in mammals indicates 

 the presence in the midbrain of cells representing the visceral 

 efferent column, but they have not yet been recognized as a definite 

 column, unless the nucleus of origin of the mesencephalic root of 

 the V nerve be that column. The somatic sensory division is 

 represented by the tectum mesencephali, the corpus geniculatum 

 laterale and mediale, and the nucleus of the medial lemniscus in the 

 thalamus. With the exception of the mesencephalic root of the 

 trigeminus, only secondary or tertiary tracts end in these nuclei. 

 The tectum mesencephali is clearly the continuation forward of 

 the primary cutaneous centers of the cord and medulla oblongata. 

 It has been modified into a center which is chiefly secondary. 

 In the diencephalon the corpora geniculata and the nucleus of the 

 lemniscus are situated in the dorsal region and hold essentially the 

 same relation to sensory tracts as do the several nuclei in the 

 tectal region of the mesencephalon. In both midbrain and inter- 

 brain a relatively indifferent region has developed special nuclei 

 for visual, cutaneous and auditory impulses. The fact is clear 

 that the position of all the centers mentioned with relation to the 

 axis of the brain and to other chief columns is the same as that 

 of the somatic sensory columns in other segments, and the con- 

 duction paths in which these centers form stations are all somatic 

 sensory conduction paths. The recent description of the brach- 

 ium conjunctivum in mammals as a tract from the nucleus 

 dentatus to the optic thalamus of the opposite side, brings the 

 brachium into close comparison with the lemniscus system and 

 adds an important fact to the grounds upon which the above 

 interpretation of the diencephalon is based. 



The visceral sensory division is not represented by any known 

 special centers in the midbrain but in the interbrain it is largely 

 developed and includes centers in gustatory pathways. The 

 visceral sensory column (or the substantia reticularis belonging 

 to it) has been distorted by the expansion and shifting of the 



