CHAPTEE X. 



The Intehbhain : The Thalamencephalon. 



The anatomical apparatus which has been described is so constructed 

 that it may be looked upon as, in the main, complete in itself. Only a very 

 few bundles pass anteriorly from the organs above described. Moreover, 

 in the lower vertebrates at least only a few small tracts pass from those 

 brain-segments anterior to the midbrain into the ganglia which lie in the 

 midbrain and oblongata, or into the spinal centers. Consequ:ently, fishes, 

 amphibians, and reptiles never show so marked derangement of functions 

 where all of the brain anterior to the posterior commissure is removed as 



C-^^s-V" *-'— - 



Fig. 74. — Brain of a Nile crocodile, natural size. The cerebrum covers the 

 thalamus anteriorly; the Tractus opticus covers it laterally; so that only a small 

 part of the hypothalamus remains visible. 



when the midbrain, with its great association-bundles and important tracts, 

 is injured or when one injures the oblongata or the spinal cord. 



Regarding the physiological significance of that part of the brain be- 

 tween the midbrain and the cerebrum — viz., the ThalamencepJialon — we 

 know practically nothing and we stand only on the threshold of morpho- 

 logical knowledge. 



Doubtless, however, the thalamencephalon is an important segment of 

 the brain. Since from Petromyzon up through the vertebrates, however 

 weakly one or the other brain-segment may be developed, one uniformly 

 finds the interbrain anterior to the midbrain. In the external view of the 

 brain it is scarcely visible, because, even in those cases where it is not en- 

 mantled by the hemispheres, still the midbrain-roof protrudes beyond it, 

 and, furthermore, it is completely covered laterally through the great fiber- 



(125) 



