Meyer, Data of Modern Ncmvlogy. 143 



skin and into the posterior horns. Each segment is connected 

 with its neighbors by ground bundles. The immense multi- 

 plicity of sensory-motor combinations would require an im- 

 mense number of long fibers running back and forth and the 

 nervous system would have the form of a fairly uniform thick- 

 walled tube. The function would be slow and complicated. 

 In reality the results of coordination are obtained much more 

 easily by centralization of the mechanisms which represent spe- 

 cial functions. 



We have seen that the auditory-facial-abducens segment 

 contains an apparatus for equilibration. The sense-organ is 

 formed by the semicircular canals of the internal ear, destruc- 

 tion of which leads to disorders of equilibration. Over this 

 segment the mechanisms relating to the appreciation of coordi- 

 nation and equilibration of all the segments are united in a spe- 

 cial structure, the ccrcbdhim. It varies in size and complexity 

 in the vertebrate series and is most developed in the fish, the 

 birds and in man. Mechanisms which are scattered all over in 

 the Amphioxus are thus centralized and can become more 

 elaborate by short association elements within the special organ. 



Further we find the optic segment (mid-brain) with a pecu- 

 liar system of connections with the afferent elements of the rest 

 of the body. In many animals which depend very largely on 

 vision, as the trout, certain reptiles and especially birds, the 

 mid-brain is very large and * sensory ' paths are connecting all 

 the rest of the neural tube with it, so that all the afferent im- 

 pulses can be elaborated into one harmonious entity. 



Between the olfactory segment and the optic segment a 

 further highly complicated mechanism developes, especially 

 from the reptiles up, the thalamus and the fore-brain, most 

 highly organized in man, and the organ of the highest reactions 

 of which a living being is capable, among others the mental ac- 

 tivity, thought, and reasoning. 



Without entering on the detailed structure of the cerebel- 

 lar, mid-brain and fore-brain mechanisms, I merely illustrate the 

 principle of the cerebellum and the fore-brain. 



Each segment of the neural tube has certain connections 



