CHAPTEE IV. 



The Development op the Beain and of Ganglia. 



Peom the preceding chapters we learn that there are scattered ganglia 

 with motor and sensory parts; but among the higher animals the greater 

 part of the central nervous system is more or less concentrated into a definite 

 location. Among vertebrates this concentrated nervous system includes a 

 longitudinal cord — the spinal cord — into which the sensory nerves come 

 from without, while from cells which lie in the cord itself the motor nerves 

 arise. Those portions of the central system which receive or send out 

 numerous large nerve-trunks in a relatively short space are much thickened. 

 As examples, one may cite the cervical and lumbar portions of the spinal 

 cord. This is especially the case in the cephalic parts of the animal. In 

 consequence of this all of the craniata possess in this segment of the system 

 an enlargement, — the medulla oblongata. From, this arise the nerves for 

 the branchial arches and for all the structures derived from them. 



Another enlargement lies farther forward where in nearly all animals 

 large optic nerves enter. Finally, one regiilarly finds at the extreme an- 

 terior end of the central nervous system a usually-large projection where the 

 olfactory nerve takes origin. 



In the spinal cord as well as anterior to it there exist connections of 

 one level with another. These produce further enlargements of the central 

 system. 



Finally, with the system as Just described there are associated other 

 structures which are not in direct relation to the different nerves, but 

 which may be, indeed, very important for certain functions of the animal. 

 For example, one finds, dorsal to the medulla in all craniata, a more or 

 less fully developed cerebellum. Ventral and anterior to the deep origin of 

 the optics is an important apparatus, — the midbrain-base with the midbrain- 

 ganglia, — which receives tracts from before and behind and which also 

 sends out tracts. Finally, there is always developed anterior to the Thal- 

 amencephalon and dorsal to the deep origin of the olfactory nerve the basal 

 ganglion of the forebrain, — the corpus striatum,- — ^with which a brain- 

 mantel may be associated. 



As is well known, the spinal cord is alone sufficient to make the lower 

 vertebrates capable of relatively complicated acts. 



One is able to determine anatomically and even experimentally how 



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