CHAPTER XXVII. 



THE NERVOUS SYSTEM (Cont'd). 



The Brain Stem and the Cranial Nerves. 



The Brain Stem. The medulla, the pons varolii, and the mid- 

 brain (Figs. 45 and 46), compose the brain stem, which is n-ally 

 an upward extension of the grey matter, and of certain of the 

 columns of the spinal cord, into the base of the brain with special 

 nerve centers and especially large bundles of inter-connecting 

 nerve fibers superadded. It is because of the crossing in various 

 directions of these bundles of fibers that the structure of the 

 medulla, pons and mesencephalon is so difficult to understand. 

 The grey matter, as in the spinal cord, lies deeply and the fibers 

 superficially. Of the latter, the pyramids and fillet, already de- 

 scribed, are the most important, and their direction is longi- 

 tudinal. The most prominent of the connecting or commisural 

 nerve bundles are the upper, middle and lower pedunchs of the 

 cerebellum, or small brain, which, it will be rememltered, lies 

 over and at the side of the pons varolii and midbrain. The 

 lower peduncles spring from the medulla and connect the spinal 

 cord with the cerebellum. They form the lower edges of the 

 fourth ventricle. The middle peduncles enter the sids of the 

 pons in which they cross at right angles with the pyramidal 

 fibers (p. 248). They connect the cerebellum of one side with 

 the cerebrum of the opposite side. The superior peduncles join 

 the encephalon just under the posterior corpora quadrigemina. 

 and the fibers composing them decussate to the other side to be- 

 come connected with certain of the so-called basal ganglia. 



The basal ganglia are the optic thalamus and the corpora stri- 

 ata, two large collections of nerve cells protruding into the third 

 and lateral ventricles of the brain and having the internal capsule 

 between them (see p. 248). The nerve cells composing these 

 ganglia receive impulses from nerve fibers arriving at them both 



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