OF THE NERVOUS SYSTEM. 385 



terior a thin layer of gray matter folded upon itself, the functions 

 and connections of which are but little understood, and are not, 

 apparently, of very great importance. 



The anterior columns of the cord present, at the lower part of the 

 medulla oblongata, a remarkable interchange or crossing of their 

 fibres (4). The fibres of the left anterior column pass across the 

 median line at this spot, and becoming continuous with the right 

 anterior pyramid, are finally distributed to the right side of the 

 cerebrum ; while the fibres of the right anterior column, passing 

 over to the left anterior pyramid, are distributed to the left side of 

 the cerebrum. This interchange or crossing of the nervous fibres 

 is known as the decussation of the anterior columns of the cord; 



The posterior columns of the cord, as they diverge on each side 

 of the fourth ventricle, form the posterior and lateral projections of 

 the medulla oblongata (3, 3). They are sometimes called the "res- 

 tiform bodies," and are extremely important parts of the brain. 

 They consist in great measure of the longitudinal filaments of 

 the posterior columns, which pass upward and outward, and are 

 distributed partly to the gray matter of the cerebellum. The 

 remainder then pass forward, underneath the tubercula quadri- 

 gemina, into and through the optic thalami ; and radiating thence 

 upward and outward, are distributed, like the continuation of the 

 anterior columns, to the gray matter of the cerebrum. The resti- 

 form bodies, however, in passing upward to the cerebellum, are 

 supplied with some fibres from the anterior columns of the cord, 

 which, leaving the lower portion of the anterior pyramids, join the 

 restiform bodies, and are distributed with them to the cerebellum. 

 From this description it will be seen that both the cerebrum and 

 the cerebellum are supplied with filaments from both the anterior 

 and posterior columns of the cord. 



In the substance of each restiform body, moreover, there is im- 

 bedded a ganglion which gives origin to the pneumogastric nerve, 

 and presides over the functions of respiration. This ganglion is 

 surrounded and covered by the longitudinal fibres passing upward 

 from the cord to the cerebellum, but may be discovered by cutting 

 into the substance of the restiform body, in which it is buried. It 

 is the first important ganglion met with, in dissecting the brain 

 from below upward. 



While the anterior columns are passing beneath the pons Varolii, 

 they form, together with the continuation of the posterior columns 

 and the transverse fibres of the pons itself, a rounded prominence 

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