108 



NERVOUS SYSTEM. 



racy through their length, notably fibres to the arm (direct pyra- 

 midal tract) in the anterior column, and to the leg (crossed pyra- 

 midal tract) in the lateral column. 



Do all the fibres of the spinal nerves pass from the brain through 



the spinal cord? 



No. It has been calculated that only about one-half as many 

 fibres enter the spinal cord from the brain as leave it through the 

 nerves ; therefore it must follow that some fibres originate from the 

 cord. The increase in gray matter in the cervical and lumbar enlarge- 

 ments, where the fibres for the large plexuses of the nerves (brach- 

 ial and lumbar) are given off, confirms this view. 



Where are the trophic centres for the anterior nerve-roots ? 



The posterior nerve-roots, we have seen, seem to be dependent 

 upon the ganglia which are found upon them for trophic influence. 

 The anterior root in a similar way seems to depend upon a trophic 

 centre in the gray matter in the anterior horn. 



What is degeneration of a nerve-fibre ? 



Division of a nerve is followed by a degeneration or breaking 

 down of the axis-cylinders of its fibres within a day or two, the 

 loss of function being an earlier and immediate manifestation. This 



FIG. 30. 



Degeneration of Spinal Nerves and Nerve-roots after Section : A, section of nerve-trunk 

 beyond the ganglion; B, section of anterior root; C, section of posterior root; D, 

 excision of ganglion ; a, anterior root ; p, posterior root ; g, ganglion. 



degeneration is centrifugal ; that is, does not proceed toward the 

 spine, but to the periphery. If the posterior root be cut, how- 

 ever, between its ganglion and its emergence from the cord, the 

 degeneration is toward the cord i. e. centripetal and the nerve 



