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THE PHYSIOLOGY OF MUSCLE AND NERVE. 



by way of the axis cylinders of the nerve fibers. By means of his 

 method Waller investigated the location of the nutritive centers 

 for the motor and sensory fibers of the spinal nerves. If an anterior 

 root is cut the peripheral ends of the motor fibers degenerate 

 throughout the length of the nerve, while the fibers in the stump 

 attached to the cord remain intact; hence the nutritive centers 

 for the motor fibers must lie in the cord itself. Subsequent histo- 

 logical work has corroborated this conclusion and showoi that the 

 motor fibers of the spinal nerves take their origin from nerve cells 

 lying in the anterior horn of gray matter in the cord, the so-called 

 motor or anterior root cells. If the posterior root is cut between 

 the ganglion and the cord, the stump attached to the cord degener- 

 ates ; that attached to the ganglion remains intact, and there is no 

 degeneration in the nerve peripheral to the ganglion (Fig. 55). If, 

 however, this root is severed peripherally to the ganglion degenera- 

 tion takes place only in the spinal nerve beyond the ganglion. The 

 nutritive center, therefore, for the sensory fibers must lie in the pos- 

 terior root ganglion, and not in the cord. This conclusion has also 

 been abundantly corroborated by histological work. It is known 

 that the sensory fibers arise from the nerve cells in these ganglia. 

 By the same means it has been shown that the motor fibers in the 

 cranial nerves arise from nerve cells (nuclei of origin) situated in 

 the brain, while the sensory fibers of the same nerves, with the 

 exception of the olfactory and optic nerves which form special cases, 

 arise from sensory ganglia lying outside the nervous axis, such, for 



Fig. 55. — Diagram to show the direction of degeneration on section of the anterior 

 and the posterior root, respectively. The degenerated portion is represented in black. 



instance, as the spiral ganglion of the cochlear nerve, or the gan- 

 glion semilunare (Gasserian ganglion) of the fifth cranial nerve. 



Nerve Degeneration and Regeneration. — ^When a nerve 

 trunk is cut or is killed at any point by crushing, heating, or other 

 means all the fibers peripheral to the point of injury undergo de- 



