246 PHYSIOLOGY FOR DENTAL STUDENTS. 



ent fibers, for if these be divided before they reach the ganglion, 

 the peripheral or far away end undergoes degeneration, whereas 

 if the cut be made between the ganglion and the cord, degenera- 

 tion occurs central-wards, that is, towards and into the cord. This 

 degeneration always occurs in the portion of the nerve fiber which 

 has been disconnected from the nerve cell. It therefore furn- 

 ishes us with a ready method for finding out whether the fiber 

 is running towards or away from the brain. In the former case, 

 the fiber is said to be ascending, and it degenerates above the 

 section; in the latter case, it is descending and it degenerates 

 below the section. Since degenerated nerve fibers give charac- 

 teristic staining reactions, we are thus furnished with a means 

 of finding out what becomes of the afferent fibers after they 

 enter the cord. 



To further trace the course and connections of the afferent fib- 

 ers in the cord, we must therefore cut the posterior roots betwc.-n 

 the ganglion and spinal cord and after a few weeks kill the 

 animal and make microscopic examination of the cord, stained 

 in special ways. If we take a series of such sections above tin- 

 level at which the posterior roots have been cut, we shall find 

 that opposite the point of entry of the cut root, the degenerated 

 fibers occupy an area near the tip of the posterior horn of grey 

 matter. As we examine sections taken higher and higher up, 

 the degenerated area will be found to shift gradually towards 

 the median fissure, occupying, first of all, the so-called postero- 

 lateral column, and later the postero-median (Fig. 42). "When 

 we get to the medulla oblongata or "bulb," the degenerated 

 areas disappear because the fibers have terminated by forming 

 synapses around the cells of the two large ganglia which form 

 the bulgings seen on the posterior aspect of this structure. The 

 fresh relay of nerve fibers do not degenerate after section of the 

 posterior roots, but by other means of investigation they have 

 been found to become collected into a bundle called the filld, 

 which crosses, or decussates, to the other side of the medulla and 

 runs up through the pons varolii and crura cerebri, sonic of tin- 

 fibers ending near the optic thalamun, whilst others run on to 

 the grey matter of the motor areas of the cerebrum. 



