580 COMPARATIVE PHYSIOLOGY. 



Exception. — It has been found that sometimes stimulation of 

 the peripheral end of the anterior root has given rise to pain, 

 an effect which disappears if the posterior root be cut. From 

 this it is inferred that certain sensory fibers turn up into the 

 anterior root a certain distance. Such are termed " recurrent 

 sensory fibers." 



Additional Experiments.—!. It is found that if the anterior 

 root be cut, the fibers below the point of section degenerate, 

 while those above it do not. 2. On the other hand, when the 

 posterior root is divided above the ganglion, the fibers toward 

 the cord degenerate, while those on either side of the ganglion 

 do not. From these experiments it is inferred that the cells of 

 the posterior ganglion are essential to the nutrition of the sen- 

 sory fibers, and those of the anterior horn of the cord to the 

 motor fibers. 



FathologicaL — Pathology teaches the same lesson, for it is 

 observed that, when there is disease of the anterior gray comua, 

 degeneration of motor fibers is almost sure to follow. These 

 cells, whether in the ganglion or the anterior horn, have been 

 termed "trophic." It is true, the functions of the ganglia on 

 the posterior roots, other than those just indicated, are un- 

 known ; on the other hand, the cells of the anterior horn are 

 distinctly motor in function. To assume, then, that the cells of 

 the ganglion are exclusively trophic, with the evidence now 

 before us, would be premature. 



The view we have presented of the relation of the nervous 

 system makes all cells trophic in a certain sense ; and we think 

 the view that certain cells or certain fibers are exclusively tro- 

 phic must, as yet, be regarded as an open question. 



It is important, however, to recognize that certain connec- 

 tions between the parts of the nervous system, and indeed all 

 of the tissues, are essential for perfect " nutrition," if we are to 

 continue the use of that term at all. 



n. THE CRANIAL NERVES. 



These nerves have been divided into nerves of special sense, 

 motor, and mixed nerves. 



The first class has already been considered, with the senses 

 to which they belong. 



The physiology of the cranial nerves has been worked out 

 by means of sections and clinico-pathological investigations. 



