Chap, hi.] GENERAL FEATURES OF NERVOUS TISSUES. 147 



ganglion, whether it be the portion which is passing from 

 the skin or other tissue to the ganglion, or the portion which is 

 passing from the ganglion to the spinal cord as part of the 

 posterior root, degenerates and dies on the side of the cut away 

 from the ganglion. This, however, is not a feature confined to 

 these spinal ganglia ; an efferent fibre of the anterior root 

 similarly degenerates when it is cut away from the nerve cell in 

 the grey matter, of the axis cylinder process of which it is as we 

 have said a prolongation. Speaking generally, a nerve cell governs 

 the nutrition of, acts as a trophic centre as it is called to, the 

 nerve fibre into which its axis cylinder process is continued. 



As regards the sympathetic ganglia, though there are some 

 results which appear to indicate that certain of these ganglia may 

 act in a simple and rudimentary way as centres of reflex action, 

 these cases are by no means clear ; and it may be distinctly 

 affirmed that these ganglia do not generally act as centres of 

 reflex action in the same way as does the grey matter of the 

 central nervous system. 



Of the fibres running in a ramus from a spinal nerve to the 

 ganglion of the same metamere, some may end in connection 

 with the nerve cells of that ganglion. In that case the nerve 

 fibre, which is a medullated one, appears to end in an arborescence 

 in contact with a nerve cell , and that nerve cell gives off one or 

 more processes which become nerve fibres, but non-medullated 

 nerve fibres. Other fibres of the ramus may simply pass through 

 that ganglion, passing by its nerve cells, and end in connection with 

 the nerve cells of some other ganglion, which nerve cells similarly 

 give rise to non-medullated nerve fibres. The nerve fibres leaving 

 a ganglion are more numerous than those reaching it from the 

 central nervous system ; and while the latter are medullated, 

 the former are increasingly non-medullated. Hence in the gan- 

 glion there is a spreading and distribution of nervous impulses ; 

 as to what changes in the nature of the impulses may be effected 

 as they pass through the nerve cells of a ganglion is not at 

 present clear. 



There s c ems at first sight evidence of some strength that these 

 sympathetic ganglia, unlike the ganglia on the posterior roots, may 

 serve as centres of rhythmic automatic action. Several organs of 

 the body containing muscular tissue, the most notable being the 

 heart, are during life engaged in rhythmic automatic movements, 

 and in many cases continue these movements after removal from 

 the body. In nearly all these cases ganglia are present in con- 

 nection with the muscular tissue ; and the presence and intact 

 condition of these ganglia seem at all events in many cases in 

 some way essential to the due performance of the rhythmic 

 automatic movements. Indeed it has been thought that the 

 movements in question are really due to the rhythmic automatic 

 generation in the cells of these ganglia of efferent impulses 

 which parsing clown to the appropriate muscular fibres call forth 



