6 9 4 SYMPATHETIC AND OTHER SYSTEMS OF NERVES. 



tissue have an action upon it, which may differ in degree, but does not 

 differ in kind ; similarly, a root of the pelvic nerve which has an action 

 on a tissue, whether excitory or inhibitory, has the same kind of action 

 as the other roots of the nerve. 



This fact gives some probability to the view that the cranial and 

 sacral autonomic centres in the central nervous system are not simply 

 anterior and posterior parts of the sympathetic autonomic centres parts 

 separated from the sympathetic main region by the development of the 

 nerves to the limbs but are distinct centres developed at a different 

 time and under different conditions. 



We have, however, seen reason to believe that the sympathetic, the 

 cranial, and the sacral autonomic systems resemble one another in a 

 fundamental point of structure. Their efferent portions consist of a 

 number of similar elements. Each element consists of two parts. One 

 part consists of a nerve cell, which has its cell body in the central 

 nervous system, and sends its axon, the pre-ganglionic fibre, outwards ; 

 the pre-ganglionic fibre divides, each division forming nerve-endings in 

 connection with a peripheral nerve cell. The other part consists of the 

 nerve cells in connection with which the pre-ganglionic fibre ends ; 

 the axons of these, the post-ganglionic fibres, form nerve-endings in 

 the contractile and secretory tissues. Whilst accepting this as a general 

 scheme, it is hardly necessary to say that the evidence for its univer- 

 sality is very inadequate. And it will be at once recalled, that nothing 

 definite is yet known as to the nerve cell connection of the cranial 

 nerve fibres to the gut. 



The relation of the enteric nervous system to those just mentioned 

 is at present only a matter of guess-work. 



The experiments on regeneration, recounted above, throw a new, 

 though doubtful, light on the question of the specific difference of nerve 

 fibres having different functions. On the one hand, the experiments 

 suggest that the nerve cells, which normally produce different efferent 

 effects, remake their normal connections more quickly and permanently 

 than abnormal ones, so that we should suppose that some difference 

 exists between the different classes of nerve cells. On the other hand, 

 it seems clear that the processes of both pre-ganglionic and ganglionic 

 nerve cells have the power, within wide limits, of making connections 

 other than the normal ones, and so acquiring non-normal functions. 

 And this makes it possible that the functions of the several autonomic 

 nerves are in part, it may be in very large part, dependent upon the 

 particular structures with which they have an opportunity of coming 

 in contact. Thus the laws of growth may have a larger role in the 

 determination of function than physiological differences in nervous 

 structures. 



On this, as on many other physiological questions, we must hold 

 suspended judgment. It is, however, hardly less important, and to 

 many minds it is certainly more interesting, to formulate the problems 

 to be solved than to dwell on the facts which are known. 



