For example, I shall first describe the semi-lunar ganglion, as we do the brain; then I shall examine the branches, among which, is that by which it communicates with the thoracic ganglions, that is to say, the great splanchnic; for it is very incorrect to consider this nerve as giving origin to the ganglion. In the same way, in the neck and the head, each ganglion will be first described; then I shall treat of its branches, among which are those of communications. The arrangement being nearly the same for the ganglions of the thorax, the pelvis and the loins, the description of each region will be similar.
This manner of describing the nerves, by placing an evident line of demarcation between the two systems, exhibits these two systems such as they really are in nature.
What anatomist, in fact, has not been struck with the differences that exist between the nerves of these two systems? Those of the brain are larger, less numerous, whiter, more compact in their texture and exhibit less variety. On the contrary, the extreme tenuity, great number, especially towards the plexuses, greyish colour, remarkable softness of texture and varieties extremely common are characters of the nerves coming from the ganglions, if we except those of communication with the cerebral nerves and some of those which unite together these small nervous centres.
Besides, this division of the general system of the nerves into two secondary ones, accords very well with that of life. We know in fact that the external functions, the sensations, locomotion and the voice are all dependent on the cerebral nervous system; that on the contrary, most of the organs which perform the internal functions derive from the ganglions their nerves, and with them the principle of their action. We know that animal sensibility and contractility arise from the first, and that where the second alone are found, there is only organic sensibility and contractility.
I have said that the termination of this kind of sensibility and the origin of the corresponding contractility are in the organ in which they are noticed; but perhaps both the termination and origin are more remote, and are in the ganglion from which the organ receives its nerves, as the termination of animal sensibility and the origin of the contractility of the same species are always in the brain. If it be so, as the ganglions are very numerous, we can understand why the forces of organic life do not refer, like those of animal life, to a common centre.
It is evident from these considerations, that there is no great sympathetic nerve, and that what has been designated by this word is only an assemblage of small nervous systems, with distinct functions, but with communicating branches.
We see then what should be thought of the disputes of anatomists respecting the origin of this pretended nerve, placed in the fifth, sixth pair, &c. in those of the neck, back, &c.
Many physiologists have entertained concerning the ganglions opinions similar to those which I have now offered, by considering these bodies as small brains; but it is essential that these opinions should enter into the description, which, as it is now made, gives a very inaccurate idea both of these nervous centres and of the nerves which go off from them.
The expression of nervous branches giving origin to such or such a ganglion, &c. resembles that in which we should consider the brain as arising from the nerves of which it is itself the origin.