These organs are remarkable for their reddish or greyish colour, for their softness, for their indistinctness, &c.; it is often difficult to distinguish them from cellular texture. The best manner of making them evident is to let the subject macerate for a day or two open in the water; they whiten then sensibly, do not soften, and appear even to increase in consistence, like the cerebral nerves in a similar case. Besides their delicacy is such, that it is impossible to submit them to any kind of reagents. Only I have observed that they possess in an eminent degree the property of horny hardening, and that they do not yield in this respect to the cerebral. This delicacy depends upon this, that all the filaments are separate from each other, instead of being like the preceding, collected into cords; it is this also that makes these nerves so numerous. If all the filaments of the brachial plexus were separated like those of the solar, they would present the same appearance and the same number in their interlacing.
Do the primitive plexuses formed by the ganglions perform a part in the nervous functions? are they centres to which are referred important phenomena? What has not been said upon this subject, concerning the solar plexus? But nothing, I believe, of all that has been advanced is founded upon accurate observation.
The plexuses of organic life are soon separated into different divisions, which go to different parts, to those especially of this life. These divisions arise from an infinite number of little filaments which go constantly separate, though placed near each other, and which never unite into cords like the preceding. They accompany almost all the arteries; thus the renal, the hepatic, the splenic, the coronary stomachic, the mesenterics, the hypogastric, the carotid and its distributions, &c. are surrounded with filaments coming from ganglions. These filaments go in two ways. 1st. Some accompany the artery without being connected with it; considerable cellular texture separates them; they go in its course without intermixing in a sensible manner with it. 2d. The others form for it, if we may so say, a new coat, exterior to the others, which adhere to it intimately, and which interlace so together, that they might be taken for a network surrounding the artery.
When the artery runs but a short course, these two orders of branches remain distinct from each other as far as the organ, as we see around the splenic, the hepatic, the renal, &c.; but if the course is longer, the external branches gradually get into the plexus, and are entirely lost there. This plexus can be followed upon the great trunks; it divides upon each branch, and can be still seen; but such is its tenuity upon the minute ramifications that it disappears there entirely. The spermatic is one of the arteries upon which it can be traced the longest. The arteries of the extremities appear to be destitute of it. In general it is upon those that go to the central organs of internal life, that this network is the most evident. When we deduct from the sum of the filaments coming from the ganglions, those by which they communicate on one part with each other, and on the other with the nerves of animal life, we see that almost all the rest is finally destined to accompany the arteries. This arrangement is wholly different from that of the cerebral nerves, whose filaments are only in apposition with the vessels. These make almost an integrant part of them, the adhesion is so intimate; this certainly supposes a use of which we are ignorant, relative to the circulation, or to the other organic functions. As these vessels distribute every where the materials of these functions, of the secretions, exhalations, nutrition, &c. the organic nerves have no doubt some influence upon them. Neither experiment or observation have taught us any thing upon this point.
The veins are not accompanied by so many organic nerves. It is the same as it respects the absorbent trunks, which go almost every where separate from this system.
The constant union of the arteries with the organic plexuses, an union that presents an arrangement wholly different from that of the ganglions, has undoubtedly an influence upon the action of these plexuses, or rather upon that of the nerves that go from them, by the motion the blood communicates to them. It should be remarked upon this subject, that as nature has placed a crowd of arteries at the base of the brain to agitate it with an alternate motion, she has put also the most considerable plexus of the whole organic system upon one of the places to which the red blood communicates the strongest impulse, viz. upon the trunk of the cœliac.
III. Structure, Properties, &c.
From what has been said above, it is evident that the nerves going from the ganglions are of two sorts as it respects organization; 1st. those that are identified with the cerebral system, by their white colour, by the possibility of dividing their trunks into distinct cords, and these into filaments, which appear to have nervous coats and medullary substance like the preceding; 2d. those which present only little separate filaments, greyish or reddish, soft, and which are seen in prodigious numbers in the plexuses. Have these a nervous coat and a medullary substance? It is impossible to determine it.
The properties of texture are ascertained with difficulty in the organic nerves. As to vital properties, it is undoubted that the animal sensibility is not as much raised in these nerves as in those of animal life. I have often laid bare the plexuses in the abdomen; then by letting the animal rest a moment, and by irritating them comparatively with the lumbar nerves, I have uniformly made this remark. We know that very frequently the ligature of the spermatic artery is not painful in sarcocele, though the branches coming from the ganglions form for it a plexus like a network, which can in no way be separated from it. If we draw out a portion of intestines by a small wound in the abdomen, the irritation of the sub-mucous layer at the side of the vessels, is hardly felt, though many nerves of ganglions are found at this place. I have had numerous occasions to act in different ways upon the carotid, to which the superior cervical ganglion furnishes branches from above; now, as long as I did not touch the par vagum, the animal remained tranquil. I am far, however, from believing in the absolute insensibility of the nerves of the ganglions; but certainly under the circumstances that I have related, the cerebral nerves would have caused much more pain to the animal.
I think that in a morbid state this sensibility is susceptible of being greatly raised. We certainly cannot deny but that the solar plexus takes a great part in the different sensations that are experienced at the epigastric region; the very acute pains that often attend the formation of aneurisms, are probably owing in part to the distension of the nervous filaments that surround the artery. I have already said that it is probable that the organic nerves are much concerned in the different sensations that are produced by some peculiar neuroses.