As the trunks advance, they furnish here and there different branches; these give out smaller ones, which send off those that are still smaller, from which arise the last divisions. All these different divisions take place at very different angles. The acute angle is the most common. It is not a real origin, but merely a separation of many cords united, that forms the branches, of one or two of these for the smaller ones, of one cord only for the still smaller, and of separate filaments for the last divisions. Thus this separation is made more or less high, in different subjects. The place where it happens is never exactly determined.
According to these divisions, the filaments which compose the cords of each nerve and these cords themselves, are of different lengths; the shortest separate first, then the middling; in fine, the longest filaments of all go the whole extent of the nerve, and only terminate where it ends. The brachial and crural nerves exhibit this arrangement in a remarkable manner.
The nervous branches are almost all accompanied by an artery and a vein, especially in the extremities; for in the trunk, there are exceptions to this rule; in the neck, for example, the arteries often cross the nerves at an angle, instead of accompanying them in their course. In the head, many arterial branches are found thus separated from the nervous. This circumstance is sufficient to make us attach less importance than some authors have done, to this juxta-position of the nervous and sanguineous systems. Moreover, if this juxta-position was so essential, it would be seen with regard to the smaller branches; but this never happens.
III. Termination of the nerves.
I call that the termination, where each filament ends and not that only where the whole trunk of the nerves terminates; so that the sciatic terminates at the thigh, at the leg and at the foot, and not merely at the extremity of this last. In fine, after what has been said already and from what will be said further, the union of filaments into cords and that of cords into trunks, is an arrangement disconnected with their functions, and each filament should be examined separately. The filaments of nerves have three different terminations. They are continued, 1st. with other filaments of the same system; 2d. with the filaments of the system of the ganglions; hence arise anastomoses. 3d. They are lost in the organs.
Anastomoses with the same system.
I have already observed, that true anastomoses should be distinguished, from the junction of a cord that passes to a nerve more or less remote from that to which it belongs, and which simply places itself by the side of its filaments, so that it contributes with them to the nervous cords. Thus there is no anastomosis in a plexus, in the union of the chord of the tympanum with the lingual nerve, &c. So that though the filaments of the different cords of a nerve pass frequently from one to the other, so as to give to the nerve a net-work-like texture, and not as anatomists say, a simple thread-like texture, still it cannot be said that the cords of the same nerve anastomose with each other; there is only juxta-position. On the other hand, the communication of the great hypo-glossal with the cervical pairs, forms a true anastomosis, because there is a continuity, and not merely contiguity of nervous filaments.
If those physicians, who have considered the anastomosis as the exclusive causes of all sympathy, had reflected how few they are in comparison with what they appear at first view, they would have been, by this simple reflection, led to a different opinion. In fact, it is very evident, that though a filament is joined to a trunk, it has no more relation to the filaments of that trunk, than these have among themselves; that is to say, that there is nothing in common but the cellular covering. The arterial and venous anastomoses are infinitely more numerous than the nervous. I believe that they can perform a part in neuralgia, in some sympathies even, a part foreign to the simple communications of the filaments.
We can generally refer anastomoses to three classes. 1st. Two branches belonging to different nerves, go on together, as in the example cited above of the great hypo-glossal, and as also the branches of the facial with those of the sub-orbitary, the occipital with the frontal, &c. 2d. The branches of the same nerve can unite together, as those of the three portions of the trigemini. 3d. Sometimes the two nerves of the same pair, or those of two different pairs, but coming from the two halves of the nervous system, unite at the median line; some examples of this may be seen in the superficial nerves of the neck, in those of the chin, &c. This union does not take place upon the abdomen, where the median line, entirely aponeurotic, has no nervous branch in its texture. It is perhaps by these anastomoses that take place at the median line, that we may explain, how certain motions can still continue in a part affected with paralysis. This sort of anastomosis is in general very rare. In the extremities it is evident, that they cannot exist; in the trunk, they are hardly ever seen behind, and not frequently before. If every pair of nerves gave examples of them, it is clear that hemiplegia would rarely take place, because the sound side of the brain or spinal marrow would through them have an influence upon the nerves of the affected side.