I say first they influence; thus a nerve being irritated in any way, a number of sympathetic phenomena take place in the system. Diseases frequently show these facts. It is thus in tic douloureux and analogous diseases, in which the nervous texture is particularly affected, sometimes the animal sensibility is raised in various remote parts, and hence the pains that are often experienced in the head, in the internal viscera, pains that cease when the cause that supported them has disappeared. Sometimes it is the animal contractility; hence the convulsions that occasionally take place in the muscles that receive branches from the affected nerve. In some cases it is the sensible organic contractility which is sympathetically excited by the nervous affections. Thus in the paroxysm of neuralgic pains there is often spasmodic vomiting, the action of the heart is hurried, &c. We can by experiments produce the same phenomena. Thus by acting upon the nerves of the superior or inferior extremities, by irritating them any way after they have been laid bare, I have frequently produced vomiting, or convulsions in muscles that were in no way connected with the nerves I irritated. In the second place, the nerves can be influenced by diseased organs; thus in many acute and chronic affections, sympathetic pains spread along the course of different nerves, particularly in the extremities. As the animal sensibility is the predominant property of the nerves, it is almost always that which is brought sympathetically into action. Physicians have not distinguished with sufficient accuracy in the pains of the extremities, that which belongs to the nerves, from that which has its seat in the muscles, the aponeuroses, the tendons, &c. _Influence of the nerves upon the sympathies of the other organs._ Authors have been much divided upon the cause that supports sympathies. How can an organ which has no relation with another that is frequently very remote, influence it so as to produce serious diseases there, merely because it is itself affected? This singular phenomenon is often witnessed in a state of health; but it is so wonderfully increased in diseases, that if we could remove from them the symptoms that are not exclusively dependant upon the derangement of the function particularly affected, we should find but little difficulty in their study or treatment. The moment an organ is affected, all the rest seem to feel simultaneously the disorder it experiences, and each seems to be agitated in its own way in order to expel the morbific matter that has seized upon one of them.