These and similar phenomena afford clear proof of the influence exerted over the secreting processes by mental states ; and it is scarcely to be imagined that this influence can be exerted through any other channel than the nervous system. If we further in quire to which division of the nervous system we are to attribute the conveyance .of this influence, we shall find reason to regard it as chiefly, if not entirely, operating through the portion commonly known as the sympathetic. For there are many secreting organs which are supplied with no other nerves than those which they receive from this division, so that they cannot possess any connection with the cerebro-spinal centres except through its medium. The mammary glands, which are supplied by the spinal nerves as well as by the sympathetic, may he considered as re quiring such a direct communication with the cerebro-spinal centres, inasmuch as their se cretion is made, for obvious purposes, greatly dependent upon sensations directly affecting themselves, which is rarely the case elsewhere. The lachrymal and salivary glands would seem to have a more direct and exclusive con nection than most others, with the cerebro spinal centres ; but perhaps this may be more apparent than real, for the fifth pair, from which they are supplied, seems in many re spects to combine the attributes of a sympa thetic with that of a proper cranial nerve ; and bearing in mind the minuteness and the universality of the distribution of the sympa thetic plexuses upon the trunks of the blood vessels, we see that even these glands, like others, may be subjected to its influence.
If we further examine into the mode in which that influence is exerted, we shall, per haps, find reason to attribute it to the effect of nervous agency, rather upon the walls of the blood-vessels and upon their contents, than upon the secretirw structures themselves. For, as already remarked, the variations in the quantity of a secretion may be accounted for by such an increase or diminution in the access of blood as we know to take place, through an alteration in the calibre of the vessels, in the act of blushing or the paleness of fright • and the feelings experienced by the nursing female harmoidse well with this sup position. On the other hand, the perversion of the quality of a secretion, which may take place as a result of mental emotion, would seem rather due to an alteration in the con stituents of the blood previously to the elimi nation of the secretion, than to the exercise of any influence upon the secreting structure itself. For we find, in the case of the pecu liar odorous matter for example, that it may be eliminated in a vaporous form by the air passages, or by the intestinal canal; or that its taint may be imparted to the liquid secre tions of the intestinal glandulw ; or, again, that it may be communicated to the urinary excretion: and this variety in the channels of escape of the same kind of material, pretty clearly indicates that it must have pre-existed in the blood. There are malty other facts
which confirm this view, by indicating that the condition of the blood whilst circulating in the vessels may be influenced by mental emotions, which probably act upon it through the medium of the sympathetic nerve ; but of these it is scarcely the place to speak.
Another class of evidence, as to the ex ertion of an influence by the nervous system upon the secretory function, is furnished by observation of the results of the interruption of that influence, either by a diseased con dition of the nervous centres or nerve trunks, or by experimental interference. One of the most familiar of these, on account of its fre quent occurrence, is the change in the cha racter of the urine in cases of paraplegia ; resulting, as it would seem, from the secre tion of an undue quantity of alkaline mucus from the lining of the bladder.* Various ex periments have been made upon the nerves of the kidney, which seem to indicate that the normal secretion of urine is dependent upon their integrity. Thus Krimer t states, that division of any of the nerves of the kidney occasioned albumen and the red colouring matter of the blood to pass into the urine, their proportion increasing as that of the re gular constituents of the urine diminished. Division of the vagus did not put a stop to the secretion of urine ; but rhubarb and prussiate of potass taken by the mouth ceased to pass off' by the urine, which at the same time acquired greater specific gravity from containinn• serum of the blood. After divi sion of the spinal cord in the dorsal or lumbar region, the urine became limpid like water ; and division of the sympathetic nerve in the neck caused it to become alkaline and albu minous. BrAchet and Milder have both ex perimented on the effects of the division of the sympathetic nerves which are distributed upon the renal artery. The former divided the trunk, and connected the divided ends by a canula, so as to allow of the continued passage of blood, whilst the nervous influence was completely intercepted ; the latter pro duced the same condition by applying a liga ture around the renal vessels, so tightly as to destroy the texture of the renal nerves at that point, and then relaxing it again, so as to permit the re-establishment of the circu lation. In both cases the effect was similar ; the secretion of true urine being interrupted, but a sanguineous fluid passing into the ureter. Milner states that a remarkable softening of the kidney was always one of the results of these experiments.