The passions and emotions have considerable influence over the renal function. Those of a depressing, yet somewhat agitating nature, are peculiarly disposed to act upon the kidneys. Among these is anxiety, or that mixture of fear and hope which anticipates or awaits some important, but doubtful event. The surgeon before he operates, the orator before a great speech, and the young soldier anticipating a battle, afford examples of this kind. Cowardly fear is notorious in its effects; tending not only to increase the secretion, but also, by relaxation of the sphincter of the bladder, to weaken the power of retention. The influence of the hysterical state, and that of the chill of febrile diseases, is probably something of the same nature, operating similarly through the nervous centres.

There is a certain opposition of function between the kidneys and skin, which, in reference to its influence upon the operation of diuretics, is of some importance in a therapeutic point of view. When one of these functions acts in excess, the other is diminished proportionally; and whatever represses one has a tendency to promote the other. The causes of this opposition are not obscure. in the first place, by an excitation of one function, the blood and nervous energy are diverted from others not in direct sympathy with it; and secondly, if one eliminating function be restrained, the circulation is necessarily overloaded, and becomes directly excitant to the other functions, having the same office, which are consequently brought into increased action. There are many substances which will operate as diuretics or diaphoretics, according as a tendency is given of the general current of excitement to one or the other of the secreting organs. By employing measures to direct action to the surface of the body, we give a tendency of the medicine to the perspiratory function; by repressing perspiration by cold or other measures, we send the medicine with the general current of excitement to the kidneys.

A similar opposition of function, though in a less degree, exists between catharsis and diuresis. Many cathartic medicines, especially the saline and vegetable hydragogues, have a disposition, in small doses, insufficient to purge, to act on the kidneys; but, when they produce their cathartic effect, they no longer prove diuretic. This arises partly, no doubt, from the fact that they are carried out of the system with the evacuations they produce; but it arises also from the general principles before stated in relation to the opposition between diaphoretics and diuretics, that a direction of excitement to one function withdraws it from the others, and that, when the liquid parts of the blood and its excrementitious matter are thrown out by one emunctory, the blood is deprived of its ordinary power of exciting the other eliminating organs. Of the effect of excessive purging and sweating in diverting from the kidneys we have an example in epidemic cholera, in the worst cases of which the urine is often nearly or quite suppressed.

From these facts we deduce the therapeutic conclusions, that, when aiming to produce diuresis, we must refrain from measures calculated to produce diaphoresis, or purgation, and, in relation to the perspiratory function especially, will sometimes find advantage in employing means to repress it.

A high state of fever, or a full plethoric state of the circulation, is unfavourable to the action of diuretics, as of other secretory excitants; because the ultimate tissue which performs the function is too highly irritated to act. So is it also in active congestion and inflammation of the kidneys. Under such circumstances, the loss of blood may facilitate the action of diuretics, by diminishing the renal excitation to a point, compatible with the performance of the function. Any other measures which lower the general or local excitement have the same effect. Besides, when the vessels are full, the medicine is less readily absorbed than when they are relatively empty; and, in this way also, depletion and other reducing measures may favour the operation of these medicines. it is supposed that the action of diuretics is often much promoted by the simultaneous use of medicines, having the property of promoting absorption; and hence, in part, the not unfrequent and often useful practice of combining the preparations of mercury and of iodine with the medicines now under consideration.