which the muscles required for the ejection of the secreted matter are called into action. This sensation arises whenever the urine has accumulated to the necessary extent, or when it possesses irritating qualities, owing to extraneous substances being contained in, or deposited from, it; or if the bladder be unusually irritable from any morbid cause, the sensation may be repeatedly, nay, almost incessantly experienced. The remarks, that have been made on the sensations accompanying the other excretions, are equally applicable here. The impression takes place in the bladder; such impression is conveyed to the brain, which accomplishes the sensation; and, consecutively, the muscles concerned in the excretion, are called into action by volition.

Physiologists have differed regarding the power of volition over the bladder. Some have affirmed, that it is as much under cerebral control as the muscles of locomotion; and they have adduced, in support of this view, that the bladder receives spinal nerves, which are voluntary; that it is paralyzed in affections of the spinal marrow, like the muscles of the limbs; and that a sensation which seems destined to arouse the will is always the precursor of its

action.

Others, again, have denied, that the muscular fibres of the bladder are contractile under the will; and they adduce the cases of other reservoirs, the stomach and the rectum, for example,—whose influence in excretion we have seen to be involuntary; as well as the fact, that we no more feel the contraction of the bladder than we do that of the stomach or intestines; and they affirm, that the action of the bladder itself has been confounded with that of the accessory muscles, which are manifestly under the influence of the will, and are important agents in the expulsion of the fluid from the bladder.

The views, last expressed, appear to be most accurate, and the catenation of phenomena seems to be as follows:—the sensation to expel the urine arises; the abdominal muscles are thrown into contraction by volition; the viscera are thus pressed down upon the pelvis; the muscular coat of the bladder is, at the same time, stimulated to contraction; the levatores ani and the sphincter fibres are relaxed, so that the resistance of the neck of the organ is diminished, and the urine is forced out through the whole extent of the urethra, being aided in its course, especially towards the termination, by the contractile action of the urethra itself, as well as by the levatores ani and acceleratores urinæ muscles. These expel the last drops by giving a slight succussion to the organ, and directing it upwards and forwards; an effect which is aided by shaking the organ to free it from the drops that may exist in the part of the canal near its extremity. The gradually diminishing jet, which we notice, as the bladder is becoming empty, indicates the contraction of the muscular coat of the bladder; whilst the kind of intermittent jet, coincident with voluntary muscular exertion, indicates the contraction

of the urethral muscles. When we feel the inclination to evacuate the bladder, and do not wish to obey it, the same muscles,—the levatores ani, the acceleratores urinæ, and the fibres around the membranous portion of the urethra and the neck of the bladderare thrown into contraction, and resist that of the bladder.

Such is the ordinary mechanism of the excretion of urine. The contraction of the bladder is, however, of itself sufficient to expel its contents. Magendie affirms, that he has frequently seen dogs pass the urine when the abdomen was opened, and the bladder removed from the influence of the abdominal muscles; and he farther states, that if, in a male dog, the bladder, with the prostate and a small portion of the membranous part of the urethra, be removed from the body, the bladder will contract after a few moments, and project the urine, with an evident jet, until it is entirely expelled.

Urine,—voided in the morning by a person who has eaten heartily, and taken no more fluid than sufficient to allay thirst,—is a transparent, limpid fluid, of an amber colour, saline taste, and a peculiar odour. Its specific gravity is a little above that of water, or 1.030. It is slightly acid, for it reddens vegetable blues. Although at first quite transparent, it deposits an insoluble matter on standing; so that urine, passed at bed-time, is found to have a light cloud floating in it by the following morning. This substance consists, in part, of mucus from the urinary passages; and, in part, of the super-urate of ammonia, which is much more soluble in warm than in cold water.

The urine is extremely prone to decomposition. When kept for a few days, it acquires a strong smell, which, being sui generis, has been called urinous; and as the decomposition proceeds, the odour becomes extremely disagreeable. The urine, as soon as these changes commence, ceases to have an acid reaction, and the earthy phosphates are deposited. In a short time, a free alkali makes its appearance; and a large quantity of the carbonate of ammonia is generated. These phenomena are owing to the decomposition of urea, which is almost wholly resolved into carbonate of ammonia.

Dr. Henry affirms, that the following substances have been satisfactorily proved to exist in healthy urine,—water, free phosphoric acid, phosphate of lime, phosphate of magnesia, fluoric acid, uric acid, benzoic acid, lactic acid, urea, gelatine, albumen, lactate of ammonia, sulphate of potash, sulphate of soda, fluate of lime, muriate of soda, phosphate of soda, phosphate of ammonia, sulphur, and silex.

The most recent and elaborate analysis of the urine has been given by Berzelius. He states it to consist in 1000 parts, of water, 933.00; urea, 30.10; sulphate of potassa, 3.71; sulphate of soda, 3.16; phosphate of soda, 2.94; muriate of soda, 4.45; phosphate of

In Vol. I. p. 17, this acid is accidentally said to be "found in the urine of man, birds, serpents, &c. and silk worms," instead of, "and in the excrements of silk worms."

VOL. II.

31

ammonia, 1.65: muriate of ammonia, 1.50; free lactic acid, lactate of ammonia, animal matter soluble in alcohol, and urea not separable from the preceding, 17.14; earthy phosphates, with a trace of fluate of lime, 1.00; lithic acid, 1.00; mucus of the bladder, 0.32; silex, 0.03.

The yellowish-red incrustation, deposited on the sides of chamber utensils, is the uric acid. This is the basis of one of the varieties of calculi.

The quantity of urine, passed in the twenty-four hours, is very variable. On the average, it is estimated at two pounds, or two pounds and a half; hence the cause of the great size of the renal artery, which, according to the estimate of Haller, conveys to the kidney a sixth or eighth part of the whole blood. Its quantity and character vary according to age, and, to a certain extent, according to sex. We have already seen, under the head of cutaneous exhalation, how its quantity varies, according to climate and season; and it is influenced by the serous, pulmonary, and cellular exhalations likewise: one of the invariable concomitants of dropsy is diminution in the renal secretion. Its character, too, is modified by the nature of the substances received into the blood. Rhubarb, turpentine, and asparagus materially alter its physical properties; whilst certain articles stimulate the kidney to augmented secretion, or are diuretics.

The urine does not appear to be intended for any local function. Its use seems to be restricted to the removal of the elements of the substances, of which it is composed, from the blood; hence it is solely depuratory and decomposing.

How this decomposition is accomplished, we know not. We have already referred to the experiments, performed by MM. Prevost and Dumas, and by Segalas, in which urea was found in the blood of animals whose kidneys had been extirpated: an inquiry has consequently arisen,—how it exists there. Prior to these experiments, it was universally believed, that its formation is one of the mysterious functions executed in the intimate tissue of the kidney.

The peculiarity of urea is, that it contains a very large proportion of azote, so that it has been imagined, the kidney may possibly be the outlet for any excess of nitrogen, or for preventing its accumulation in the system.

The quantity of azote discharged in the form of urea, is so great, even in those animals, whose food does not essentially contain this element, that it has been conceived a necessary ingredient in the nutrition of parts, and especially in the formation of fibrine, which, we have seen to be a chief constituent of the blood, and of every muscular organ. The remarks, made on the absorption of azote during respiration, indicate how it is received into the system; and it has been presumed, that the superfluous portion is thrown off in the form of urea.

The experiments of MM. Prevost and Dumas, and of Segalas, would certainly favour the conclusion, that urea may exist ready formed in the blood, and that the great function of the kidney may be to separate it along with the other constituents of the urine.

Adelon ascribes the source of the urea to the products of interstitial decomposition. He conceives, that, in this shape, they are received into the blood, and that the office of the kidneys is to separate them. All this is necessarily conjectural, and it must be admitted, that our knowledge of the subject is by no means ample, and that we must wait for farther developments.

Certain it is, that the removal of the constituents of the urinary secretion from the blood is all important. Experiments on animals have shown, that if it be suppressed by any cause for about three days, death supervenes, and the dangers to man are equally imminent.

In consequence of the rapidity with which fluids, received into the stomach, are sometimes voided by the urinary organs, it has been imagined, either that vessels exist which communicate directly between the stomach and bladder, or that the fluid passes through the intermediate cellular tissue, or by means of the anastomoses of the lymphatics.

In support of the opinion that a more direct passage exists, the assertion of Chirac,—that he saw the urinary bladder become filled with urine, when the ureters were tied, and that he excited urinous vomiting, by tying the renal arteries, is adduced. It has been farther affirmed, that the oil, composing a glyster, has been found in the bladder. Darwin, having administered to a friend a few grains of nitrate of potassa, collected his urine at the expiration of half an hour, and had him bled. The salt was detected in the urine, but not in the blood. Brande made similar experiments with the prussiate of potassa, from which he inferred, that the circulation is not the only medium of communication between the stomach and the urinary organs, without, however, indicating the nature of the supposed medium; and this view is embraced by Sir Everard Home, Wollaston, Marcet, and others. Very recently, Lippi, of Florence, thinks he has found an anatomical explanation of the fact. According to him, the chyliferous vessels have not only numerous inosculations with the mesenteric veins, either before their entrance into the mesenteric glands, or whilst they traverse the glands, but, when they attain the last of those glands, some of them proceed to open directly into the renal veins, and into the pelves of the kidneys. At this place, according to him, the chyliferous vessels divide into two sets; the one, ascending and conveying the chyle into the thoracic duct; the other, descending and carrying the drinks into the renal veins and pelves of the kidneys. He affirms, that the distinction between these two sets is so marked, that an injection, sent into the former, goes exclusively into the thoracic duct,

whilst if it be thrown into the latter it passes exclusively to the kidneys. These direct vessels Lippi calls vasa chylopoietica urinifera.

If the assertions of Lippi were anatomical facts, it would be obviously impossible to doubt some of the deductions; other anatomists have not, however, been so fortunate as he; and, consequently, it may be well to make a few comments.

Some of these chylopoietica urinifera, he affirms, open into the renal veins. This arrangement, it is obvious, cannot be invoked to account for the shorter route, the royal road to the kidney: the renal vessel is conveying the blood back from the kidney, and every thing, that reaches it from the intestines, must necessarily pass into the vena cava, and ultimately attain the kidney through the renal artery. The vessels, therefore, that end in the renal veins, must be put entirely out of the question, so far as regards the topic of dispute; and our attention be concentrated upon those that terminate in the pelvis of the kidney. Were this termination proved, we should be compelled, as we have remarked, to bow to authority; but not having been so, it may be stated as seemingly improbable, that the ducts in question should take the circuitous course to the pelvis of the kidney, instead of proceeding directly to the bladder.

We know, then, anatomically, nothing of any canal existing between the stomach and the bladder; and we have not the slightest evidence,—positive or relative,—in favour of the opinion, that there is any transmission of fluid through the intermediate cellular tissue. We have, indeed, absolute testimony against it. MM. Tiedemann and Gmelin, having examined the lymphatics and cellular tissue of the abdomen in cases where they had administered indigo and essence of turpentine to animals, discovered no traces whatever of them, whilst they could be detected in the kidney.

The facts, again, referred to by Chirac, are extremely doubtful. If the renal arteries be tied, the secretion cannot be effected by the kidney; consequently, no urine ought to be found in the economy, and urinous vomitings would be impossible. On the other hand, if the ureters be tied, the secretion being practicable, death will occur if the suppression be protracted; and, in such case, the secreted fluid may pass into the vessels and give a urinous character to the perspiration, vomited matters, &c. &c.

Again, the experiments of Darwin, Brande, Wollaston, and others only demonstrate, that these gentlemen were unable to detect that in the blood which they found in the urine. Against the negative results attained by these gentlemen, we may adduce the positive testimony of Fodera, an experimentalist of weight, especially on those matters. He introduced into the bladder of a rabbit a plugged catheter, and tied the penis upon the instrument to prevent the urine from flowing along its sides. He then injected into the stomach a solution of the ferrocyanate of potassa. This being done, he frequently removed the plug of the catheter, and received the drops of urine on filtering paper: as soon as indications of