layer, giving off on their way branches, which form a fine arterial meshwork around the tubes, and ending in a similar plexus from which the venous radicles arise.

Besides the small afferent arteries of the Malpighian bodies, there are, of course, others which are distributed in the ordinary manner, for the nutrition of the different parts of the organ; and in the pyramids, between the tubes, there are numerous straight vessels, the vasta recta, some of which are branches of vasa efferentia from Malpighian bodies, and therefore comparable to the venous plexus around the tubules in the cortical portion, while others arise directly as small branches of the renal arteries.

Between the tubes, vessels, etc., which make up the substance of the kidney, there exists, in small quantity, a fine matrix of areolar tissue.

Nerves.-The nerves of the kidney are derived from the renal plexus.

The Ureters.-The ducts of each kidney, or ureter, is a tube about the size of a goose-quill, and from twelve to sixteen inches in length, which, continuous above with the pelvis of the kidney, ends below by perforating obliquely the walls of the bladder, and opening on its internal surface.

Structure.It is constructed of three principal coats (a) an outer, tough, fibrous and elastic coat; (b) a middle muscular coat, of which the fibres are unstriped, and arranged in three layers—the fibres of the central layer being circular, and those of the other two longitudinal in direction; and (e) an internal mucous lining continuous with that of the pelvis of the kidney above, and of the urinary bladder below. The epithelium of all these parts (fig. 250) is alike stratified and of a somewhat peculiar form; the cells on the free surface of the mucous membrane being usually spheroidal or polyhedral with one or more spherical or oval nuclei ; while beneath these are pear-shaped cells, of which the broad ends are directed towards the free surface, fitting in beneath the cells of the first row, and the apices are prolonged into processes of various lengths, among which, again, the deepest cells of the epithelium are found spheroidal, irregularly oval, spindle-shaped or conical.

The Urinary Bladder.-The urinary bladder, which forms a receptacle for the temporary lodgment of the urine in the intervals of its expulsion from the body, is more or less pyriform, its widest part, which is situate above and behind, being termed the fundus:

and the narrow constricted portion in front and below, by which it becomes continuous with the urethra, being called its cervix or neck. Structure. It is constructed of four principal coats,-serous, muscular, areolar or submucous, and mucous. (a) The serous coat, which covers only the posterior and upper half of the bladder, has the same structure as that of the peritoneum, with which it is continuous. (b) The fibres of the muscular coat, which are unstriped, are arranged in three principal layers, of which the external and internal have a general longitudinal, and the middle

Fig. 250.-Epithelium of the bladder; a, one of the cells of the first row; b, a cell of the second row; c, cells in situ, of first, second, and deepest layers (Obersteiner).

layer a circular direction. The latter are especially developed around the cervix of the organ, and are described as forming a sphincter vesica. The muscular fibres of the bladder, like those of the stomach, are arranged not in simple circles, but in figure-of-8 loops. (c) The areolar or submucous coat is constructed of connective tissue with a large proportion of elastic fibres. (d) The mucous membrane, which is rugose in the contracted state of the organ, does not differ in essential structure from mucous membranes in general. Its epithelium is stratified and closely resembles that of the pelvis of the kidney and the ureter (fig. 250).

The mucous membrane is provided with mucous glands, which are more numerous near the neck of the bladder.

The bladder is well provided with blood- and lymph-vessels, and with nerves. The latter are branches from the sacral plexus (spinal) and hypogastric plexus (sympathetic). A few ganglion-cells are found, here and there, in the course of the nerve-fibres.

The Urine.

Physical Properties.-Healthy urine is a perfectly transparent, amber-coloured liquid, with a peculiar, but not disagreeable odour, a bitterish taste, and slight acid reaction. Its specific gravity varies from 1015 to 1025. On standing for a short time, a little mucus appears in it as a flocculent cloud.

Chemical Composition.-The urine consists of water, holding in solution certain organic and saline matters as its ordinary constituents, and occasionally various other matters; some of the latter are indications of discased states of the system, and others are derived from unusual articles of food or drugs taken into the stomach.

Reaction. The normal reaction of the urine is slightly acid. This acidity is due to acid phosphate of sodium, and is less marked

soon after meals. The urine contains no appreciable amount of free acid, as it gives no precipitate of sulphur with sodium hyposulphite. After standing for some time the acidity increases from a kind of acid fermentation, due in all probability to the presence of mucus and fungi, and acid urates or free uric acid is deposited. After a time, varying in length according to the temperature, the reaction becomes strongly alkaline from the change of urea into ammonium carbonate, due to the presence of one or more specific microorganisms (micrococcus urea). The urea takes up two molecules of water-a strong ammoniacal and fœtid odour appears, and deposits of triple phosphates and alkaline urates take place. This does not occur unless the urine is freely exposed to the air, or, at least, until air has had access to it.

Reaction of Urine in different classes of Animals.—In most herbivorous animals the urine is alkaline and turbid. The difference depends, not on any peculiarity in the mode of secretion, but on the differences in the food on which the two classes subsist; for when carnivorous animals, such as dogs, are restricted to a vegetable diet, their urine becomes pale, turbid, and alkaline, like that of an herbivorous animal, but resumes its former acidity on the return to an animal diet; while the urine voided by herbivorous animals, e.g., rabbits, fed for some time exclusively upon animal substances, presents the acid reaction and other qualities of the urine of Carnivora, its ordinary alkalinity being restored only on the substitution of a vegetable for the animal diet. Human urine is not usually rendered alkaline by vegetable diet, but it becomes so after the free use of alkaline medicines, or of the alkaline salts with carbonic or vegetable acids; for these latter are changed into alkaline carbonates previous to elimination by the kidneys.

Average daily quantity of the chief constituents of the Urine (by healthy male adults).

Variations in the Quantity of the Constituents.—From the proportions given in the above table, most of the constituents are, even in health, liable to variations. The variations of the water in different seasons, and according to the quantity of drink and exercise, have already been mentioned. The water of the urine is also liable to be influenced by the condition of the nervous system, being sometimes greatly increased, e.g., in hysteria, and in some other nervous affections; and at other times diminished. In some diseases it is enormously increased; and its increase may be either attended with an augmented quantity of solid matter, as in ordinary diabetes, or may be nearly the sole change, as in the affection termed diabetes insipidus. In other diseases, e.g., the various forms of albuminuria, the quantity may be considerably diminished. A febrile condition almost always diminishes the quantity of water; and a like diminution is caused by any affection which draws off a large quantity of fluid from the body through any other channel than that of the kidneys, e.g., the bowels or the skin.

Method of estimating the Solids.-A useful rule for approximately estimating the total solids in any given specimen of healthy urine is to multiply the last two figures representing the specific gravity by 2:33. Thus, in urine of sp. gr. 1025, 2'33 × 25=58·25 grains of solids, are contained in 1000 grains of the urine. In using this method it must be remembered that the limits of error are much wider in diseased than in healthy urine.

Variations in the Specific Gravity.-The average specific gravity of the human urine is about 1020. The relative quantity of water and of solid constituents of which it is composed is materially influenced by the condition and occupation of the body during the time at which it is secreted; by the length of time which has elapsed since the last meal; and by several other accidental circumstances. The existence of these causes of difference in the composition of the urine has led to the secretion being described under the three heads of Urina sanguinis, Urina potus, and Urina cibi. The first of these names signifies the urine, or that part of it which is secreted from the blood at times in which neither food nor drink has been recently taken, and is applied especially to the urine which is evacuated in the morning before breakfast. The terms urina potus indicates the urine secreted shortly after the introduction of any considerable quantity of fluid into the body: and the urina cibi, the portions secreted during the period immediately succeeding a meal of solid food. The last