The urine secreted by the kidneys enters the pelvis of these organs and then trickles down the ureters (fig. 131, a) to the bladder. It is remarkable that a kind of alternation in functional activity takes place between the two kidneys, so that first one and then the other kidney secretes a few drops of urine and rests awhile. The ureters pierce the coats of the bladder obliquely, a disposition of parts which, whilst it presents no obstacle to the flow of urine from the kidney to the bladder, effectually prevents any influx of fluid from the bladder towards the kidney.
The bladder (g, fig. 131) is the receptacle which receives and retains for some hours and then expels the urine which is excreted. It weighs about a pound, and when distended it is capable of containing about four quarts of fluid. It is situated above the pubes and below the rectum, and is covered in front and behind by the peritoneum; inferiorly this coat is wanting. When greatly distended the bladder projects forwards into the abdomen, but in the ordinary condition of being partially filled it is contained within the bony pelvis. It is a musculo-membranous bag. The muscular tissue is arranged in several layers, some fibres being longitudinal, others transverse, and others again oblique, and by their contraction the contents of the cavity can be completely expelled through the urethra. Just beyond the point where the bladder and urethra are continuous, one with the other, the muscular fibres are so arranged as to form a strong muscular band encircling the urethra, to which the name of sphincter of the bladder is applied. The office of this muscle, which is in part under the influence of the will, is to close the orifice of exit from the bladder, and it therefore acts in antagonism to the general muscular coat of the bladder. It is necessary that it should relax before any urine can be expelled. If it loses its tone, the urine can no longer be retained, and incontinence of urine occurs.
PLATE XXVIII. CHAMPION STALLION: AMERICAN TROTTER, CRESCENS.
PLATE XXVIII. PACING STALLION: AMERICAN TROTTER, STAR POINTER.
Copyright photo. by Sclireiber, 1895.
The internal coat of the bladder is the mucous membrane. This is a soft, pale rose-coloured layer, which is continuous before and above with the membrane lining the ureters, and behind and below with that lining the urethra. In the empty bladder it presents folds or rugae, which are obliterated as the urine accumulates and distends the organ. It is composed of an external layer of connective tissue, which is in accurate contact with the muscular coat, and of several layers of epithelial cells. The bladder is well supplied with blood-vessels, nerves, and lymphatics. The arteries are derived from the branches of the internal pudic, and the veins return the blood to the internal pudic veins. The lymphatics form a close network on and internal to the muscular layers, and discharge their contents into the sublumbar glands. The nerves are derived from the hypogastric plexus, and partly proceed from the spinal cord and partly from the great sympathetic nerve. The nerve centre controlling the act of staling is situated in the spinal cord at the lower part of the lumbar region, as is shown by the fact that after division of the spinal cord in the dorsal region the bladder can be stimulated to discharge its contents by the application of cold to the buttocks; ordinarily, however, the sensation of fulness of the bladder excites consciousness, and impulses proceed from the brain which, on the one hand, cause the sphincter guarding the opening from the bladder to relax and allow the urine to escape, and, on the other hand, cause the muscular tissue forming the walls of the bladder to contract and force it out.