336 HEALTH AND DISEASE 



kidney at the hilus break up into arches which are situated at the junction 

 of the medullary with the cortical zone. From this region numerous large 

 branches run to the cortex, where they break up into minute afferent 

 branches, one of which runs to each little capsule of Malpighi, and, deeply 

 indenting the wall, divides into a little ball of branching and intertwined 

 capillaries (fig. 133, a), which unite together again to form an efferent vessel. 

 This efferent vessel (fig. 133, v), which would elsewhere be termed a vein, 

 after a short course again divides, as before, like an artery into a net-work 

 of capillaries which are distributed over the outer side of the cortical and 

 convoluted portion of the renal tubules. These, reuniting, form the proper 

 renal veins by which the blood is conveyed out of the gland. 



Mr. Bowman pointed out that the peculiar arrangements of the blood- 

 vessels are well adapted for the secretion and excretion of such a fluid as 

 the urine. The little balls or glomerules of capillaries which are formed by 

 the first divisions of the renal arteries are well adapted to permit the escape 

 of the watery parts of the blood, which, if the expression may be used, 

 flushes the renal tubuli through their entire length, and in doing so dis- 

 solves and washes away the urea and hippuric acid and salts which are 

 secreted by the cells lining the convolute portions of the tubules, and which 

 have been separated by them from the blood coursing, under comparatively 

 low pressure, through the second plexus of capillaries which surrounds them. 



THE ARRANGEMENTS FOR THE RETENTION AND 

 DISCHARGE OF THE URINE 



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 con- 

 tained within the bony pelvis. It is a musculo-membranous bag. The 



