EXCRETION 47 1 



is much greater in cardiac disease with dropsical effusions than 

 in health. Caffein, when injected into the blood, affects the 

 pressure but little. It causes dilatation of the renal vessels after 

 a passing constriction, and an increase in the flow of urine after a 

 temporary diminution. The vascular dilatation is not the chief 

 reason for the diuretic effect, for the latter is still obtained when the 

 vaso-motor mechanism has been paralyzed by chloral hydrate, and 

 even after the secretion of urine has been stopped by the fall of 

 pressure consequent on section of the spinal cord. Caffein, there- 

 fore, acts directly on the renal epithelium. The action of urea, 

 potassium nitrate, and the saline diuretics is probably also a direct 

 action on the secreting structures, although some have supposed that 

 their primary effect is to cause vaso-dilatation in the kidney, and a 

 consequent local increase in the capillary pressure. The influence 

 of anaesthetics on diuresis is of practical importance. Ether generally 

 increases, while chloroform generally diminishes the flow of urine 

 in dogs. A.C.E. mixture has a variable effect, but there is always 

 a marked after-increase. A mixture of ether and chloroform con- 

 stitutes the ideal anaesthetic for experiments on the kidney, since 

 it alters the diuresis only slightly (Thompson) . 



Summary. Our knowledge of renal secretion may be thus 

 summed up : The water and salts of the urine are chiefly separated 

 by the glomeruli ; the process is not a mere physical filtration, but 

 a true secretion. Substances like sugar, peptone, egg-albumin, and 

 hemoglobin when injected into the blood are probably excreted mainly 

 by the glomeruli ; and so is the sugar of diabetes. Urea, uric acid, 

 and presumably the other organic constituents of normal urine, 

 with a portion of the water and salts, are excreted by the physio- 

 logical activity of the ' rodded ' epithelium of the renal tubules. The 

 rate of secretion of urine rises and falls with the pressure, and 

 still more with the velocity, of the blood in the renal vessels. No 

 secretory nerves for the kidney have been found ; the effects of section 

 or stimulation of nerves on the secretion can all be explained by the 

 changes produced in the renal blood-flow. Some diuretics act by 

 increasing the blood-flow, others directly on the epithelium of the 

 tubules or the glomeruli. 



Micturition. The urine, like the bile, is being constantly 

 formed ; although secretion varies in its rate from time to time, 

 it never ceases. Trickling along the collecting tubules, the urine 

 reaches the pelvis of the kidney, from which it is propelled along 

 the ureters by peristaltic contractions of their walls, and drops 

 from their valve-like orifices into the bladder. When this 

 becomes distended, rhythmical peristaltic contractions are set 

 up in it, and notice is given of its condition by a characteristic 

 sensation, which is perhaps aided by the squeezing of a few drops 

 of urine past the tonically contracted circular fibres that form 

 a sphincter round the neck of the bladder, and into the first part 

 of the urethra. The desire to empty the bladder can be resisted 

 for a time, as can the desire to empty the bowel. If it is yielded 

 to, the smooth muscular fibres in the wall of the viscus are thrown 



