EXCRETION. 433 



less present in the spinal cord. It was found by Bernard that punc- 

 ture of the medulla was occasionally followed by a profuse secretion 

 of urine without the presence of sugar. The route of the vaso-motor 

 impulses which influence the renal blood- supply is down the cord 

 through the splanchnics and through the renal plexus. 



Influence of Variations in the Composition of the Blood. 

 As it is the function of the kidneys to excrete water and various end- 

 products from the blood and thus maintain a general average com- 

 position, it is highly probable that as soon as they accumulate be- 

 yond a certain percentage they themselves act as stimulants to renal 

 activity, either by acting directly on the renal epithelium or by in- 

 creasing the glomerular pressure. There is evidence at least that 

 urea acts in the former manner. An excess of water in the blood 

 from copious drinking or from a sudden checking of the skin from a 

 fall of temperature will act in the latter way. The introduction into 

 the blood of inorganic salts, such as potassium nitrate, sodium 

 acetate, etc., will in a short time lead to increased activity of the 

 kidneys, as shown by an increase in the quantity of urine excreted. 

 The manner in which these agents and other members of their class, 

 the so-called saline diuretics, increase renal activity is yet a subject 

 of discussion. On the one hand, it is stated that they promote an 

 absorption of water from the tissues to such an extent that a condition 

 of hydremic plethora is produced, which in itself increases not only 

 the general blood^pressure but the local renal pressure as well, and 

 that it is this factor which is the cause of the increased flow of urine. 

 On the other hand, it is asserted that though the salts increase 

 the local pressure and the volume of the kidney, they nevertheless 

 act specifically on the renal epithelium, and therefore may be re- 

 garded as secreto-motor agents. An increase in the percentage of 

 sugar or urea in the blood has a similar influence on the kidney. 



MICTURITION. 



Movement of the Urine. The urine, as soon as it is poured 

 into the uriniferous tubules, flows through the tubules into the pelvis, 

 through the ureter into the bladder. The immediate cause of this 

 movement is undoubtedly a diflererice_of_pressure^ between the 

 terminal portions of the JubulfiSi ., jamijt^e terrmnaT portion of the 

 u^rg^c, aided by the peristaltic contraction of the muscle wall of 

 the ureter. 



The bladder is a reservoir for the temporary reception of the urine 

 prior to its expulsion from the body. When distended it is ovoid in 

 shape and is capable of holding from 600 to 800 cu. cm. The bladder 

 is composed of four coats: viz., serous, muscle, areolar, and mu- 

 cous. The muscle coat consists of an external longitudinal and an 

 internal circular and oblique sets of fibers of the non-striated variety 

 28 



