EXCRETION 473 



peripheral ends is followed by a constriction of the vessels and a cessation 

 of the flow of urine. 



The vaso-motor center for the blood-vessels of the kidney is in all proba- 

 bility situated in the medulla oblongata in close proximity to the general 

 vaso-motor centers, though subordinate centers are doubtless present in the 

 spinal cord. It was found by Bernard that puncture 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 to local vaso-motor centers, thence through the 

 splanchnics to the renal ganglia, thence through the renal plexus to the 

 blood-vessels. 



The Influence of Variations in the Composition of the Blood. As it 

 is the function of the kidneys to excrete water, inorganic salts, and various 

 end-products of metabolism from the blood and thus maintain its general 

 average composition, it is highly probable that as soon as they accumulate 

 beyond a certain percentage they themselves act as stimuli to renal activity, 

 either by acting directly on the renal epithelium or by increasing the glomeru- 

 lar pressure. There is evidence at least that 'urea acts in the former manner 

 and that an excess of water in the blood, from copious drinking or from a sud- 

 den checking of the skin from a fall of temperature, acts in the latter manner. 

 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 never- 

 theless act specifically on the renal epithelium, and therefore may be regarded 

 as secreto-motor agents. An increase in the percentage of sugar or urea in 

 the blood has a similar influence on the kidney. 



The Storage and Discharge of Urine. Urination. The urinary con- 

 stituents, as soon as they are eliminated from the blood, pass into and through 

 the uriniferous tubules and by them are discharged into the pelvis of the 

 kidney. They then enter the ureter by which they are conducted to the 

 bladder. The immediate cause of this movement is undoubtedly a difference 

 of pressure between the terminal portions of the tubules and the terminal 

 portion of the ureter, aided by the peristaltic contraction of the muscle wall 

 of the ureter. 



The Bladder. 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 mucous. The 

 muscle coat consists of external longitudinal and internal circular and oblique 

 layers of fibers of the non-striated variety which collectively encircle the en- 

 tire organ. As these fibers by their contraction expel the urine from the 

 bladder, they are known collectively as the detrusor urina muscle. At the 



