484 TEXT-BOOK OF PHYSIOLOGY. 



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

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

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

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

 ular 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 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 entire organ. 

 As these fibers by their contraction expel the urine from the bladder, they 

 are known collectively as the detrusor urina muscle. At the exit of the bladder 

 the circular fibers are somewhat increased in number, giving rise to the 

 appearance of a distinct muscle band which has been termed the sphincter 

 vesicce muscle. The presence of this muscle has, however, been denied and 

 the retention of the urine has been attributed to mechanic conditions at the 

 neck of the bladder. The urethra just beyond the bladder is provided with 

 a distinct circular muscle composed of striated fibers, the sphincter urethra 

 muscle. At the close of an act of urination or micturition the bladder is 

 small, contracted, and its cavity is almost obliterated, but as urine is con- 

 tinually descending the ureter and entering the bladder at its base, the 

 detrusor muscle gradually relaxes or becomes sufficiently inhibited from 

 moment to moment to receive it. The escape of urine into the urethra is 

 prevented either by mechanic conditions or by the contraction of the sphincter 



