EXCRETION 469 



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



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

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

 sverage 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- 

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

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

 Ln 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, 

 odium 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 

 jnanner'in which these agents and other members of their class, the so-called 

 aline diuretics, increase renal activity is yet a subject of discussion. On the 

 bne hand, it is stated that they promote an absorption of water from the 

 issues to such an extent that a condition of hydremic plethora is produced, 

 vhich in itself increases not only the general blood-pressure but the local 

 enal pressure as well, and that it is this factor which is the cause of the 

 ncreased flow of urine. On the other hand, it is asserted that though the 

 ,alts increase the local pressure and the volume of the kidney, they never- 

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

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

 he 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 

 -he urinif erous tubules and by them are discharged into the pelvis of the 

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

 aladder. 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 



the ureter. 



The Bladder. The bladder is a reservoir for the temporary reception of 

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

 n 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 unna muscle. At 1 

 exit of the bladder the circular fibers are somewhat increased in number 

 laiving rise to the appearance of a distinct muscle band which has been terme 

 the sphincter vesica muscle. The presence of this muscle has, however, been 

 denied and the retention of the urine has been attributed to mechanic condi 

 tions at the neck of the bladder. The urethra just beyond the bladder is pro 

 vided with a distinct circular muscle composed of striated fibers, the sph^ter 

 urethra muscle. At the close of an act of urination or micturition the bladd 

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

 Itinually descending the ureter and entering the bladder at its .base the 

 detrusor muscle gradually relaxes or becomes sufficiently inhibited from 

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



