k 



THE EXPULSION OF THE URINE. MICTURITION 1077 



are developed at times within the bladder during its periods of con- 

 traction, cannot possibly interfere with the vascular supply nor 

 the secretory function of the kidneys. ^ Very different conditions, 

 however, arise if the pressure in the ureter itself is raised excessively, 

 a condition commonly associated with the stagnation of the urine in 

 consequence of renal calculi. A very decided reduction in the blood 

 flow through the corresponding kidney then results which cannot 

 remain without effect upon its secretion. 



Physiologically, the bladder must be considered as a hollow muscu- 

 lar organ, the contraction of which places its contents under a consider- 

 able pressure. 2 Since the orifices of the ureters are closed, the pressure 

 so developed must be directed toward the internal and external sphinc- 

 ters. The resistance of the latter, however, cannot be overcome by 

 pressure alone, and hence, the voiding of urine or act of micturition 

 must also necessitate the relaxation of these bands of muscle tissue, 

 the inner one by reflex action and the outer one volitionally. A third 

 factor at work during this process is the abdominal press. It will be 

 remembered that the latter consists in an inspiratory action which 

 is immediately followed by a closure of the laryngeal orifice and a con- 

 traction of the abdominal muscles and diaphragm. The increase in 

 the abdominal pressure produced thereby is propagated unto the pelvic 

 organs and favors micturition as well as defecation. In most in- 

 stances, however, it is not brought into play until the final stages of 

 these acts. 



Under ordinary conditions, micturition does not result until the 

 pressure in the bladder has risen to about 150 mm. II2O, i.e., at a time 

 when this organ has been distended sufficiently to contain between 

 230 and 250 c.c. of urine. To begin with, of course, the pressure 

 increases very slowly, owing to the constant relaxation of the walls of 

 the bladder. Eventually, however, as the tissues have about attained 

 their maximal degree of stretching, the pressure rises more rapidly 

 and finally evokes a series of slight rhythmic oscillations which are 

 soon succeeded by more forcible contractions. But, much depends 

 upon the rapidity with which the bladder is being filled, because a slow 

 influx of urine enables the different muscle fibers to lengthen more 

 gradually, while a more rapid influx causes them to react antagonisti- 

 cally by tonic or rhythmic contractions, thereby evoking micturition 

 much sooner. Furthermore, since the external sphincter is under the 

 control of the will, the forceful contraction of this ring of muscle tissue 

 may overcome these reflexes, at least for a short time. When aided 

 by the abdominal press, a pressure of 2 m. H2O may be produced. 



The Nervous Control of the Bladder. — The reflex center for mictu- 

 rition is situated in the lumbosacral segment of the spinal cord, whence 

 connections are formed with the higher centers. In this way, volition 



1 Burton-Opitz, Pfliiger's Archiv, cxxiii, 19. 



2 Rehfisch, Virchow's Archiv, xl, 1897, iii; also: Mosso and Pellacani, Arch, 

 ital. de biol., i, 1882, 291. 



