398 ESSENTIALS OF PHYSIOLOGY. 



or other predominating according to circumstances. The drinking of a 

 large quantity of water, for instance, is followed by its rapid excretion 

 through the glomeruli, by which most of the water of the urine is 

 normally excreted. 



SECTION III. 

 MICTURITION. 



The urine formed in the kidneys passes along the ureters to the 

 bladder, where it accumulates, the bladder being emptied from time to 

 time by the process of micturition. The flow of urine along the ureters 

 is assisted by rhythmic waves of contraction, passing down from the 

 pelvis of the kidney to the bladder at intervals of a few seconds ; they 

 can still be observed in the ureter when it is isolated from the central 

 nervous system. 



The wall of the bladder consists of unstriated muscle fibres arranged 

 in three layers, an outer and an inner longitudinal layer, and a middle 

 layer of fibres running circularly ; it is lined by transitional epithelium. 

 When the muscular walls contract they lessen the size of the cavity. 

 The escape of urine from the relaxed bladder is prevented by two 

 sphincters, namely, first, circular unstriped muscular fibres, forming 

 a loop round the orifice of the bladder and called the trigonal sphincter, 

 and, secondly, the sphincter urogenitalis, or compressor urethrse, which 

 encloses the second part of the urethra, and is composed of striated 

 muscular fibres. The bladder receives its nerve supply from (1) the 

 nervi erigentes, stimulation of which causes it to contract, and (2) 

 sympathetic fibres from the hypogastric plexus, stimulation of which 

 is followed in some animals by inhibition and in others by contraction 

 of the bladder wall ; afferent fibres also pass from the bladder in the 

 nervi erigentes to the spinal cord. 



Micturition is normally carried out as a reflex action which, in the 

 adult, is controlled and can be inhibited by impulses from the higher 

 parts of the brain. Distension of the bladder wall gives rise to impulses 

 which, travelling to the spinal cord, reflexly bring about emptying 

 of the bladder by the contraction of its muscular coat, the nerve cells 

 concerned in the reflex lying in the sacral region of the cord ; the 

 escape of urine is made possible by the simultaneous relaxation of the 

 sphincter muscles. The intensity of the afferent impulses varies with 

 the rate of filling of the bladder. When the bladder fills slowly, its 

 muscular wall relaxes, and it may contain a considerable amount of 

 urine before any appreciable tension is placed upon its muscular fibres. 

 On the contrary, when urine is being formed rapidly, the tension within 



