474 TEXT-BOOK OF PHYSIOLOGY 



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 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 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 

 muscle at the vesic orifice. When the accumulating urine reaches a certain 

 volume, it gives rise to an intra- vesic pressure. When this pressure rises to 

 about 80 cm. of water the detrusor urina muscle acquires a certain degree 

 of tension or tonus. This is followed by rhythmic contractions of the 

 detrusor muscle which increase in extent and vigor as the urine continues 

 to accumulate until finally a general contraction develops, the force of which 

 overcomes the constricting influences at the bladder orifice and the fluid 

 is discharged. This action of the detrusor muscle is generally reinforced 

 by the contraction of the abdominal muscles. The latter portions of the 

 urine are ejected through the urethra by the rhythmic action of the ac- 

 celerator urinae muscles. 



The Nerve Mechanism of Urination. The expulsion of urine is pri- 

 marily a reflex act though subject to a variable amount of volitional control. 

 The reflex character of the act is especially noticeable in young children in 

 whom, by reason of the imperfect development of the brain, there is a lack 

 of volitional control. During the intervals of urination the orifice of the 

 bladder is closed by the tonic contraction of the sphincter vesicae and sphinc- 

 ter urethrae muscles, thus preventing the immediate exit of urine after its 

 descent into the bladder. The tonic contraction of both muscles is main- 

 tained by the activity of nerve-centers in the lumbar region of the spinal 

 cord. The detrusor muscle is at the same time in a more or less relaxed 

 condition, the result of an inhibition of its motor centers in the spinal 

 cord. 



When the accumulating urine reaches a certain volume it causes, as 

 previously stated, an intra-vesic pressure, an increased tonus of the detrusor 

 muscle, followed by slight rhythmic contractions of its fibers. 



When the desire to urinate is experienced impressions are being made on 

 the afferent nerve endings in the mucous membrane of the bladder. The 

 nerve impulses thus developed are transmitted to the urination center in the 

 spinal cord and to the cerberum and influence in one direction or another 

 their activities. In a young child the arrival of the transmitted impulses in 

 the spinal cord is immediately followed by an inhibition of the sphincter 

 centers and a stimulation of the detrusor center, as a result of which the 

 sphincter muscles relax and the detrusor muscle contracts, thus expelling 

 the urine. In the adult, if the act of urination is to be permitted the same 

 mechanism is brought into action. In its expulsive efforts the detrusor mus- 

 cle is assisted by the contraction of the abdominal muscles and possibly the 

 diaphragm in response to volitional efforts. After the discharge of urine 



