47 o TEXT-BOOK OF PHYSIOLOGY 



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 delrusor urin& acquires a certain degree of tension 

 or tonus. This is followed by rhythmic contractions of the detrusor urinse 

 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 accelerator 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 governing center 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 

 there is a return to the former condition, namely, a contraction of the sphincter 

 muscles and a gradually inhibition of the detrusor muscle. If the act of 

 urination is to be suppressed volitional impulses descend the cord and cause 

 an increased contraction of the sphincter urethrae muscle, whereby the action 

 of the reflex mechanism is for a while opposed. 



The nerve mechanism therefore involves both efferent and afferent nerves 

 as well as nerve-centers in the lumbo-sacral region of the spinal cord. 



Efferent Nerves. The efferent nerve-fibers for the sphincter urethrae 

 muscle have their origin in the spinal cord from which they pass by way of 

 the third and fourth sacral nerves, the pelvic nerve and the inferior hemor- 

 rhoidal nerve directly to the muscle. 



The efferent nerve-fibers, for the detrusor muscle, including the specialized 

 portion, the internal sphincter, have their origin in nerve-cells in the lumbo- 



