INTRAVESICAL TENSION. 341 



bladder that there is any fall of the pressure at which urine escapes. 

 These facts show that the resistance to the outflow of urine is situated 

 at the neck of the bladder, and must be due either to a tonic contraction 

 of circular fibres at this point, or to the elastic retraction of this orifice, 

 aided, perhaps, by the apposition of the mucous surfaces along the whole 

 length of the urethra, and the tendency of the weight of the contained 

 urine in the relaxed bladder to press the two walls of the urethra 

 together. Griffiths concludes that this resistance is in the main physical, 

 from the fact that it remains the same after death (on the first occasion 

 tried) as during life. 1 



Intravesical tension. — In the empty bladder the wall of this organ 

 is contracted, so that the cavity is obliterated. The pressure in the 

 empty bladder can be regarded as practically zero. As urine is gradu- 

 ally secreted, the tension in the bladder rises. Since, however, the wall 

 of the bladder is normally in a condition of greater or less contraction, 

 it follows that the rise in internal pressure of the bladder is not pro- 

 portional to the increased amount of fluid in its cavity. If the tonus of 

 the bladder wall is well marked, the intravesical pressure may be raised 

 to a considerable extent by the injection of only a small amount of 

 fluid. On the other hand, the bladder is able to accommodate large 

 quantities of fluid without any corresponding rise in pressure, in con- 

 sequence of a relaxation of the tonic contraction of its muscular wall. 

 The amount of fluid which the bladder is able to accommodate depends 

 largely on the rate at which the fluid enters the bladder. Thus, in man, 

 the state of tension of the bladder wall which is associated with a 

 desire to micturate, equals about 15 cms. water. This state of tension 

 is attained by the injection of about 150 c.c. of fluid, although, when 

 the bladder is allowed to fill naturally with urine secreted slowly by 

 the kidneys, an intravesical pressure of 15 cms. corresponds to about 

 250 c.c. of urine. 2 When, on account of a continued accession of urine, 

 the tension reaches a certain height, slow rhythmical contractions of the 

 bladder wall begin, weak at first, but growing gradually stronger, with 

 increasing distension. Thus in the bladder, as in most other hollow 

 viscera, the effect of tension on the sheet of unstriated muscle forming 

 the wall of the organ is twofold. 



1. A relaxation (not a stretching), accommodating the capacity of 

 the viscus to the incoming fluid. 



2. An excitatory effect, bringing about rhythmic contractions of the 

 wall, the extent and strength of the contractions being directly pro- 

 portional to the intravisceral tension (i.e. to the stretching of the muscle 

 fibres). 



Both these effects seem to be independent of the nervous system, 

 and may be observed in most hollow viscera after separation from all 

 nervous connections with the body, and must therefore be dependent 

 on local nervous mechanisms or on fundamental properties of the muscle 

 fibres themselves. 



Each rhythmic contraction of the bladder lasts on an average from 



1 This experiment of Griffiths does not put a muscular tonic action absolutely out of 

 court. The retractor penis muscle in the dog retains its irritability and contractility for two 

 or three days after the death of the animal, and would, when the body had cooled down, 

 be in a strongly contracted condition. It seems possible, therefore, that the unstriated 

 muscle at the neck of the bladder might similarly be in a state of tonic contraction one or 

 even two days after death. 



2 Genouville, Arch, de physiol. norm, et path., Paris, 1894, tome vi. 



