182 ANALYTIC SECT. XXI. 
ccccti. The internal orifice of the urethra is 
at the side of the bladder, by which arrange- 
ment it is less subject to pressure from the urine, 
than if it had been placed at the lowest part of 
this organ. The orifice of the urethra does not 
appear to be closed solely by that part of the 
muscular coat of the bladder called the sphincter 
vesice. It is clear, that when this part of the 
muscular coat contracts, the fibres of which it is 
a continuation, should contract likewise, and ex- 
pel the urine, in place of retaining it in the blad- 
der. While the bladder is filling, its muscular 
coat is strained by the mechanical operation of the 
urine; the sphincter vesicee must be strained by 
the same means, and thus probably: contributes 
feebly to close the orifice of the urethra. 
cccciu. The prostate gland, and the expan- 
sible tissue about the mouth of the urethra, seem 
to close it in the same manner as the anus. At 
the commencement of making water, an effort is 
required to overcome this expansible tissue ; but 
when the urine has begun to flow, it keeps the 
orifice open to admit the following current pressed 
upon it by the contraction of the bladder. When 
the prostate gland is much enlarged, pressure upon 
the periphery of the bladder, and contraction of 
its coats, are inadequate to open the mouth of 
the urethra, and the urine has to be abstracted by 
a catheter. 
