EXCRETION OF MATTER FROM THE BODY 407 



an increased flow of urine, although the blood pressure in the 

 kidney is not raised. This is taken advantage of in cases of 

 heart disease, when the secretion of urine is almost arrested 

 from low arterial pressure, and when dropsy is rapidly ad- 

 vancing. Until the heart is toned up, the kidneys may be 

 stimulated to get rid of water by means of such diuretics as 

 caffeine. 



EXCRETION OF URINE. 



1. Passage from Kidney to Bladder. The pressure under 

 which the urine is secreted is sufficient to drive it along the 

 ureters to the bladder. If these are constricted the pressure 

 behind the constriction rises, and may distend the ureter and 

 pelvis of the kidney, but when it reaches about 50 mm. Hg 

 in the dog, the secretion of urine is stopped. The muscular 

 walls of the ureters show a rhythmic peristaltic contraction, 

 which must also help the onward passage of the urine to the 

 bladder. 



2. Micturition. As the urine accumulates in the urinary 

 bladder the rhythmic contraction of the non-striped muscle 

 becomes more and more powerful. These contractions are 

 chiefly excited by the fibres of the nervi erigentes of the 

 second and third sacral nerves, although fibres passing down 

 from the inferior mesenteric ganglion also probably act either 

 in exciting or inhibiting in different animals. Very different 

 results as regards the action of these nerves have been 

 observed, and at present no definite conclusion as to their 

 mode of action is possible. The backward passage of the 

 urine into the ureters is prevented by the oblique manner 

 in which these tubes pass through the muscular coat of the 

 bladder. 



The passage of urine into the urethra is at first prevented 

 either by the oblique manner in which the urethra leaves the 

 bladder, or more probably by the contraction of a strong 

 band of non-striped muscle, the sphincter trigonalis. This 

 muscle or the striped fibres which surround the membranous 

 part of the urethra are under the control of a centre in the 

 lumbar enlargement of the spinal cord, and the expulsion of 

 urine must be preceded by their relaxation. In some cases 

 of inflammation of the spinal cord the increased activity of 



