in the Structure of Calculi. 237 
deposited upon extraneous substances introduced into the 
bladder, but appear never to form small kidney calculi. 
In what is commonly called a fit of the gravel, a small uric 
calculus is formed in the kidney, and passes along the ureter 
into the bladder. 
It is found from observation, that for some time after a 
stone has passed from the kidney, the urine is generally un- 
usually loaded with uric acid, and deposites that substance 
upon the nucleus now in the bladder. When this period, 
which is longer or shorter in different individuals, has elapsed, 
the subsequent addition to the calculus consists principally of 
the phosphates. 
Where the disposition therefore to form uric acid in the 
kidnies is very great and permanent, the calculus found in the 
bladder is principally composed of uric acid ; but where this dis- 
position is weak and of short duration, the nucleus only is uric 
acid, and the bulk of the stone is composed of the phosphates. 
Where the increased secretion of uric acid returns at in- 
tervals, the calculus is composed of alternate layers of uric 
acid and the phosphates. 
Other small calculi being formed in the kidney, make their 
way into the bladder, and afford fresh nuclei ; so that several 
calculi are sometimes found in the same bladder, and their 
composition is usually nearly the same. 
In other cases it happens, that a constant increased secre- 
tion of uric acid is going on from the kidnies, only in small 
quantity, which will be more uniformly mixed with the phos- 
phates deposited in the bladder, and where the uric acid pre- 
dominates, the species of calculus denominated improperly, 
urate of ammonia, will be produced. 
