72 FORMATION OF SHELLS OF ANIMALS, ETC., 
The first of the natural products which I shall consider 
are the minute calculi found in the urine of the horse. 
As these structures are well known to microscopists, 
it will not be necessary to describe them. It may 
suffice merely to say that calculi of a similar composition 
can be formed artificially, so exactly resembling these as 
not to be distinguished from them by the most careful 
microscopic examination. Those artificial calculi, the 
most like the calculi found in the urine, are the calculi 
deposited in the bottle, and not those adherent to the 
lower surface of the glass slide. (See the account of the 
process for making artificial calculi.) These natural 
calculi also, if examined in a good specimen containing 
calculi of various forms and sizes, will be seen to present 
appearances indicative of coalescence, in all respects the 
same as those of the analogous forms of the artificial 
calculi. These urimary calculi, existing in a fluid excreted 
from the blood by -a living organ, are considered by 
physiologists to be vital products. It is observed in a 
review of Professor Quekett’s ‘ Lectures on Histology,’ that 
“ henceforth a calculus will not be regarded as merely a 
mass of earthy or crystalline matter, collected by simple 
attraction of aggregation; but it shows evidence of the 
existence of cell-formation.” Professor Quekett also 
observes, concerning these calculi, that they are not mere 
sediments, but are passed from the kidneys, being either 
made up of small calculi, or of concentric laminz ; being 
in fact urmary calculi in miniature, the secreting cells 
remaining after the calcareous element has been dissolved, 
as in the larger calculi; and that a calculus found in 
connexion with the body is not a simple mass of inorganic 
substance, containing organic matter as an accidental 
constituent, but that a calculus, when in contact with 
