(A) CALCULI.- True calculi, answering to the definition just laid down, may be deposited from almost all the secreted fluids. But of these fluids, the urine is, perhaps, the only one of which the saline and other actual constitu ents, independently of any materials naturally foreign to their composition, form the sub stance of calculi; when calculous formations occur in other secretions, foreign ingredients may almost invariably be detected. The saline substance thus met with in calculous masses, and which does not enter naturally into the composition of the secretion, (or enters in excessively small proportion,) is most com monly the phosphate of lime. So frequent is the occurrence of this salt in calculous masses on mucous surfaces, as to lead irresistibly to the conclusion that mucous membrane has a specific tendency to secrete this salt, under certain conditions of local irritation.
(a) Urinary calculi. — Various constituents of the urine are capable of accumulating indi vidually, or in association with each other and with certain animal substances, (mucus, fibrin, albumen, fatty matters, colouring matters, &c.,) so as to form masses of variable form and size ; these masses are according to their bulk termed calculi, milieny calculi, and gravel. The sante materials unaggregated into masses form the substance of sediments, clouds, and pellicles. The following are the substances which to various amounts have been recognized as the constituents of urinary calculi : uric acid, urates of ammonia, of soda, of magnesia and of lime, oxalate and benzoate or hippurate of ammonia*, oxalate of lime, xanthin or uric oxide, cystin, phosphate (neutral and basic) of lime, triple phosphate of ammonia and mag nesia, carbonate of lime, carbonate of magnesia, silica, peroxide of iron, fat, extractive malter, colouring matters, fibrin, albumen, and mucus.
The coalescence of the component parts of urinary calculi is effected in three chief ways.
I. When the materials forming them are crys talline, minute crystals, the basis of the future calculus, go on increasing in number, though not individually in size, and by their accretion, depending upon mutual attraction, form masses. Animal matter may aid in cementing together the constituent parts, but in this form of coales cence its occurrence to any aniount is acci dental, and tends rather to diminish the firm ness of union. Pure uric acid calculi are formed
on this model. 2. When the substance form ing calculi is primitively amorphous, no attrac tion exists between the minute particles form ing the deposit ; hence a medium of union or cement is necessary. This is furnished by animal matters secreted with the urine, or thrown out by the surfaces along which it passes. The quantity and quality of these matters being liable to vary, the general aspect of the resulting calculus, and its properties of density, &c., must be subject to similar variety. Impure urate of ammonia calculi illustrate this mode of formation. 3. In the third species of aggregation, saline • — FOI 10 O. 0,01.,l0/1/11.4 Olt:WU 111.1 a sort of thick magma, as particu larly insisted on by M. Civiale* ; the condensation of this magma produces a uniform mass, or small spherical bodies, or simply a pulverulent mat ter. This mode of formation is chiefly observed in oxalate of lime calculi, but occurs also in the uric acid species mixed with various salts (e.g. in a calculus in the University College Collection composed of uric acid, urate of ammonia, triple phos phate and phosphate of lime), and in the phosphatic.
The first deposition of matter from the urine in these cases depends upon some one or more of the causes we ,, : 1.: e Cc11./y G1111111e/ ‘11.C1.1 III bt/C41%111g of the precipitation of the saline stituents of secreted fluids generally.
If this matter be not expelled from the body, it acts in various ways as a source of further de position and accumulation around itself ; it is for this reason called the nucleus, and the matter accumulated around it the cortex, of the entire mass. Every calculus may hence be theo retically resolved into a nucleus and cortex ; but it is not the practice to give the central part the former name, unless it be distinctly different in composition, or, at least, in aspect, from the matter immediately investing it ; there are, therefore, practically speaking, non-nuclear calculi, of which the pure uric acid and eystin species furnish examples.