8. The xanthic oxide (uric oxide, xanthin, urous acid) calculus is of extreme rarity ; four examples only (analysed by Marcet, Laugier, Stromeyer, and Dulk,) have as yet been met with. The external surface is described as smooth and polished, and of light brown co lour. Some fragments of the calculus analysed by Stromeyer are prescrved in the University College Collection; their fracture is sharp, their colour pinkish brown ; they are com posed apparently of easily separable concen tric lamina!, and are very hard ; they become waxy-looking h en rubbed. Marcet's speci men weighed 8 grains, that of Stromeyer 338 grains, that of Dulk 7 grains; those examined by Laugier were very small. In Dulk's case the oxide formed the cortex of a uric acid nu cleus ; in the others it was the sole ingredient. Unger has discovered minute traces of a sub stance which he considers closely allied to, if not identical with, xanthin in guano : its pre cise chemical relations, however, appear to be as yet not fully determined.
Observations are wanting concerning the characters of uric oxide sediments : Berzclius says they are pulverulent and grey.
9. The carbonate of lime calculus, very rare in man, is not uncommon in graminivorous animals. Dr. Prout has seen some small cal culi from the human subject consisting of this salt, of perfectly white colour and very friable. The carbonate may, however, be implire, and the mass accordingly vary in colour from yel low to brown and red. This species has been observed by Smith with the appearance of a mulberry calculus*, by Brugnatelli, Fromni herz, Walther, Loir, and others. Wood f has described two of pearly, and Rampold one of metallic lustre.
10. Carbonate of magnesia, according to Berzclius, very probably exists in all calculi composed of carbonate of lirne. It appears to have been actually detected in two instances only, —once by Moscati, once by Lindberg cln.
11. Urate of nurgnesia has been found form ing the chief mass of two calculi by Scharling; urate of ammonia was likewise present.
12. Urates of soda, potassa, and lime, never form the entire mass of a calculus. The former was found in large quantity by Lindbergson in the calculus just referred to. It is uncertain whether urate of soda exists naturally in the urine ; it occurs in association with uric acid and urate of ammonia in sediments, as already mentioned.
13. Phosphate of magnesia is, according to Brugnatelli, of common occurrence, either mixed with triple phosphate, or forming alter nate lavers with it.
11. C'hloride of sodium never forms the sub stance of calculi, and the conditions under which this salt crystallizes in the urine are not well ascertained : partial evaporation of the fluid must first take place. The crystals are
octahedral, and have their planes indented like steps of stairs.
The nature of the so-called fibrinous eaten- . lus (originally described by Dr. Nlarcet) has been made matter of question by Berzelius. It appears that the material supposed to be fibrin by that analyst was soluble, though not readily so, in nitric acid,—a character not be lonqing to either fibrin or albumen. This, with other of its properties as detailed by Marcet, leads Berzelius to regard the matter as inspis sated vesical mucus.
The Museum of University College contains a " fibrinous calculus " taken from the bladder of a cow (fig. 85). It is of irregular elon gated shape, measuring two and a half by one and a half inches; very light ; elastic ; of brownish grey colour internally, whitish exter nally, and coated with a white earthy crust.
A new substance has recently been added to the list of constituents of' urinary calculi by Heller*, under the title of urostealith. This substance is said to form a soap with alkalies, and to have been discharged in small masses varying in size from that of a hemp seed to that of a small nut.
Each particular division of the urinary pas sages is the seat occasionally of calculous for mations, and the characters of these are in each site more or less peculiar. Into the description of these characters we cannot here enter seri atim ; of the varieties thus depending upon the seat of the product —namely, renal, ureteral, vesical, prostatic, urethral, and preeputial,— the most important, the vesical, may be considered to have been specially kept in view in the pre ceding pages. As respects renal calculi we must content ourselves with illustrating by a figure (fig. 86) the curious branched form they sometimes assume, as they gradually mould themselves to the interior of the pelvis and infundibula.
Renal calculi sometimes attain great bulk. Among numerous examples of the fact we may refer to a case seen by Wilson+, in which the kidney, perfectly atrophous, and replaced by a multilocular membranous sac (the dilated pelvis and infundibula) contained an oxalate of lime calculus weighing seven ounces.and a half. Renal calculi derive much of their prac tical interest from their tendency to produce such atrophy of the kidney, with pyelitis (U. C. Mus.) or, more rarely, hydronephrosis.