(b) In laminated calculi the cortex is, as the word implies, composed of successive layers. Although each layer may and generally does differ in thickness from others, its own thick ness at its different parts may be said to be commonly pretty uniform ; nevertheless to this there are exceptions, and it is manifest that whenever the cause of accretion is in greater activity towards one aspect of a calculus than another, there the lamina in course of deposi tion must be thicker than elsewhere. Now in conformity with a general principle already laid down, we should expect the phosphates, which are formed in consequence of local irritation in particular places, to furnish the most fre quent instances of irregular thickness ; and such is in truth the fact, though, as is proved by the annexed cut (fig. 75), not always so.
The mode of union of the larninx differs ; it may be so intimate that the line of separation is lost at certain points of the apposed sur faces ; in other cases the union is irnperfect and loose, and interspaces of some width are left between those surfaces.
The section of a laminated calculus is some times marked by lines radiating from the centre to the periphery, cutting the latninw into seg ments of unequal size. This is obvious in certain cystin calculi, in some of uric acid, and in many of the mulberry species.
The tint of the different strata of a calculus, composed essentially of one substance, is not always the same throughout ; nor are strata, the same in composition and separated by ma terials of other kinds, generally of the same hue. These varieties must depend upon irregular admixture of impurities.
Calculi composed, at least essentially, of a single substance, are termed simple ; of several, compound. When the different materials are applied in successive laminm, the calculus is said to be alternating ( fig. 76) ; when they are irregularly mingled it is called mixed. The relative frequency of the three kinds may be deduced as follows from tables printed by Dr. Prout. Of 1520 calculi there were The mode of succession of various substances in the formation of alternating calculi (they may consist of two, three, four, or several suc cessive strata), is pathologically important, and has been specially investigated by Dr. Prout. The nature of this work prevents us from en tering into the subject, but we may mention (as evidence of the generality of Dr. Prunes law, that the existence of mixed phosphates in a calculus excludes the subsequent deposition of other matter), that of 566 alternating calculi composed of two layers, two only were examples of a nucleus of phosphates with a cortex of another kind of salt ; that of 172 calculi formed of three layers, not one had a phosphatic nu cleus, and in only three was the middle atraturn composed of phosphates ; and that, lastly, of 25 calculi containing four distinct layers, not one had a nucleus of phosphates ; in one only was the second la) er, and in three only was the third layer, thus composed. It is, however,
right to observe that if' small quantities of phosphates, not forming actual layers, were taken into consideration, the exceptions to the law would be much more numerous.
The degree of rapidity with which calculi form and acquire bulk depends upon the con stitutional condition of the individual in whom they form, much more than upon the nature of their own ingredients ; for, if it be true that oxalate of lime and uric acid caltuli commonly enlarge slowly, and the phosphatic species with great quickness, instances of the direct contrary are far from uncommon. The cases in which calculous matter accumulates round a foreign body are obviously those, and those alone, in which perfect accuracy as to dates can be obtained ; now cases are on record showing that some weeks suffice in one case for as abundant accumulation of phosphates as several months in another. The slower the enlargement, the greater, caleris paribus, the density of the mass. In this latter quality calculi vary exceedingly, some being as re markable for their porousness and openness of texture, as others for their compactness and closeness. Their specific gravity, accord ing to Fourcroy, varies from 1213 to 1976, water being 1000 ; Scharling found it in one instance to amount to 2014.
Calculi are commonly solitary ; from some calculations, which have been made on a limited scale, it would appear, however, that in one of every five or six cases of calculous disease two or more calculi are met with. The number and size of these bodies discovered in the bladder are sometimes almost marvellous ; thus Rodrigue de Fonseca refers to a case in which that viscus contained fifty as large as nuts ; fifty nine were found in Buffon's bladder ; and Moraud counted six hundred and seventy-eight in the bladder of an old man, and nearly ten thousand in his kidneys. Probably miliary calculi only accumulate in such extraordinarily great numbers.