URINARY CALCULI. 445 



IV. Calculi of cystin. Calculi of cystin, although rare, are 

 more common than those of uric oxide. Although sometimes 

 mixed with other constituents, cystin most commonly forms the 

 sole ingredient. These calculi seldom attain any great size ; 

 they are usually small, round, smooth, and of a yellow colour. 

 In consistence such a calculus is soft; the cut surface presents a 

 semi-transparent, confusedly crystalline appearance ; not how- 

 ever laminated. When broken, it appears to be made up of 

 small crystals of a waxy lustre, the margins of which are rounded. 

 The microscope affords the best means of recognizing the ex- 

 istence of cystin : if we dissolve a fragment in caustic ammonia, 

 and allow it to evaporate spontaneously, crystals are deposited 

 in six-sided tables or prisms. The peculiar behaviour of cystin 

 before the blowpipe distinguishes it not only from calculi with 

 fixed ingredients, but also from those of uric acid and uric 

 oxide. It is also distinguished from the latter (with which, 

 however, it has never yet been found associated) by its solu- 

 bility in carbonate of potash, and in dilute hydrochloric acid ; 

 and from the former by its solubility in dilute hydrochloric, 

 phosphoric, and even oxalic acid. When cystin is associated 

 in calculi with other constituents, it is most commonly found to 

 alternate with uric acid. Thus Henry found a nucleus of 

 cystin, and an external layer of uric acid ; and Yellowly found 

 an external layer of cystin and a nucleus of uric acid. It is 

 worthy of remark that Bley, in examining two calculi taken 

 from the same man, found in one, cystin associated with car- 

 bonate of magnesia and ammoniaco-magnesian phosphate; in the 

 other the cystin was displaced by uric acid. The calculus that 

 contained the cystin was reniform, compressed, and flattened ; 

 of a yellow colour and weighed T75 grains. It exhibited a 

 stratified appearance internally, and when exposed to heat left 

 scarcely a trace of ash. After the removal of the earthy matters 

 by hydrochloric acid, there remained a residue soluble in pot- 

 ash, which, on the addition of acetic acid and evaporation, de- 

 posited small six-sided crystals. 1 



It need scarcely be mentioned that if in a calculus contain- 

 ing cystin there is any perceptible difference between the 



1 It seems strange that the cystin did not dissolve in the hydrochloric acid. The 

 case is recorded in Buchner's Repert, 2d series, vol. 2, p. 165. 



