The nucleus of a urinary calculus, although commonly formed of sedimentary saline matter, may likewise consist of various materials not naturally existing in the urine, and these ma terials may be either formed in the body or introduced from without.
First : sedimentary nuclei may be composed of any one of the more important materials (with perhaps a single exception) detected in urinary calculi ; of these uric acid and oxalate of lime are the most common, while the phos phate of lime and triple phosphate hold the opposite position in the scale. A law esta blished by Dr. Prout, that " a decided deposi tion of the mixed phosphates is not followed by other depositions," is, with few exceptions, universal. Cystin has not (as far as we are aware) been found playing the part of a nu cleus in any recorded case; to this statement a large calculus of cystin surrounded with a very thin coating of phosphates (Univ. Coll. Museum) cannot fairly be considered to sup ply an exception.
A calculus commonly contains a single nu cleus only ; but instances are not wanting of calculi containing two, three, and more nuclei. Masses of the latter kind are probably simply aggregations of smaller on es, as appears to have been the case with that exhibited infig. 74.
The mode of connection of the nucleus and cortex varies. (1.) The union may be inti mate and general by every point of the ap posed 'surfaces ; this is the most common case ; (2.) the nucleus may adhere to the cortex by asperities on its siu•face only ; empty spaces, or spaces filled with grey gritty matter, being interposed between them : (3.) the nucleus may be free in the centre of the mass. Of the latter rare state a striking specimen exists in University College Museum ; the surface of the nucleus is covered with dark-coloured matter in powder (dried and altered blood), some of which helps to fill the cavity existing between the nucleus and cortex.
The nucleus (when of the present species) is generally the hardest part of a calculus. It forms either in the kidney, or, much more rarely, in the bladder.
Secondly : animal matter, having such cha racters as render it impossible, according to Berzelius, to determine whether it is composed of mucus or of albumino-fibrinous substance, almost constantly occurs in calculi, and some times forms the nucleus of a mass. Howship has figured a remarkable specimen, voided from the urethra of a negress, in which the nucleus consisted of "mucus," associated with a very little phosphate of lime,— the cortex of more solid phosphates. Clots of blood occa
sionally form the central part of calculi ; a fact noticed first by Frere Come.
Thirdly : foreign bodies, introduced from without, not very unfrequently constitute the nuclei of urinary calculi. In the great majority of cases these bodies are directly pushed into the bladder ; but in some well-authenticated cases have reached that viseus after having been swallowed or otherwise introduced. Among bodies acting as nuclei have been met pins (Univ. Coll. Mus.), needles, tooth-picks, ear picks, pieces of wood, stems of plants, ears of corn, grains of corn, stones of various fruits, tubes of various kinds, glass or earthen, &c., pieces of bougies and catheters, balls and other metallic bodies, a globule of mercury,* pebbles, &c. When the bodies thus introduced are sharp, as pins, they sometimes protrude beyond the calculous matter, and (a fact surgically im portant), are fixed in the surrounding tissues.
Instead of a nucleus the centre of a calculus may present one or more cavities of variable size and shape, almost invariably- lined with a black pulverulent or laminar matter, and sometimes containing powdery substance ; in other instances, there is neither surrounding nor contained rnatter of this kind. It has been supposed that in all these cases an origi nal vegetable or animal nucleus had been gradually removed by a process of decom position and subsequent filtration through chinks in the cortex.
The prorninent physical characters of the cortices of calculi divide them into two na tural classes ; the (a) granular or irregular, and the (b) laminated.
(a) Of granular calculi the best example is supplied by oxalate of lime, but the appear ance of these masses is not always identical. Sometimes the mass looks homogeneous and non-granular, manifestly from the close aggre gation of the original granules. In the more ordinary cases the granules remain cbstinct, whence the well-known tuberculated or mul berry-like aspect. Uric acid calculi, rendered impure by association of certain saline matters, assume the granular form ; the phosphates are sometimes granular ; and the pulverulent cha racter of the fusible calculus allies it to this species.