TUMOUR WITHIN THE CRANIUM. 151 
fully defined facets, freely floating in a limpid fluid which 
distended the walls of the cell. There appeared to be no 
obstacle to the passage of the crystal from side to side of the 
cavity or in any other direction when the specimen was placed 
in different positions, its weight quickly carrying it to the 
most depending part. The walls of the cell have every in- 
dication of being composed of layers of areolar tissue spread 
out in a membranous form ; they are not, however, of uniform 
thickness throughout, although everywhere very translucent. 
Towards the circumference or periphery of the cell on one 
side there exists a bell-shaped spot [See plate, a, fig. 1), which 
is thinly covered with membrane, but surrounded with many 
fibres, far more dense than in any other part. Besides the 
crystal within the interior, there is a small mass of granular- 
like matter, which can also be made to vary its position ; this 
mass is marked b . 
<e The occurrence of this deposition of the oxalate of lime 
in this situation is the more interesting from the circumstance 
that this salt of lime is very rarely met with in the urine of 
the horse, in which the carbonates, on the contrary, are very 
common. Various forms of the carbonate of lime are noticed 
in the urine of the herbivora, produced by causes disturbing 
its ordinary mode of crystallization ; but none of these forms 
can be confounded with the octohedral arrangement of the 
oxalate. 
({ The priority of the formation of the cell or the crystal is 
not easy to be determined, it being possible that the blood 
of the animal, from impregnation with the oxalate of lime, 
deposited this salt in the place it was found, and that sub- 
sequently a cell enclosed it to prevent any serious ill conse- 
quences to the surrounding organism ; or it may be that the 
cell was first formed, and then the salt w r as effused into its 
interior, where it led to the exudation also of fluid. It is 
perhaps right to mention, in conclusion, that several capillary 
vessels are to be observed ramifying upon the w r alls of the 
cyst, and that it was firmly held in its place by fibres of 
areolar tissue. I may also add that the crystal has not been 
measured to ascertain its exact size, but that it can very 
readily be seen by unassisted vision.”] 
