These are tumors formed largely of the peculiar fat which is found in
bile and brain matter, and that crystallizes in flat oblong scales with
a notch at one corner. The tumors are usually connected with the choroid
plexus and developed beneath the pia mater, and may be of any size from
a pea to a hen’s egg, or in exceptional cases a sheep’s kidney. As a
basis there is a stroma of connective tissue permeated by blood vessels
from the plexus. Groups of spherical or polygonal cells fill the
interstices while fusiform cells are found in the stroma. There is a
variable amount of phosphate or carbonate of lime which in oldstanding
cases may give a cretaceous character to the mass. These constitute
sandy tumors (psammomata).
Cholesteatomata are especially common in old horses and are manifestly
connected with congestion of the choroid plexus and exudation. In a
recent case or in a case which has shown a recent cerebral hyperæmia, we
may find a central mass of yellowish cholesterine, and surrounding this
an abundant yellow gelatinoid exudation. This latter is rich in
cholesterine which fails to dissolve along with the rest of the exudate
on the occurrence of resolution, and is therefore laid up as the solid
fatty material. For the same reason the fatty element is usually laid on
in layers, one corresponding to each access of local hyperæmia and
exudation. The great tendency to calcareous degeneration has been
attributed to the abundance of phosphate of lime in the cerebral
exudate.
The _symptoms_ of these tumors are exceedingly uncertain. Many such
tumors of considerable size have been found after death in animals in
which no disease of the brain had been suspected during life. In these
it is to be inferred that the accretions were slow, gradual, and without
any serious congestion. In other cases the tumor is attended by
paroxysms of vertigo, or indications of hyperæmia or meningitis, which
will last for several days and gradually subside. It is reasonable to
suppose that the tumors are largely the result of such recurrent attacks
of encephalitis, and are no less the cause of their recurrence. The
intervals of temporary recovery correspond to the subsidence of
hyperæmia and the reabsorption of the liquid portion of the exudate. The
manifestations during an access correspond directly to those met with in
encephalitis. As in that affection there is usually an initial period of
excitement and functional nervous disorder tending to more or less
somnolence, stupor, paralysis or coma, with long intermissions of
apparently good health. In other cases the stupor or paretic symptoms
may persist up to the fatal issue.