Returning once more to the vascular effects of a localized compression, it is obvious that whilst the compressing force is producing a surrounding anæmia--depleted veins and arterioles--yet that the neighbouring but more distant brain is venously engorged--from venous retardation. Insomuch, moreover, as the venous tension and cerebro-spinal pressure are equal, it is obvious that, as the result of venous engorgement, there is some surrounding serous transudation into the perivascular, pia-arachnoid, and cerebral regions. The brain bordering on the anæmic area is therefore œdematous, the œdema diminishing and fading away in direct proportion to the distance of the area involved from the primary focus of compression.
If the compression force is of a progressive nature, the engorged area becomes in turn anæmic, and still more distant regions become engorged and œdematous. This state is known as that of _progressive œdema_. Its complete development is largely dependent on brain infection--abscess of the brain, &c.
The rapidity with which the symptoms of compression develop are largely dependent on the producing cause. Thus, three examples may be cited:--
(1) Compression of slow development, requiring weeks or months before typical symptoms are observable--e. g. tumour formation.
(2) Compression of medium development, requiring days only: e. g. abscess, subdural hæmatocele, and some forms of middle meningeal hæmorrhage.
(3) Compression of acute development, almost immediate: e. g. diffuse subdural hæmorrhage, some cases of middle meningeal hæmorrhage, depressed fractures, and intracranial foreign bodies.
The special symptoms of compression are, as Leonard Hill pointed out, referable to the effects produced on the bulbar centres. That such is the case is suggested by the following facts:--
(1) The same compression symptoms result wherever the compressing force acts.
(2) Experimentally, an increased pressure in the posterior fossa produces compression symptoms earlier than when the compressing force is exercised in some more distant region of the brain.
(3) A fracture involving the posterior fossa gives rise to compression symptoms earlier than a fracture in some more distant region.