(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.
(4) A far smaller body kills in the bulbar region than in the cerebral chamber.
(5) The general pressure effects are in no way due to excitation of the part of the brain pressed on, for, after division of the mesencephalon, the pressure was just as active in calling forth changes in respiration and circulation (Sir Victor Horsley).
Leonard Hill also pointed out that the first effect of anæmia of the bulbar region was in the nature of stimulation, the vaso-motor centre being influenced in such a manner as to produce a general rise of blood-pressure. A further degree of compression may again produce the same result, the arterial pressure rising considerably above the normal. Later on, the medullary centres—and more especially the vaso-motor—become exhausted, with consequent grave fall in blood-pressure. The pulse, previously slow and full, becomes rapid, small, readily compressible, and irregular in rhythm, whilst the respiration, previously deep and regular, becomes gasping, irregular, and Cheyne-Stokes in character.
The sequence of events may be depicted in another manner. In cerebral compression, the battle—as pointed out by Schüster—largely resolves itself into a life and death struggle between the attacking compression force on the one hand and the defending vaso-motor centre on the other.