3. By foreign bodies, which may enter the skull from without;
4. By pathological conditions—collections of blood or pus, tumors, etc., which may be produced either from the brain substance, its containing bone or membranes, or its vessels.
In every one of these conditions the size and tension of the brain are affected. The cerebrospinal fluid is mainly involved in acute not in chronic conditions. A slow reduction of the diameters of the skull produces such slow alterations of pressure as to cause a minimum of disturbance. So far as compression from traumatic influences is concerned we distinguish mainly between compression—
1. By extravasation of blood (see [Plate XLIII]);
2. By fractures of the skull with depression, or by foreign bodies penetrating from without;
3. By products of acute infectious inflammation due to septic infection from without.
The result common to all of these is increase of intracranial tension, and its consequence is a less rapid flow of blood and an altered blood supply to the brain and its membranes.
Experiment has established that in compression of the brain cerebrospinal fluid is forced by pressure into the spinal canal, whose membranes are more elastic, and which thus help to accommodate it; it has been also established that compression of the brain by one-sixth of its volume, by any material, is fatal, and that much less is at least serious. That fractures with depression produce sometimes serious, at other times trifling, symptoms is due to the varying accommodation of the spinal canal. Both experiment and observation seem to confirm the view that consciousness pertains to the cortex as a whole, and that unconsciousness is an inhibitory or paralytic condition which is produced in compression.
Temperature is a matter of great importance in studying compression and foretelling its consequences. Elevation of temperature is an early, continuous, and constant symptom in these cases. If temperature be subnormal and subsequently rise, prognosis is bad. Variations of temperature are more reliable guides than conditions of consciousness. As Phelps has remarked, in no condition except sunstroke is temperature so uniformly high as in cases of serious encephalic lesions.