It is not at all improbable that concussion is produced after a manner somewhat resembling the following. The brain has a natural tendency to remain at rest, but is liable to be brought into a state of commotion by impulses on the cranium being communicated to it. When a slight blow is inflicted on the skull, only a slight commotion of the brain is induced, the cranial contents are, as it were, slightly jumbled, and a temporary and trifling confusion of its functions follows. When, however, the stroke is more severe, the brain is separated from its cranial attachments, both at the point struck and at the part directly opposite,—it is thrown upon itself towards its centre; its substance is thereby condensed, its diameter in the direction of the impulse is diminished, and a separation between the brain and cranium is formed at each extremity of that diameter. By post mortem examinations, it has been ascertained that condensation of the substance of the brain does exist in cases of severe concussion. Such commotion may be sufficient to cause instant extinction of life, or the brain may gradually resume its former condition, or with only such slight incited action as may be required to reunite the dura mater with the inner table of the skull. Extravasation of blood or serum is extremely liable to occur in such cases, the vessels being either compressed, stretched, or otherwise thrown out of their natural relations along with the other cranial contents, reparation can only take place by absorption of the extravasated fluid, and gradual deposition of plastic matter. When extravasation takes place to a greater extent, compression is the consequence, as will be more fully explained further on. Perhaps the brain does not recover itself gradually, but suddenly; the impulse, which was at first directed from the circumference towards the centre, now acting from the centre towards the circumference; and then the propulsions and recoilings may be repeated, though gradually lessening in their intensity, until the effect of the original impulse is lost, and all vibration consequently ceases. But concussion may be caused by an impulse received not immediately on the cranium, but on some other part of the body, as when a person falls from a considerable height and alights on the feet or buttocks; and in such a case also its effects may be indirectly communicated to it through the brain, and may produce equally violent effects, without there ever being any appreciable lesion of the cerebral matter.