In _favourable cases_ the injury being of a comparatively mild nature, the vaso-motor centre soon recovers tone, and, as the result of previous depression, the vascular conditions are now reversed--_the stage of depression_ giving place to the _stage of reaction_. The blood-pressure rises, the carotids pulsate forcibly, the face is turgid and flushed, the skin hot and moist, whilst the temperature rises, and the general cerebral hyperæmia is evidenced by headache, restlessness, and perhaps by delirium. In the _more severe cases_ the vaso-motor depression is unduly prolonged, or persists till death, the patient remaining concussed for a considerable period of time, or dying in that condition. In these fatal cases the post-mortem examination _usually_ reveals brain laceration, but _always_ evidences considerable engorgement of superficial cerebral veins, increase of cerebro-spinal fluid and œdema, both of the pia-arachnoid region and of the brain substance itself. These latter phenomena are accounted for by the fact that, insomuch as the venous pressure and cerebro-spinal tension are equal, the greater the engorgement of the cerebral veins the more extensive is the transudation of fluid into the pia-arachnoid and cerebral systems. In a fatal case of concussion--recently under my care--the surface of the cortex, when exposed by operation, was obscured by a layer of œdematous pia-arachnoid ¹⁄₄ inch in depth. Arguing from the presence of the very definite pathological changes observed in all fatal cases--contusions, lacerations, hæmorrhages, and œdema--one may arrive at the conclusions (1) that lesions of a similar but less serious nature are existent in the serious but non-fatal cases, and (2) that the prolongation of the stage of depression is directly dependent on the presence of such lesions. On investigating the numerous cases that come under the care of the surgeon, certain features present themselves which are not always readily explained on the definition of concussion as given above. Thus, the variability of the symptoms evinced in cases of concussion admitted into hospital are well exemplified by the following table, compiled from a consecutive series of 50 cases admitted into St. Bartholomew’s, diagnosed and treated as concussion:-- =12= _cases in the hospital for 7 days or less_ (_cerebral shock only_). _Temperature_ on admission: 8 subnormal. 2 normal. 2 slightly raised. _Pulse_ on admission: 3 subnormal. 3 normal. 6 accelerated. =26= _cases detained for 7 to 10 days_ (_probably cerebral shock only_). _Temperature_ on admission: 6 subnormal. 9 normal. 11 slightly raised. _Pulse_ on admission: 4 subnormal. 12 normal. 10 accelerated.