The following facts appear noteworthy as bearing upon the morbid condition going by the names “hæmatoma of the dura mater” and “pachymeningitis hæmorrhagica interna.” In the course of recent experiments I have had occasion to incise the dura mater of (anæsthetized) rabbits and apply Sp. Vin. Gallic. or diluted cantharidin to the cerebral cortex through the aperture so made. On killing one of the animals after the lapse of 48 hours and opening the skull almost the whole of the right hemisphere (that operated upon) was seen to be covered with dark-red clot. This term is justified by the appearance of the exudate, whatever its origin may have been. On incising and reflecting the dura mater it was found that the clot lined the inner surface of that membrane, and had no connection with the subjacent structures. The vessels of the dura were unduly prominent and numerous, but there was no swelling of the membrane, and, apart from the clot, its inner surface was free from exudate. The clot was raised without difficulty, and washed gently in water; the colouring matter came gradually out, and left a delicate, greyish-pink, translucent, continuous pseudo-membrane of sufficient consistence to permit of manipulation. This was divided into two portions, which were stained with hæmatoxylin and safranin respectively. Microscopically the pseudo-membrane was found to consist of red and white corpuscles (the former in considerable majority), a meshwork of fibres (fibrin, apparently), and an amorphous substance, uniformly stained.