Both the dura mater and the pia mater are subject to inflammation and hemorrhages. The arachnoid being practically a part of the pia mater is involved in its diseases, so that no mention is made of it as being separately affected.

Inflammation Of The Dura Mater; Pachymeningitis

The outer surface or inner surface of the dura may be involved, constituting pachymeningitis externa or interna.

Pachymeningitis Externa

The external surface is most often affected by injuries from without, or by extension of diseases from the adjoining bone. In cases of fracture the inflammation which accompanies healing frequently causes the dura to become densely adherent to the overlying skull. This is noticed particularly when trephining operations are performed for the relief of focal or Jacksonian epilepsy. Should the fracture be compound or open, the occurrence of sepsis will tend to involve the adjacent dura mater. The same occurs in cases of necrosis. Syphilitic disease of the bones is most apt to affect the vault of the skull, while the dura towards the sides and base is most often involved by suppurative ear disease. The dura also becomes involved in tumors and gummata.

Inflammation of the dura is not apt to be a marked disease. It is a very dense membrane with few blood-vessels, therefore it is quite resistant to inflammatory processes. It acts as a barrier to the farther extension of an inflammation rather than as a carrier. Therefore we see epidural collections of pus existing for a considerable time without brain symptoms supervening.

The dura mater contains the large cerebral venous sinuses, and when the inflammatory process occurs in those regions, the sinuses become inflamed and thrombosis or clotting occurs. The clot becoming infected breaks down, the pus and debris pour into the general circulation, and general septicaemia, and even death, is caused. This is most liable to occur in the region of the ear, where the infection is apt to reach and involve the lateral (transverse) sinus. Infection of the longitudinal sinus is much more rare.

Pachymeningitis interna is an inflammation of the inner surface of the dura. It occurs, to a certain extent, in cases of gumma or other new growths involving the inner surface of the dura or extending from the pia mater below. The name pachymeningitis interna, also called hemorrhagica, is usually restricted to a chronic inflammation of the inner surface of the dura, with the formation of one or more hemorrhagic membranous layers. Adhesions to the pia do not occur. The disease has been seen in purpuric and infectious diseases, as well as in alcoholic and demented individuals.

Dural Hemorrhage

Hemorrhage arising from injury to the dura through fracture of the skull has already been discussed (see page 18). Epidural hemorrhage may, however, occur from an injury to the skull and detach the membrane from the bone without a fracture being present. The possibility of this occurring is proved by the remarkable case reported by Dr. J. S. Horsley (New York Med. Jour., Feb. 9, 1901). A man was struck on the head with a wooden club. He was momentarily stunned, but soon recovered and felt perfectly well. An hour and a half later he became drowsy, and in a few hours was in a state of stupor. The right side of the body and face was paralyzed, and the left arm and leg were in constant jerking convulsions. He was trephined over the left parietal eminence and four to six ounces of blood clot removed. There was no evidence of fracture or wound of the dura. Recovery was prompt. There have also been other recorded cases.

In operations involving the separation of the dura from the bone, bleeding may be quite free. This comes from rupture of the veins passing from the bone to the dura, and sometimes from the rupture of a vein passing over or in the dura itself.

Subdural hemorrhages always originate from the pia mater.