The dura mater participates in the diseased states of the cranial bones. Cancer or fungoid disease affecting the calvaria or any part of the cranial wall which is covered by dura mater, will extend to the dura mater and subjacent parts.
When there has been a solution of continuity and a loss or removal of any portion of the cranium, the exposed surface of the dura mater is apt to thro%v out a growth of granulations which constitute the fungus of the dura tnater, analogous to that which sprouts from the sur face of a similar fibrous membrane--the tunica albuginea of the testicle. In point of structure this fungoid growth is the sante as the granula tions on the surface of external ulcers.
Effusion of blood, constituting a form of rneningeal apoplexy, may occur on the external surface of the dura mater separating it from the bone; or on its internal surface, dissecting away thearachnoid membmne from its adhesion to the dura mater. The former kind is mostly if not always tmumatic, that is, resulting from the application of violence to the exterior of the cranium. The latter kind is of extremely rare occurrence, and must be carefully distinguished from that variety of effusion into the arachnoid sac in which the effused blood appears to be covered by a serous membrane. This mem brane, however, results from the condensation of the supeificies of the clot by its friction against the parietal arachnoid, and it may be distinguished from a true serous membrane by the absence of epithelium from its free surface. Effusions of either kind generally occur on some part of the surface of the cerebral hemi spheres above the level of the petrous bone.
The arachnoid membrane. — The ameh noid membrane is sometimes the seat of rtcute inflammation, and presents the same signs of that process as are met with in other serous membranes. The chief and, indeed, the only unequivocal sign of his condition as of recent occurrence is the exudation of plastic lymph upon the free surface of either or both layers of the membrane, with or wahout pus. This is attended with a highly injected state of the subjacent tissue (pia mater or dura mater, genemlly the former). The arachnoid itself, it will be remembeted, contains no blood vessels, but derives its nourishrnent from the vessels of the subserous tissue. Its apparent
vascularity is due to its great tenuity and trans parency, which allow the bloodvessels lying underneath to be seen through it as if they be longed to the membrane itself.
An opaque condition of the arachnoid, vary ing both in degree and extent, is a very common appearance of this membrane, especially at the middle and advanced periods of life. This occurs sometimes in patches; at other times it is generally diffused over the whole membrane. It is most conspicuous on the convex surface of the brain, especially towards the great longi tudinal fissure, and it is frequently associated with large and numerous Pacchionian bodies. It occurs, however, very commonly at the base, and frequently opposite the confluxes of the subarachnoid fluid.
The opacity of the arachnoid is commonly attributed to a former acute inflammation of the membrane, or to a chronic inflammation going on up to the time of death. But this state of the membrane is of such frequent oc currence, and is so often found in persons who evinced no sign of important organic change during life, that it seems scarcely correct to attribute it to such a cause. It is not meant to deny that previous inflammation or chronic inflammation is capable of causing these opaque spots, but undoubtedly other causes may produce them as well. The friction of two opposed surfaces may do it, and deposits upon the free surface of the membrane, an al tered condition of the epithelium, may have the same effect. Some recent microscopical examinations convince me that morbid deposits similar to those which are formed on the coats of arteries, may be found here, and occur in those morbid states of the blood, and conse quently of the whole system, which are favour able to the deposition of a morbid material throughout the arterial system, or in the sub stance of viscera, In confirmation of this view it may be stated that opacities of the arachnoid are most com mon after the middle period of life, and that they are then almost uniformly associated with a morbid state of the arteries of the brain and of other portions of the arterial system.