Adhesion between the opposed surface of the visceral and parietal layers of the arachnoid is (and the fact is curious) not of frequent oc currence, excepting at the convex border of the falx cerebri, where the Pacchionian bodies are found. And the intrusion of these bodies into the longitudinal sinus frequently increases the closeness of that adhesion. The cellular ad hesion so common in other serous membranes is rarely found in the arachnoid.
Small plates of cartilage or ofbone are some times found in connection with the arachnoid. Their formation is generally the result of a pre vious morbid deposit which has subsequently become converted into cartilage or bone.
. ' kyusion.—Effusions take place either !Tao ,/ the subarachnoid or the arachnoid cavity. The existence of serum, in undue quantity, in the former situation, must be looked on as an in crease in the fluid which naturally occupies that space, and as we have already remarked in a former part of this article, it takes place in consequence of the failure of the normal pres sure upon the vascular surface, and may rather be regarded as tending to preserve the functions of the brain than as pmducing an injurious pressure upon it. Indeed I have always foun that in cases where an abnormal quantity fluid existed in the subarachnoid cavit brain afforded no indication of its having rienced undue pressure previous to death. such cases the brain seems to contain le blood than natural, and its anaemia is mo obvious in the grey matter. Sometimes the is hyperxmia of the veins in the white matt of the hemispheres, as if the heart's force, nto necessary to the venous circulation within 11 cranium than even to that of other parts of t. body, had been prevented from exerting 1 1 influence through the capillaries upon th blood in the veins.
The existence of serous fluid in the arach noid cavity is of very rare occurrence. In some instances old adhesions of the two layers of arachnoid to each other circumscribe a space in which fluid accumulates.
Blood is sometimes effused into the subamch noid cavity. This is frequently the case in injuries of the head, the blood escaping from broken vessels of the pia' mater. Sometimes the blood effused into either lateral ventricle will escape into the subarachnoid cavity, break ing down the membrane of the ventricle. Han apoplexy occur near the surface of the brain, the laceration of the cerebral substance may extend quite to the surface, and the blood may pass through the pia mater into the subarach noid space.
In some instances we find blood in the cavity of the arachnoid (the arachnoid sac). The blood is either loose in the sac, or it is more or less closely connected with the inner surface of the membrane lining the dura mater.
In a recent communication from Mr. Pres cott Hewitt published in the last volume of the Medico!Chirurgical Transactions, the prin cipal facts relating to this subject have been collected and arranged in an interesting form. Mr. Hewitt describes these effusions of blood as existing in four forms.:— " 1. The extravasated blood may be either liquid or coagulated ; if in the latter state, it may be in clots, or spread out in the shape of a thin membranous layer, covering a greater or less extent of the surface of the brain.
" 2. Sometimes the extravasation presents itself under the shape of a false membrane, possessing more or less of the original colour of the blood.
" 3. The blood may be fixed to the free surface of the arachnoid and there maintained by a membrane, which to the naked eye pre sents all the characters of the serous membrane itsel f.
" 4. The blood is frequently found enclosed in a complete cyst of various degrees of thick ness, which may be removed unbroken from the cavity of the serous membrane.
" The four divisions above referred to," adds Mr. Hewitt, " may be and often are combined with each other, but in whatever state the extra vasated blood has been found, it has, in the majority of cases, corresponded to the upper surface of the brain, and has been rarely met with in the cerebellar fossw." It is impossible that these effusions of blood can have any other source but the minute bloodvessels of the pia mater or the dura mater, which becoming ruptured allow the blood to burst through the serous membrane by which they are covered. They occur mostly in per sons of a scorbutic or hemorrhagic habit, or in whom the arteries have become brittle from abnormal deposits in them; and it is not im probable that whilst the imperfect nutrition of the arteries is going on, the serous membrane itself suffers, becomes wasted, and therefore easily yields to the force of the blood as it escapes from the bloodvessels. • Pus is found in the subaraehnoid cavity where there has been inflammation of the pia mater and amchnoid, and more rarely in the arachnoid sac.