=Pacchionian Bodies.= Along either side of the great fissure may be noticed within the pia mater of adults, several small white bodies, varying in number and size—the _Pacchionian bodies_. They may cause absorption and perforation of the dura mater, and even of the bones of the skull. Being looked upon as the result of mere senile changes, they do not indicate the presence of disease, though repeated congestions of the brain appear to favor their more rapid development. =Inflammation= of these membranes, or _meningitis_, is a very common affection, and is accompanied with an accumulation within its substance or beneath its layers, of serum, lymph, or fibrine in various proportions. It may be confined to circumscribed portions, or involve a large portion of the membrane, and even extend to the spinal cord. Adhesions between the two surfaces of the arachnoid sometimes result from this form of inflammation. Insanity in its various forms is most frequently accompanied with some morbid condition of these membranes. In twenty-two cases of insane persons whose brains were inspected by Dr. Marshall, in twenty-one, serous fluid, varying in amount from one to twelve ounces, was found in the ventricles, and in seventeen of these twenty-one cases, similar effusion was found in the sub-arachnoid space, or within the substance of the pia mater. While red injection of the membrane was found only in four cases, yet other conditions—the effusions, &c.—were evidently the result of previous inflammation. In nine cases were the arteries of the brain opaque, thickened, steatomatous, or ossified; conditions highly favorable for deranging the capillary circulation of the membranes or of the brain.[5] The following statement gives the principal morbid changes of these membranes which have been found in cases of insanity: 1. Injection, more or less intense, of the _pia mater_, giving a red or scarlet appearance; or, where infiltrated with serous fluid, presenting a pale gray color and increased in thickness. 2. The arachnoid (the visceral layer) becomes opaque and thickened, resembling the dura mater or macerated parchment. 3. The meningeal injection may terminate in serous effusion, either from the free surface of the arachnoid into the sub-arachnoid tissue, (pia mater,) or from the choroid plexus into the ventricles. 4. Albuminous exudations may be found upon the free surface of the arachnoid of the _dura mater_, covering its whole extent, or confined to definite portions. 5. Adhesions of the two surfaces of the arachnoid may rarely be found. It is most common in the great fissure, and has been found in the ventricles. 6. Blood may be effused upon the surface of the arachnoid or in the substance of the pia mater.