=Serous Effusion.= As has been already intimated, this may be found either in the cavity of the arachnoid—between the reflected and visceral layers—or within the ventricles. In the former position, the quantity is never large, while in the latter, it may amount to twelve or sixteen ounces, and be present for many years. When in such large quantity, there will be great distension of the ventricles and thinning of the corpus callosum and fornix, with destruction of the septum lucidum, as well as more or less separation of the cranial bones. It is only in children and before the bones of the head have become united, that such large accumulations are possible, as at a later period, from the unyielding condition of the walls of the skull, the presence of a single ounce, particularly if suddenly formed, would produce death. In all cases, the danger to life will be in proportion to the rapidity of the formation; a very slow and gradual accumulation permitting either of an expansion of the cranial bones, or a gradual absorption of brain substance, thus preserving an approximation to the normal pressure on the brain tissue. Serous effusions, like sanguineous, are generally the result of over distension of the cerebral vessels, either from mechanical obstruction, or a weakened condition of the coats of the vessels, with increased force in the action of the heart. It generally attends tubercular meningitis, and may be favored by an anæmic condition of the system. The symptoms during life, attending a rapid effusion of serum, are not so readily distinguished from those of sanguineous effusion, as to enable us to pronounce with certainty in any given case as to the cause of the cerebral pressure.