In considering the subject of oedema of the brain, a question already several times suggested, has again arisen, as to a mode of death, which is known by the name of "Serous Apoplexy." I have postponed answering it, till a more convenient opportunity; but now, having taken a survey of all the effusions of serum that occur within the cranium, to which, according to the present state of our knowledge, the production of serous apoplexy is, or might be, attributed, we are in a condition to enter into the subject.

The appearances usually mentioned as characterizing serous apoplexy are as follow: serous effusion into the sac of the arachnoid (especially a large collection, after the brain is taken out, at the back of the skull); infiltration of the meshes of the pia mater with serum; and a similar effusion between it and that part of the cerebral layer of arachnoid which stretches like a bridge over it; serous effusion into the ventricles; and withal, more or less distinct injection of the vessels of the pia mater with blood. To complete the picture, I must add, that the parenchyma of the brain is also infiltrated with serum. The essence of the whole condition would be an acute exudation of a large quantity of serum produced by congestion, and resulting in palsy of the brain.

The question then is, whether such a disease as serous apoplexy really exist, and whether it can be recognized in the body by the post-mortem appearances only, without reference to the symptoms attending the death of the individual? I would refer, on this subject, to what has been said on the difficulty of the question, at p. 259, and also to p. 291, where, when speaking of a disease similar to this in its final result of paralyzing the brain, viz., vascular apoplexy, I mentioned our limited power of determining, by what is found in the body after death, the influence of a disease in producing death (Lethalität). And I would now add that a. In the first place, we must exclude from consideration all cases in which hyperemia originates from any pre-existing disease of the brain, such as inflammations, adventitious products, etc, as well as all acute effusions of serum resulting from them: all cases of hydrocephalus also, all secondary or subordinate (unwesentliche) chronic effusions of serum (those, for instance, which are produced by a vacuum within the cranium), cases of oedema of the brain in old and in insane persons, which destroy life by gradual palsy of the brain, and, lastly, all those effusions, which are proved by daily experience to take place without injuring the brain, and have no evident connection with the death, must be disregarded.

b. As it is a fact that substantive acute effusions of serum within the cranium do occur, their fatal influence will be "prima facie" less doubtful, in proportion to their amount. They may supervene where gradual effusion has been already taking place. (Edema of the lungs presents the closest analogy to them.

c. The great frequency of chronic effusions, and their similarity, render it one of the most difficult of the duties of pathologist to recognize fatal effusions of this kind, and to determine that serous apoplexy has occurred in any particular case. The present state of our knowledge allows us to assert conscientiously that death has taken place in this manner only.

When the effusions are considerable in some particular situation, and the membranes and brain are still distinctly vascular:

When the brain is swollen by the oedema; and, especially, when white softening exists in the neighborhood of the ventricles, but cannot be referred to previous hydrocephalus:

When it is not known that the individual has suffered from a previous affection of the brain:

When the morbid appearances which are frequently associated with it in other organs, especially in the lungs, such, for instance, as hyperaemia, hypostasis, oedema, are nothing but what the symptoms and the course of the agony show to have been merely consequences of the palsy of the brain, and cannot be looked upon as the actual mode of death, or as the cause of the congestion and exudation of serum within the cavity of the skull.