Hyperemia ex vacuo. These congestions give rise to the transient or protracted attacks, which simulate apoplexy, and are so frequent in old age.

B. Actual apoplexy, hemorrhage, finds one of its chief causes in atrophy of the brain, and the congestion to which atrophy gives rise.

y. (Edema of the brain is a very common appearance in the atrophied brain of the aged and imbecile (Calmeil); it may be chronic, or, to a certain extent, acute; and it is attended by effusions into the ventricles.

Atrophy, when it involves the whole brain, and has reached a certain degree, terminates fatally, either by paralyzing the brain, or through some of the consequences described above.

Partial atrophy is very rarely a primary and idiopathic disease; much more commonly it is secondary; it presents two orders of cases:

(1.) Those of the first order are represented by two pretty frequent examples; on the one hand, by shrinking of the optic thalamus and corpora quadrigemina, in consequence of blindness: and, on the other hand, by that of various sets of white fibres after attacks of apoplexy and inflammation, especially when these attacks have taken place at the surface of the brain. The atrophy of one or more parts of the brain, therefore, is consecutive, and arises from disease of the central or of the peripheral extremities of the nerves. The diseases of the peripheral nervous system, which produce such a result, are primary and secondary paralyses, the original physical cause of which, though various, and for the most part unknown, may, in the end be reduced to wasting of the texture of the nerves. The atrophy of the optic thalamus, etc, in consequence of paralysis of the retina, makes it probable, that many of the atrophies of particular parts of the brain are not the cause of peripheral paralysis, but rather the consequence of it; i. e., that the atrophy is propagated from the periphery to the corresponding centre in the brain. The principal diseases of the central extremities of nerves are wasting of the cerebral substance after inflammation and apoplexy, of which I shall have to speak hereafter.

(2.) To the second order of cases belong those losses of substance, those atrophies, which result from inflammation and apoplexy; in which so much of the substance of the brain as was broken down by extrava-sated blood, or disorganized by inflammatory processes, is altogether removed by absorption, together with the extravasation and the exudation. These are instances of secondary atrophy, occasioned by previous disease of texture. And they are cases, which, especially when the loss of substance is at the periphery of the brain, give rise to the atrophy of different sets of the fibres, as I mentioned among the first order of cases: they may even be so extensive as to occasion wasting of an entire hemisphere, and of the fibres continuous with it in the crus, the pons, the medulla oblongata, and the medulla spinalis.

As these cases of atrophy proceed, they always give rise to well-marked induration (Sclerosis) of the portion of brain which they involve: and that half of the brain in which they occur, and even sometimes the whole brain takes part in the induration too. Atrophy of an entire hemisphere of the cerebrum produced in this manner, is sometimes concentric; sometimes its lateral ventricle is dilated, and then it is excentric. The former, or the case in which the hemisphere shrinks upon itself, is the more usual of the two. In the latter there is a one-sided dropsy of the ventricles.

When the atrophied portion is seated on the surface of the brain, the vacuum is filled up by thickening of the adjoining part of the membranes, by oedema and bulging of the pia mater, by enlargement of the collateral ventricle, and even by the wall of the skull sinking in. When it is deeply seated, its place is, under certain circumstances, after apoplexy, for instance, occupied by serum; and the cavity thus formed exists for some time, and may even continue permanently.

Cases of partial atrophy sometimes bear a very close resemblance to those in which the brain being originally of small size, or defective in some of its parts, consequently presents an originally unsymmetrical appearance. But the atrophy may be distinguished by the induration which it produces in the part which is diminished in size, as well as by the induration of the cerebral substance around, while the vacuum and the complementary serous effusions distinguish it when the skull is found symmetrical, and the rusty-brown, or yeast-yellow coloring of the brain determine when apoplexy has preceded the atrophy, etc.