Disregarding, for the time, the displacements within the skull to which the brain is subjected by various growths, we find the most striking anomaly in respect to the position of the brain to be hernia, - encephalocele, - extrusion through an aperture in the skull.

Congenital hernia of the brain is occasioned by an extreme increase of the organ in bulk. In most cases it is undoubtedly the consequence of hydrocephalus (hydrencephalocele), the pressure of which interferes with the development of the bones generally, and, at some particular spot, arrests it altogether. The size and form of the congenital hernia of the brain bears a certain relation to the dimensions of the aperture in the skull, as well as to the quantity of the protruded cerebral mass, and of water accumulated in it: the size of the hernia, however, and the dimensions of the aperture in the skull, are very often proportioned inversely to one another. The hernials sometimes as large as the head, or larger; more frequently it is below that size. Its form is that of a round tumor, or of an appendage to the skull; and when it is large, and the aperture in the skull small, it appears attached by a neck or pedicle.

The protruded mass of brain is covered externally by the general integuments, which are mostly thin, and without hair; internally, the inner cerebral membranes are in immediate contact with it, while, between the two, the pericranium and dura mater are intimately united with each other. True hernia of the brain must not be confounded with saccular protrusions through the skull, which, though similar, are merely herniae of the arachnoid: they are sometimes so far combined with hernia of the brain, that, as they increase, a portion of that organ may project within their pedicle.

The protruded portion of brain is sometimes in the same state of destruction, and consecutive malformation or vitiation of its growth (Ver-bildung), as in the case of hemicephalus, the hernia is then combined with hemicephalus, the former passing into the latter.

The situation in which hernia occurs is also very various. Most commonly it is at the occiput; and next, though much less frequently, it occurs further upwards in the mesial line, at the anterior fontanelle. More rarely still, it happens in the lateral regions of the skull, and on the forehead: while sometimes, but most unfrequently, the brain protrudes into the nostrils, or sphenoidal sinuses, and forms a tumor at the root of the nose, or in the pharynx.

The skull, in these cases, is altered in size and shape. As more of the brain protrudes, the cranium becomes generally smaller, and its vault flatter; and if, at the same time, the aperture be large, that form of the head predominates which is exhibited in hemicephalus. But this rule has its exceptions; for if the hydrocephalus be very large, the great quantity of the serum may, in spite of the size of the tumor, not only preserve the skull at its normal dimensions, but even enlarge it beyond them. When the hernia protrudes through the cribriform plate into the nostrils, the vault of the skull sinks, in the form of a saddle.

Hernia of the brain rarely comes on after birth, for it is then only through accidental openings of the skull and dura mater, or those made designedly by art, that the brain can protrude. The hernia is effected by the congestion and turgescence of the brain, which are excited by the external injury, by swelling of the brain arising from acute oedema, by acute hydrocephalus, etc. The protruded portion of brain takes the form of a sausage; it sometimes reaches a considerable size, and measures several inches in length. It is liable to be strangulated by the aperture in the skull and dura mater, and then frequently becomes congested, and mortifies; and injuries to its free extremity may be followed by inflammation and suppuration.