1. Deficient And Excessive Development Of The Brain

Acephalus, or deficiency of the head, affords an instance in which the brain is entirely wanting. In such a case more or less of the spinal marrow and vertebral column, especially, of their upper part, is generally wanting too. And with this deficiency is combined absence of the heart, of great part of the vascular system, of the lungs, and of the principal abdominal organs, so that, while the urinary and genital organs exist, nothing else can be found within the peritoneal sac, except a rudimentary intestinal canal.

In cases allied to this an extremely rudimentary and simple brain is enclosed in a very small cranium, in a mere shapeless, and very small, bony capsule: monstrosity of the face exists also.

Sometimes a part of the brain is wanting. It may be the whole cerebrum or any large section of it, as the anterior lobes with the organ of smell, the optic thalami and optic nerves, the posterior lobes, the fornix, the septum, the corpus callosum, the cornua ammonis, etc. The skull is then small in proportion to the deficiency, and the face wanting or malformed: or, again, it may be some smaller and less essential part of the brain, as the hippocampus minor, or the gray commissure; or single convolutions, so that the white substance is exposed, etc.

Again, the brain may be generally of small size, though it exists in all its parts; and the skull is diminished to a corresponding extent: but of this state, microcephalus, as well as of several other instances of deficient development of the brain, a further account will be given amongst the anomalies in its form.

There is, besides, one instance of congenital deficiency of more or less of the brain, which I have not put in the same series with those already mentioned, because, to say the least, it is highly probable that it owes its origin to an attack of hydrocephalus at some period (generally a very early period) of foetal life. It is the instance known as anencephalus, hemicephalus, and also as acrania.

There is much difference in the extent to which different cases of hemicephalus proceed, depending partly on the extent of the previous hydrocephalus, but principally upon the period of foetal life at which the disease of the brain commenced. Sometimes that organ is wholly wanting, and only the membranes are found at the base of the skull, with the cerebral nerves sunk into them: sometimes a few rudiments of the brain exist, particularly those structures which compose its base; while it is covered with a membrane, formed of much attenuated skin, and dura mater, which exhibits traces, more or less distinct, of having been ruptured. The vessels of the inner membranes are generally numerous and gorged, the membranes themselves are filled with extravasated blood, they present a honeycombed arrangement of their structure (which has been compared to hydatids), and contain some grayish-red cerebral substance. The brain itself is unusually vascular and soft, and appears as if it had been macerated. The roof of the skull is almost entirely wanting; for the usually expanded frontal and parietal bones form mere small and slender streaks, or irregular triangular plates of bone, and are sunk down upon the base: and the broad occipital plate is shrunk to a few rudiments, or severed by wide fissures. A vault may be formed to the skull by these rudiments of the bones, but it is very low, and divided by wide fissures from before backwards. The bones at the base, if they are not divided also, are small like the occipital bone, but very thick and coarse.

In other cases, only a small part of the cerebrum is destroyed; and, as the greater part of the brain remains, the cavity of the cranium is proportionally capacious, and the deficiency of bone is confined to its uppermost part.

Hemicephalus is allied to a certain stage of hydrocephalus. Within a skull of normal size or enlarged, but which is closed, there exist no cerebral hemispheres, but a sac, surrounded with the cerebral membranes, and filled with serum, while the base of the brain lies at the lower part of the skull, more or less rudimentary and misshapen.

Such a case exhibits clearly the alliance which subsists between hemicephalus and hydrocephalus, and the foundation of the former in the latter; but the combination of hemicephalus with encephalocele, that is, its origin from hydrencephalocele, exhibits it more clearly still. In this instance of hemicephalus, a part or even the whole of the brain, destroyed in the manner above described, lies outside the skull; the cranial vault is split along the mesial line by a greater or less fissure, and is low in every case, but it is sometimes quite sunken to the base.

But further, we have an opportunity sometimes of demonstrating this cause of hemicephalus, by direct observation at the time of its occurrence. The skull of a foetus, at such a time, is found distended and hydrocephalic, and at the vertex a slough is seen, produced by the pressure and stretching.

Lastly, hemicephalus is very frequently combined with the same instances of arrested development as the higher degrees of. congenital hydrocephalus. It is often accompanied, too, by fissure of the vertebral column (spina bifida): when the occipital bone is split completely through, the cervical vertebra are nearly always fissured also.

When the brain is developed in excess, it becomes more or less completely double. It is very rarely found that any one part is double while the remainder of the brain is single: though such is sometimes the case, with the gray commissure, for instance. The cerebrum and cerebellum sometimes have an unusual number of lobes, and thus appear to be developed in excess.