Malformations of the brain may be associated with deformities of the skull, or may occur independently. Acrania is an absence of the skull, but usually with preservation of the membranes and a small mass of nerve tissue. Hemi-crania is an undeveloped condition of the skull and brain on one side. Anencephaly refers to a condition in which there is almost complete lack of brain-substance; it is usually associated with acrania. Cephalocele is a hernia of the brain-substance through fissures or openings.

Hypoplasia or microcephaly is a condition in which the brain is unusually small, but properly proportioned. It is frequently associated with some degree of external hydrocephalus. Macrocephaly, or increase in size of the brain, is generally due to a hyperplasia of the neurogliar tissue. Porencephaly refers to the presence of definite holes or depressions in the brain-substance. It may be the result of softening following infarction.

Hydrocephalus is a collection of fluid either within the ventricles of the brain or in the subarachnoid space. It may be external to the arachnoid, or internal, and may be congenital or acquired.

External hydrocephalus is frequently exvacuo to supply by an exudate a loss of cerebral tissue. 35

Internal hydrocephalus is a collection of fluid within the third and lateral ventricles of the brain. The amount of fluid may vary greatly. The process generally begins before birth, and may cause serious obstruction to labor. The condition is generally bilateral. After birth, if the accumulation of fluid persists, there is a very typical deformity of the skull. The presence of the fluid within prevents the bones of the brain from uniting. The sutures are pushed far apart, giving a peculiar bulging to the forehead. The head becomes quite large and round, the face small, and the eyes may project. The cerebral tissue, on account of the pressure, shows a marked flattening of the convolutions.

The dura and pia may be thin or thick, and the choroid plexuses of the ventricles may be hyper-trophied or cystic.

The collection of fluid will or will not interfere with the mentality of the individual according to the amount that is present. In very marked cases it is incompatible with life, but, if less severe, the individual may live, although more or less of an imbecile.

The cause of this condition is not known, but by some it is thought to be due to alcoholism in the parents; to inflammatory conditions of ependyma and choroid plexus; to closure of the transverse fissure, causing obstruction to the escape of fluid from the ventricles; and to changes in the pressure within the cerebral veins.

Acute acquired hydrocephalus is generally found as a result of basilar meningitis. The brain is pale, soft, and the convolutions flattened; the contained fluid is frequently gelatinous. The ependyma and choroid plexuses are injected, and if the process was tubercular, tubercles will be found. The substance of the brain will show under the microscope the presence of small areas of suppuration.

Fig. 183. - Congenital Internal Hydrocephalus, with Marked Atrophy of the White Substance (from Bollinger).

Chronic acquired hydrocephalus generally occurs late in the course of epidemic meningitis or as a consequence of a chronic granular ependymitis.

Ependymitis may be either acute or chronic. The acute form is associated with acute meningitis, and in it there is a thickening and leukocytic infiltration of the ependyma and pia. In chronic ependymitis the surface is granular, the ependyma is thrown into folds and becomes much thicker on account of a hyperplasia of the contained neurogliar fibers.

Circulatory Disturbances

Anemia of the brain is characterized by a pallor of the cortex and white substance as a result of the diminished amount of blood. It is due to general anemia, severe hemorrhage, disease of the bloodvessels, particularly atheroma, increased intercranial pressure, or to spasmodic contraction of the blood-vessels.

Acute Hyperemia

The amount of blood in the brain is increased during its activity. Pathologically, it is found in beginning inflammations, in infectious diseases, acute delirium, sunstroke, etc. The blood-vessels in the pia are injected, the cortex is darker than normal, and minute hemorrhages may be present.

Passive hyperemia occurs in any of the conditions that prevent the blood escaping from the cerebral veins, as heart disease or local growths. The veins of the membranes are much distended, and the cortex and medulla are of a bluish tinge.

Edema of the brain is generally secondary to conditions causing passive congestion. There may be a slight edema that is soon taken up by the lymphatics. The fluid is most marked in the subarachnoid and ventricular spaces. The membranes are elevated, and the convolutions are flattened. The edematous fluid contains more albumin and is more cloudy than the normal. Indications of inflammation are usually present. This condition may be a terminal phenomenon. It is found in renal disease and in alcoholism. In the latter there is frequently a great excess of the fluid. Microscopically, there may be some proliferation of the endothelium, and around the blood-vessels there is some round-cell infiltration. Local edema is sometimes found in the neighborhood of areas of softening. In acute hydrocephalus the internal capsules may be involved and transient hemiplegia follow.

Hemorrhage occurs in two forms, minute (punctate) or massive.

Punctate hemorrhages are small collections of blood formed by diapedesis of the erythrocytes or by rupture of a small vessel, due to fatty degeneration of its walls. They are generally found in the cortex, and occur in the course of inflammation of the brain, in various infectious diseases, and in toxic conditions, particularly lead-poisoning. They seldom cause any secondary disturbances.

Massive hemorrhages, unless traumatic, are commonly found to affect the branches of the middle cerebral artery. In these cases there has generally been a pre-existing disease of the vessel, the rupture usually taking place in a small aneurysm. It is commonly known as apoplexy. The internal capsule is almost always involved. The pons is quite frequently the seat of hemorrhage, and occasionally the cerebellum, but very rarely the medulla.

The size of the involved area depends upon the amount of blood extravasated and upon the density of the tissue. It is more diffused in the white than in the gray matter. The effects of the hemorrhage may be primary or secondary.

Primary effects are tearing and compression of the brain-substance. If the patient does not immediately die, softening occurs. As a result of the staining by the retained hemoglobin the area is known as red softening. Shortly after the blood escapes it undergoes coagulation, forming a cerebral hematoma. This acts as a foreign body and sets up an inflammatory reaction, with more or less hyperplasia of the surrounding neuroglia. The fluid portion finally becomes absorbed, the corpuscles broken down, and the pigment liberated, which stains the walls of the cavity. Occasionally a cyst filled with a clear fluid may form. If all fluids are absorbed, the walls of the cavity may come in contact and a scar result.

Other primary effects are distant ones in the form of paralysis, both motor and sensory, and generally on the side of the body opposite to the seat of the hemorrhage. Although there may be a considerable return of the lost faculties, yet in a majority of cases permanent damage is done. This is due to the secondary effects - the secondary degenerations. These are systemic, and follow the direction of the nerve impulses. The commonest degeneration is one of the pyramidal tracts. The optic tract and fibers from the temporo-sphenoidal area may also be involved. In the brain there is finally loss of nerve-cells, without destruction of the neurogliar fibers.

As a result of traumatism, hemorrhages may take place in any part of the brain, with or without fracture of the skull. The hemorrhage may be found on the side of the head opposite to that where the blow was received.

Thrombosis And Embolism

Thrombosis is most common as a result of embolism or of endarteritis. It may be found anywhere, but is probably more frequent in the basilar artery. From obstruction to the nutrition encephalomalacia, or softening of the brain, ensues.

Embolism commonly results from a breaking off of a part of a verrucosity or of a leaflet of one of the heart valves, especially the aortic. The greater number of the emboli pass along and finally lodge in the artery of the Sylvian fissure. If the embolus is so large as to remain at the beginning of the artery, there will be a large degenerated area. The corpus striatum, a large part of the internal capsule, and the anterior part of the optic thalamus will be involved. As different branches are obstructed, the areas of degeneration will vary. If the emboli are so small as to enter the posterior perforated space, the optic thalamus only will show small areas of degeneration.