quently in the cerebral hemispheres than in the basal ganglia or the cerebellum. It always springs from the neurogliar tissue of the central nervous system, and is remarkable for its extreme richness in cells, that by suitable staining-methods may be shown to possess protoplasmic processes (astrocytes). These protoplasmie processes, either wholly or in part, form the so-called matrix of the tumor, which appears to be composed of numerous fine, interlacing fibers, in the midst of which, by the ordinary staining-methods, the cell appears to lie. Usually the tumor is exceedingly vascular, the vessels consisting of small capillaries or larger spaces lined with endothelium, and this vascularization may be so excessive that the tumor in parts has a delicate pinkish or reddish color, giving to the cross-section a mottled appearance. Stroebe considers as one of the characteristics of this tumor the fact that it does not infiltrate the pia mater, and can invariably be shown not to spring from it. In some cases the consistency of the tumor is quite hard, and in these the cellular elements are increased in amount proportionately to the fibrous tissue. Ordinarily true nervous tissue is absent from the midst of the tumor, the mass in the center being made up of the neuroglia-tissue. This gradually diminishes toward the periphery, and ultimately fades into the true nervous tissue, that may be somewhat edematous. In some cases, however, as in ordinary sclerosis of the central nervous system, myelinated nerve-fibers may be found in the midst of the tumor, and it frequently happens that the secondary degeneration extending from a glioma is much slighter than the apparent extent of the process would lead one to expect. Occasionally, either glioma-cells, or possibly ganglion-cells, that have undergone proliferative changes in the midst of the growth, may be found. These are much larger than the ordinary cells and contain numerous branched processes, and often one or several large nuclei. This is the so-called ganglionar neuroglioma.

Some pathologists hold that the starting-point of glioma is always one or more ependymal cells that have been displaced in embryonal existence and have failed to assume the type of glia-cells, basing their theory in part upon the atypical shape of many of the glioma-cells.

Sarcoma of the brain is probably the next most frequent tumor. It usually occurs in middle adult life, although it is quite frequent in children. The commonest seat is the cortex, which it probably invades from a primary focus of proliferation in the membranes. The tumors are ordinarily nodular growths, usually distinctly circumscribed from the surrounding tissue, and in some cases even surrounded by a fibrous capsule, from which they can be readily removed. They are rather firmer than the brain-tissue, and sometimes slightly umbilicated if at the surface. The surface of section is pale and dry, but frequently mottled on account of the presence of hemorrhages. If primary, the sarcomata are single; if

secondary, more frequently multiple, and in these cases they are very apt to be found upon the convex surface of the hemispheres, forming, if large, irregular flattened masses. Histologically, almost any type of sarcoma may be found, the most frequent, perhaps, being the round-cell, non-pigmented form. Giant-cells are very frequently found. In the midst of true sarcoma, proliferated fibrous tissue is not found, but many of the tumors are traversed by fibrous trabeculæ, and some of them contain considerable masses of fibrous tissue, giving rise to the so-called fibrosarcoma. Ordinarily the tumors are extremely vascular, and occasionally contain interstitial hemorrhages. The surrounding brain-tissue shows the symptoms of marked compression, is edematous, and may contain small interstitial hemorrhages. The true nervous substance is ordinarily degenerated, and we find extensive secondary degeneration as a result of the presence of the tumor. Sarcoma springs primarily from the brain, and nearly always grows toward the surface and infiltrates the pia mater.

Certain forms of tumors have been described that appear to spring from the adventitia of the blood-vessels. They consist of masses of cells usually sharply circumscribed, somewhat cylindrical in shape, having in their center a small lumen in which bloodcells may sometimes be detected. These are the so-called perithelioma.

Many authors believe that a combination of glioma and sarcoma may occur, and gliosarcomata have been frequently described. As the neurogliatissue has the functions of connective tissue, and in many respects resembles it in its pathologic processes, it is natural that a glioma should be similar to a sarcoma, and this has possibly caused an error of diagnosis in some cases. As the two tumors arise from tissue of different natures, and develop in different situations, their combination is theoretically unlikely.

Fibroma occurs as a hard, circumscribed tumor, sometimes found in the hemispheres, but it is extremely rare.

Lymphangioma appears to arise from the pia.

Osteoma is usually an extension inward from the skull or membranes, but occasionally occurs as a tumor, apparently originating in the brain-substance, forming hard masses from the size of a pea to that of a cherry, and somewhat irregular in shape.

Psammoma.-This special term has been given to tumors containing calcareous granules. It has already been mentioned in connection with the choroid plexus and the membranes. Occasionally circumscribed fibrous tumors containing calcareous granules are found in the brain-substance, but these are extremely rare (Fig. 352).

Carcinoma of the substance of the brain is invariably a metastatic growth. It appears either as small, round, circumscribed nodules, of firm consistency and pale color; or as larger, more or less infiltrating masses with softened interior. Histologically, the tumor may present any type of carcinoma, according to

the nature of the primary growth. It usually replaces entirely the nervous tissue, either pushing it aside or destroying it, and gives rise, therefore, to more or less pronounced secondary degeneration. Multiple metastatic carcinomatous growths, however, may exist in the brain without causing clinical symptoms.

THE CHOROID PLEXUS.

The choroid plexus, as has been stated, consists of a plexus of vessels derived from the mesoblast, that are everywhere cov

FIG. 352.-Psammoma, showing calcareous spicules and whorls.

ered by a layer of cuboidal or columnar epithelium. Histologically, therefore, it consists of an inner layer of endothelial cells and an outer layer of epithelial cells, separated by a small amount of areolar fibrous tissue. This condition is most distinct and typical in the lower vertebrates, particularly in amphibians, in which the choroid plexus consists, in some parts, of a single vessel covered by epithelial cells. The structure of the choroid plexus, therefore, bears a very close resemblance to the structure of the glomeruli of the kidneys, and, as it is obvious that the blood-vessels of which it is composed can have no nutritive function, excepting in so far as they supply the epithelium that covers them, it has been supposed that the choroid plexus is a secretory organ, its object being to secrete the cerebrospinal fluid. This theory is sustained by the fact that the cerebrospinal fluid differs in composition from the lymph, or from the fluid poured out in serous exudations. It would naturally be supposed, therefore,

that in all conditions in which the alterations in the composition of the cerebrospinal fluid have been detected, there is primarily some disease of the plexus.

Hypersecretion.-In certain cases in which communication between the cavities of the brain and the interior exists-as, for example, through the ethmoid plate of the nose-there is distinct hypersecretion of the cerebrospinal fluid. In a case described by St. Clair Thomson, as much as a pint was discharged in twentyfour hours. In these cases, if for any reason the flow is checked, symptoms of intracranial pressure occur. Regarding the pathologic changes in the choroid plexus that are associated with this condition we have at present no information. Hypersecretion probably also takes place in all inflammatory conditions.

Inflammation. In inflammatory conditions, particularly meningitis, encephalomyelitis, and ependymitis, either separate or combined, and associated with the presence of an abnormal quantity, of albumin in the cerebrospinal fluid, more or less inflammation of the choroid plexus is usually found. This is indicated particularly by round-cell infiltration about the vessels and beneath the epithelium. Sometimes there is a slight amount of exudation upon the epithelial surface, and occasionally small foci of suppuration can be detected.

Infectious Diseases.-Among the infectious diseases the only one of importance is tuberculosis. In this condition miliary tubercles or large cheesy foci may be found.

Chronic inflammation appears to be exceedingly rare, or, what is probably more accurate, has rarely been described, for there is every reason to believe that the choroid plexus shares in all conditions that involve the pia mater. Atheromatous conditions seem to be very infrequent, but a certain amount of thickening of the walls of the vessels may be observed in old age. A certain amount of overgrowth of connective tissue, sometimes associated with slight round-cell infiltration, is also occasionally found in old people.

Hemorrhage may take place into the choroid plexus, and is usually an agonal phenomenon.

Degenerations.-Calcification is the most frequent of the infiltrations. This may be recognized by the gritty sensation upon cutting into the choroid plexus. Microscopically, the calcareous nodules appear as minute dark granules that stain purplish with hematoxylin. Masses of cholesterin are by no means uncommon; they rarely present the form of typical cholesterin-plates, but are somewhat irregular in outline, of a faint yellowish color, and shine brightly when examined by oblique light. Both conditions are common in old people.

Pigmentary infiltration probably also occurs, certainly in malaria, and perhaps in old age.

Amyloid degeneration occurs in cases of amyloid disease.

Tumors. A great variety of tumors occur in the plexus. Cysts may be either retention- or extravasation-cysts, or parasitic. The retention-cysts may be lined with epithelium, and probably represent adhesions at the edges of fissures with persistence of secretory activity. More commonly they are lined with endothelium and represent dilated lymph-spaces. They may be single or multiple, and often are more numerous upon one side of the brain than on the other. They appear as small translucent sacs, filled with a clear, slightly yellowish liquid, or else a colloid-like, viscid mass. Parasitic cysts are usually due to Tænia solium. They may be either single, the Cysticercus cellulose, which may be attached to the plexus or lie free in the ventricular cavity, or multiple, forming the so-called Cysticercus racemosa of the brain.

Among the benign tumors, fibromata, possibly but organized thrombi, and lipomata have been described. The latter is exceedingly rare, and is usually associated with multiple lipomata in other parts of the brain. A dermoid cyst has been reported by Lebert. Endotheliomata may spring from the lymphatic space. They are small, white, infiltrating growths, of firm consistency. They possess, microscopically, the ordinary alveolar structure, the cavities being lined by endothelial cells, which have proliferated and may lie loose in or fill up the lumen. Degenerative changes in the stroma or in the cells themselves, particularly of a hyaloid nature, are very common in these tumors. Epitheliomata may arise from the epithelial covering. Two varieties have been described: the papillomatous epithelioma is a small, often microscopic, tumor that appears as a villous outgrowth upon the surface of the plexus, and is composed of flat or polygonal epithelial cells, with distinct evidence of proliferation that may, in the lower layers, cause a breaking through of the basement-membrane. These tumors, however, are apparently non-malignant. The so-called cylindrical carcinomata also occur. These grow very rapidly and usually become attached to the wall of the ventricle, and from this situation invade the substance of the brain. They are alveolar in structure and soft in consistency. There is some doubt as to whether these tumors do not arise from the ependyma, and it has also been suggested that they are sarcomatous in nature and spring from the adventitia of the blood-vessels. They are usually single growths and do not give metastases. The choroid plexus, as a whole, may be altered, by the presence of cysts, into a mulberrylike mass, or by a general fibrous change, into a pale, solid body of cylindrical shape with a finely granular surface.