ous sinus and the facial veins by means of the orbit explains the liability of patients suffering from a facial erysipelas to a complicating meningitis. Leeching the nose will relieve headache, if congestive in type, because the longitudinal sinuses communicate with the veins of the nose. Depletion back of the ears may also be employed to deplete the lateral sinus through the mastoid vein. The liability of suppuration of the middle ear to a complicating meningitis is well recognized, and it is to be explained by the thinness of the bone between the dura and the tympanic cavity. As the sinuses of the brain receive tributaries from without chiefly through the sutures, the operation of trephining should never be performed over a suture if it can be avoided. Scalp-wounds are especially liable to become complicated by meningeal symptoms from the venous anastomosis that exists between the exterior and the interior of the skull. The escape of cerebrospinal fluid from the ear, in case of fracture of the base of the skull, is a valuable sign that the dura is lacerated in the internal auditory canal, and the tympanic cavity also involved. Displacement of the cerebro-spinal fluid from the subarachnoidean space at the base of the skull by tumors of the dura or skull, the occurrence of meningeal hæmorrhage, severe concussion, etc., may tend to explain the occurrence of vertigo, nystagmus, noises in the ears, and some forms of paralysis. This would be particularly the case if an excess of fluid were crowded into the cerebellar fossa.

THE ARACHNOID.-Between the dura and the pia there may be demonstrated a delicate non-vascular membrane of the fibrous type, called "the arachnoid." It is continuous with the membrane, filling the same relative position within the spinal canal, known as the "spinal arachnoid." It can be easily demonstrated by means of a blowpipe, the injected air lifting it from the pia. It forms sheaths for the cranial nerves, embraces the basilar artery, bridges over the more important sulci of the brain, covers the exposed portion of the corpus callosum, and forms the limiting membrane for the lymph-spaces of the more important vessels of the cranium.

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THE ARACHNOID AND PIA MATER.

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Between the arachnoid and the dura is a space, called the "subdural space," and between it and the pia is another space, known as the "subarachnoidean space." The latter is traversed by a delicate network of fibers, that subdivide it into compartments. The subdural space is lined with an endothelium. Both spaces are filled with a fluid that is similar to that which enters the ventricles by means of the foramen of Magendie. The normal quantity of this fluid that is found outside of the brain varies from a few drachms to about two Hilton compares this fluid at the base of the brain to a water-bed for protection against transmitted violence, as when a subject falls and strikes upon the feet. The effect of any lesion that tends to decrease the cubical contents of the cranial cavity must be to displace this fluid. But, since the fluid is not evenly distributed over the base of the skull, some regions are more exempt from this displacement than others. Again, lesions of the character described may prevent the escape of the cerebro-spinal fluid through the aqueduct of Sylvius, and thus disturb the beautifully adjusted relationship between the amount of fluid within the ventricles and the circulatory apparatus. Duret attributes the loss of consciousness that accompanies sudden lesions of the basal ganglia or the white substance of the cerebral hemispheres to a rapid displacement of the cerebro-spinal fluid. Blachez reports a case where a rupture of the basilar artery filled the entire area of the base of the brain with blood (Allen).

THE PIA.-This is a fibro-vascular structure that lies in direct contact with the cortex of the brain. It is continuous with the pia of the spinal cord, but differs from it in that it is more vascular and does not form ligaments. In the skull it consists of two layers, the outer being a receptacle of large vascular trunks and the inner for the smaller twigs that enter the cortex. The inner layer is continuous with the neuroglia. The pia sends prolongations into the ventricles of the brain, chiefly by means of the transverse fissure that lies between the cerebrum and the cerebellum. Allen states that it is so closely adapted to the walls of the fissure by which it enters

the ventricles as to resist the pressure of the cerebro-spinal fluid, and it thus prevents its escape from the ventricles.

The prolongations of the pia within the substance of the brain constitute the parts known as the "velum interpositum" and the choroid plexuses.

The velum interpositum has a triangular form and lies immediately beneath the fornix, and forms the curtain-like roof of the third ventricle. Its base corresponds to the transverse fissure, and its apex lies between the foramina of Monro. It incloses the pineal gland, and overlaps the optic lobes. Two large veins can be seen within its substance that empty into the straight vein of Galen. It is the largest prolongation of the pia.

The superior choroid plexuses are formed from the lateral margins of the velum interpositum and appear in the lateral ventricles. In each hemisphere the choroid plexus of the ventricle is prolonged into the middle or descending horn, lying upon its floor. The vascular loops that compose this plexus are covered with a layer of pavement epithelium.

In the third ventricle, two prolongations from the velum can be demonstrated. These are sometimes prolonged into the fourth ventricle.

The inferior choroid plexus lies upon the floor of the fourth ventricle (the posterior surface of the medulla), and consists of a median tuft of vessels that envelops the under surface of the worm of the cerebellum, and two lateral processes that run out into the angles of the ventricle. This plexus of vessels is generally derived directly from the pia, which penetrates into the fourth ventricle through the socalled "inferior transverse fissure." This is situated at the line of junction of the under surface of the cerebellum and the medulla. In this region, the arachnoid membrane becomes perforated, forming the so-called "foramen of Magendie," through which the cerebro-spinal fluid reaches the ventricular cavities of the brain.

The pia is supplied with nerves from the third, sixth, seventh, eighth, and eleventh cranial nerves and from the

THE CEREBRAL CIRCULATION.

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sympathetic system. Its blood-vessels are derived from the vertebral and internal carotid arteries.

THE BLOOD-VESSELS OF THE BRAIN.

The vessels of the brain are of great interest to the surgeon, because they have a direct bearing upon the pathology and symptomatology of injuries of the head. To the general practitioner also the vessels of the encephalon furnish many suggestions of value respecting those diseases that attack the brain substance or the meninges. It is not out of place, therefore, to call attention to the more important facts that have been published by those observers who have devoted special care to the investigation of the anatomy of the vessels of the brain and the peculiarities of its circulation.

The blood is sent to the brain and its coverings chiefly by means of two large trunks upon either side, the vertebral and internal carotid.' The vertebrals enter the skull by means of the foramen magnum and unite to form the basilar. The carotids enter farther forward, by means of the carotid canals in the petrous portion of each temporal bone. The branches that derive blood from the vertebrals are called, when collectively considered, the posterior or “vertebral system.” Those that spring from the carotids are called the anterior or "carotid system."

The "vertebral system" is distributed to the posterior portions of the cerebrum, the cerebral peduncle, the cerebellum, pons, medulla, corpora quadrigemina, and the posterior part of the thalamus.

The "carotid system" is distributed to all the important parts of the cerebrum lying anterior to the cerebral peduncle in the region of its base, the frontal lobes, the anterior and outer parts of the temporo-sphenoidal lobes, the insula, the two nuclei of the corpus striatum, and the anterior portion of the thalamus.

The coverings of the brain derive blood from other sources as well; chiefly from the internal maxillary, ascending pharyngeal, and occipital arteries.

It will be necessary to consider the separate branches of the internal carotid, vertebral, and basilar arteries, in order to give the reader a clear conception of the areas of braintissue that are nourished by each.

The INTERNAL CAROTID ARTERY, on escaping from its bony canal and entering the cavity of the skull, turns sharply upward and backward and gives off the following branches: 1, the ophthalmic; 2, the anterior choroid; 3, the anterior cerebral; 4, the middle cerebral; and, 5, the posterior communicating.

The ophthalmic artery passes directly into the orbit and distributes its blood by many branches to the eye and its appendages. This fact enables the neurologist to determine often, by means of the ophthalmoscope, the condition of the cerebral vessels, because similar changes may be detected in the blood-vessels of the retina.

The anterior choroid artery passes backward to the transverse fissure of the brain and assists in supplying the vessels of the choroid plexus. It lies in relation with the extremity of the temporo-sphenoidal lobe, which conceals it from view for a part of its course.

The anterior cerebral artery winds around the edge of the optic chiasm and meets its fellow about one twelfth of an inch in front of the chiasm, where the two become joined by a short branch, the anterior communicating artery. From this point the two vessels run side by side, following the curve of the corpus callosum from its beak to its posterior extremity. This vessel supplies the optic chiasm, the lamina cinerea, the anterior portion of the caudate nucleus, the corpus callosum and the adjacent fornix, and the convolutions upon the inner surface of the hemisphere of the same side, as far as the cuneus. The importance of the relation of this vessel to the edge of the optic chiasm, as a factor in the production of that rare condition known as "bi-temporal hemianopsia" has been shown in a paper by Prof. H. Knapp, of this city. This condition is discussed later in the volume.