THE THIRD AND FOURTH VENTRICLES.

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unites the thalami. It is often torn across in removing the brain.

Posteriorly, the third ventricle unites with the fourth by means of a narrow tubular canal, the "aqueduct of Sylvius (iter e tertio ad quartum ventriculum). This canal passes beneath the corpora quadrigemina, and may be regarded as a homologue of the central canal of the spinal cord, which has been expanded in the region of the fourth ventricle, in order to allow of the many nuclei of origin of the cranial nerves found in its floor. Anteriorly, the third ventricle communicates with the lateral ventricle of each hemisphere by the foramina of Monro.

The third ventricle, the aqueduct of Sylvius, and the fourth ventricle are lined with a continuation of the gray matter that surrounds the central canal of the spinal cord, the so-called "central tubular gray substance."

The relation of the third ventricle to the pillars of the fornix, the lamina cinerea, and the structures that form the interpeduncular space at the base of the cerebrum, are made very apparent in the preceding cut.

The gray substance that lines the third ventricle has been described in connection with the thalamus, to which the reader is referred for information respecting it.

The

THE FOURTH VENTRICLE.--This cavity is properly regarded as an expansion of the central canal of the spinal cord. It communicates with the third ventricle above, and the central canal of the cord and the subarachnoidean space below. latter communication takes place through the "foramen of Magendie." It allows of the entrance and escape of cerebrospinal fluid. The importance of this as a means of equalizing pressure upon the brain substance (when the vascular supply is increased or diminished) will be considered in detail in connection with the spinal meninges.

The fourth ventricle lies below the level of the cerebral hemispheres. Its gray matter contains the nuclei of origin of the more important cranial nerves. These have been discussed in connection with the architecture of the medulla. Its roof

is formed by the under surface of the cerebellum and the socalled "valve of Vieussens."

THE COMMISSURES OF THE BRAIN.

In connection with the ventricles, the corpus callosum and fornix have been mentioned. They deserve further consideration, as they have not been separately described in previous pages.

THE CORPUS CALLOSUM.-This commissural band has been discussed to some extent in those pages that treat of the commissural fibers of the cerebral hemispheres. It is about three inches long by three quarters of an inch in breadth, and lies at the bottom of the great longitudinal fissure. It is the great commissural band between the hemispheres, and forms the roof of the lateral ventricle of each. Anteriorly, it curves downward to reach the base of the brain, and posteriorly it dips downward to form the "splenium." The anterior bend is termed the "genu" or "anterior flexure" of the callosum. The splenium reaches as far as the transverse fissure of the cerebrum, and bears intimate relationship with the pineal gland and optic lobes. The so-called " peduncles of the callosum" reach to the anterior perforated spaces at the base of the cerebrum. Fibers of the callosum can be traced to the following parts:

1. The white substance of the cerebral hemispheres. 2. The gyrus fornicatus.

3. The fornix.

4. The occipital lobe.

5. The temporo-sphenoidal lobe, along the descending horn of the lateral ventricle.

The fibers of the callosum are both longitudinal and transverse. They serve to unite the component parts of the cerebral hemispheres. The transverse fibers probably assist in uniting homologous parts of each hemisphere. The function of the longitudinal fibers is not well understood.

The corpus callosum is sometimes defective or absent.

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When so, the septum lucidum and the fornix are also, as a rule, defective. The mental condition of subjects with a defective corpus callosum, according to the researches of Knox, is impaired in proportion to the imperfections found in it and the other commissural systems. Idiocy and imbecility have occurred as a consequence of this form of congenital deformity of the brain. The anterior commissure appears to be relatively large in those animals that have the corpus callosum imperfectly developed. It is possible that this band may take the place of the corpus callosum in those rare cases where that body is rudimentary or absent in man. Ward reports a remarkable observation ("London Medical Gazette," March, 1846), where the brain of a child of about one year of age separated into two equal halves when it was removed from the skull, on account of the absence of all transverse commissural systems in the cerebrum and pons.

THE FORNIX.-The arched fibers of this structure serve apparently to unite the tip of each temporo-sphenoidal lobe with the thalamus of the corresponding hemisphere, and, by their fusion in the mesial plane, to join the two hemispheres with each other. Each lateral half of the fornix presents an anterior pillar (which passes to the base of the brain and then doubles upon itself in order to unite with the thalamus (Fig. 67), and a posterior pillar (which enters the middle or descending horn of the lateral ventricle as a flattened ribbon-like band, called the "corpus fimbriatum "). This band terminates in the so-called "corpus dentatum" of the descending horn.

The arched fibers of each lateral half of the fornix become united with those of its fellow in the mesial line, thus forming the so-called "body of the fornix."

The body extends, anteriorly, from the point of commencement of the anterior pillars of each side to that of the posterior pillars. It helps to form the roof of the third ventricle, lying above and in close contact with the reflection of the pia mater called the "velum interpositum." It becomes fused posteriorly with the corpus callosum. The shape of

the body of the fornix is triangular, the apex pointing forward. Its upper surface appears upon the floor of the central cavity of the lateral ventricle. In order to expose the third ventricle of the cerebrum from above, it is necessary to divide the body of the fornix close to the anterior pillars, and to turn it backward. The velum interpositum with its vessels then comes into view. This membrane has to be also removed before the ventricular cavity is seen.

COMMISSURES OF THE THIRD VENTRICLE.-The posterior commissure of the third ventricle has been considered in connection with the tegmentum cruris, and the anterior commissure will be discussed in connection with the olfactory nerves. The middle commissure is an integral part of the thalami, which it serves to unite.

THE MEMBRANES OF THE BRAIN.

The brain has three coverings, called, from without inward, the dura mater, the arachnoid, and the pia mater. The exterior, or dura, is essentially protective in function; although it serves some other purposes, such as the formation of venous channels, the support of certain parts, the nourishment of the bones, etc. The arachnoid is a fibro-serous membrane, and is structurally related to the lymph channels, as are all serous membranes. The pia is a vascular membrane, and serves to nourish the parts with which it comes in contact. It will be necessary to consider each of these membranes separately.

THE DURA. This is a dense fibrous membrane, closely adherent to the base of the skull and along most of the cranial sutures. It is loosely attached, however, to the convexity of the skull, save at the sutures. Small vessels pass from its exterior surface into the diploë, or middle layer of the bony skull-cap. Its inner surface is smooth, and is lined with pavement epithelium. It is in relation to the so-called "subdural space." Around the margins of the cranial foramina, the petrosal ridges, and the crista-galli process, the dura is

THE CEREBRAL SINUSES.

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particularly firm in its attachments. The "sella turcica," that holds the pituitary body, is covered over by a process of this membrane, which binds the pituitary body firmly in place and conceals it from view.

Processes of the Dura.-The dura assists to form three processes, called the cerebral falx, the cerebellar falx, and the tentorium. The falces of the cerebrum and cerebellum prevent lateral oscillation of the cerebral and cerebellar hemispheres, while the tentorium supports the posterior part of the cerebrum and prevents it from injuring the cerebellum by its weight.

The Cerebral Sinuses.-Along the upper and lower borders of the falx cerebri the two reduplicated layers of the dura assist to form the superior and inferior longitudinal sinuses. Where it joins the tentorium, the straight sinus is formed. The attachment of the tentorium to the skull forms the lateral sinuses, by splitting of the dura into two layers. The occipital sinuses run along the sides of the falx cerebelli. At the base of the skull we encounter the transverse and circular sinuses, both of which cross the median line, and also three pairs of sinuses, viz., the superior petrosal, the inferior petrosal, and the cavernous.

There are some clinical suggestions of value that may be made in connection with the dura. Inflammatory affections of that membrane may induce thrombosis of the cerebral sinuses, although that condition can occur also from extension of inflammation from other parts by means of the communicating veins, and as the result of pressure exerted upon them by intracranial lesions. The vessels of the dura may be the seat of extravasation of blood, and suppuration between the dura and the skull may follow traumatism. The nerves of the dura cause a headache, when that membrane is the seat of disease, or is pressed upon; as, for example, in the case of a cerebral tumor. Encephaloid cancer seems to occur most frequently in the dura about the foramen magnum. In this case, the symptoms would be closely allied to those of a lesion of the medulla. The communication between the cavern