RECAPITULATION OF 'THE TRACTS OF THE CEREBRO SPINAL AXIS.
In the foregoing pages we have studied the several parts of the cerebrospinal axis in segments, and have connected each by its immediate relations, and by the continuity of the tracts of which it is composed.
It will now be of advantage to review the axis as a whole, and to trace its tracts in continuity through its entire extent, in connection with the nerve centers with which they are associated. In so doing, we will attempt to arrange the several tracts and nerve centers into general systems, to accord with their supposed functions, in order to assist the memory in retaining a knowledge of the anatomy, and to simplify what at first sight appears difficult to the student.
Beginning, therefore, with the spinal cord, we will follow each system from its origin below to its destination above.
The ventricular system begins as a minute canal, extend ing through the length of the spinal cord and terminating above in the fourth ventricle, at the middle of the posterior surface of the medulla oblongata. The fourth ventricle is a broad cavity, situated behind the upper half of the medulla and the Pons Varolii. At its upper extremity the fourth ven tricle is contracted into a small canal, the iter, or aquaduct of Sylvius. The iter is about three-fourths of an inch in length, lies beneath the upper extremity of the valve of Yieussens, the commissure of the corpora quadrigentina, and the posterior commissure of the third ventricle. The iter terminates above
63 in the third ventricle. The third ventricle is a vertical cavity' between the thalami optici, extending from the base of the brain to the fornix above, and laterally, upon each side, to the edges of the fornix where it comnannicates with the lateral ventricles through the fissure of Bichat and the foramina of Monro. The lateral ventricles are large cavities in the hem ispheres of the cerebrum, and each ventricle sends projections into the frontal, temporal, and occipital lobes, called the an terior, middle and posterior cornua of the lateral ventricles. The fifth ventricle is enclosed by the two layers of the sep tum lucidum, and has no communication with the other cavi ties of the brain. The floor of the third ventricle is con nected with a, cavity in the pituitary body by a canal through the infundibulum.
The ventricular system communicates with the subarach noid spaces along the great transverse fissure, and by a foramen in the pia 'Dater above the calanaus scriptorius, called the foramen of Majendie.