THE VENTRICULAR, SYSTEM, —In the centre of the mass throughout its whole length is a cavity or ventricle which is also modified at different points along its course. In the spinal cord it is a slender canal which, behind the medulla oblongata, con nects with the posterior median fissure by an interruption of the posterior commissnre of the spinal cord. At this point, on account of the lateral separation of the posterior columns of the cord, the minute canal expands into a broad cavity, the fourth ventricle, which is situated behind the upper half of the medulla oblongata and the polls Varolii, and is beneath the cerebellum. Above the pons, the cavity of the fourth ventricle is contracted into a canal about the size of a, small quill, the iter, which is about five.eighths of an inch in length, lies beneath the optic lobes and between the crura of the cerebrum, and connects the third and fourth ventricles of the brain.
The iter terminates above in a vertical slit, the third ventricle, situated between the opposed sides of the thalami optici, or basal ganglia, of the cerebrum. The third ventricle reaches below to the. base of the brain, and above extends lat erally outward over the thalami to the edges of the foruix, beneath the margins of which it is continuous with the lateral ventricles of the hemispheres. The lateral ventricles are large cavities, one in 111.-_, center of each hemisphere, pro longations of which extend into the lobes of the hemispheres, and are called the anterior, middle and posterior cornua of the lateral ventricles.
Thus it may be said that the cerebro spinal axis is a hol low tube of nervous matter divided into lateral halves which are separated by a fissure and united by a commissure. The cavities will be again des:cribed in connection with the parts that enclose them.