VENTRICLES OF THE 'BRAIN. The lateral ventri cles represent the expanded anterior end of the embryonic neural canal. and are the cavities of the hemispheres. They are lined by a single layer of simple cylindrical epithelium, the epen dynia. derived from the epiblastic lining of the neural canal. The two ventricles arc separated from each other in the median line by a vertical septum known as the septum hieidum. The shape of the ventricles is irregular, each ven tricle consisting of a main cavity or body and extending olr from this three smaller cavities called cornua or horns. The anterior horn passes outward and forward into the anterior lobe. The posterior horn extends backward into the posterior lobe, while the middle horn descends into the substance of the middle lobe. The main ea% ity of the ventricle is roofed in by the under surface of the corpus callosuni. its floor is made by the corpus striatum and optic thalamus, al though the ta'nia semicireularis, ehoroid plexus, corpus timbriatum, and fornix are also situated in its floor. The third ventricle is a long nar row cavity lying between the optic thalami. Above it is bounded by the under surface of the velum interpositum, to which is attached the choroid plexus of this ventricle. Its lloor is made by the lamina cinerea, tuber einereum, in fundibulum, corpora albicantia, and posterior perforated space. Three commissures cross the cavity of the ventricle, known respectively as the anterior, the middle, and the posterior coin missures. Four openings connect the third ven tricle with other cavities. In front the fora mina of Monroe, one on each side, serve to con nect the third ventricle with the lateral ven tricles. Posteriorly, the aqueduct of Sylvius con nects the third ventricle with the fourth. A fourth opening in the anterior part of the floor leads into the cavity of the infundibulum. The so-called fifth ventricle of the brain is. prop erly speaking, not a true ventricle, being simply a part of the great longitudinal fissure \villa has been cut in the development of the brain, by the union of the two hemispheres, through the corpus callosum and the fornix. It is not lined by epithelium as are the other ventricles. The fourth ventricle is described above in con nection with the medulla oblongata.
Certain structures remain to lie described which have already been mentioned as lying in the floor of the lateral ventricles. The two largest of these, often called the basal ganglia, are the corpora striata and the optic thalami.
The corpora striata are two masses of mingled gray matter and white matter, the superior surfaces of which appear in front of the body of the lateral ventricles. The extra-ventricular portion is known as the lenticular nucleus. The intra-ventricular part known as the caudate nu cleus is smaller than the extra-ventricular, from which it is separated by the internal capsule, a large band of fihres which represents the con tinuation upward of the main tracts of the cord and crura on their way to the cerebrum. A layer of white matter which covers the extra ventricidar portion of the lenticular nucleus is known as the external capsule.
The optic thalamus lies to the inner side and behind the lenticular nucleus, from which it is separated by a part. of the internal capsule. Each thalamus is composed of white matter ex ternally, of gray matter internally, and rests upon the corresponding erns cerehiri. It forms a part of the lateral wall and floor of the holy of the lateral ventricle. As the middle nr de scending horn of the ventricle descends the thalamus collies to form the roof of this portion of the ventricle. in front is a prominence known as the anterior tubercle, while its posterior part presents two small rounded eminences. the in ternal and the external genicnlate bodies.
The I:cilia semieireularis is a narrow band of white fibres which lies in the depression between the caudate nucleus and the optic thalamus. An teriorly, it follows the descending pillar of the fornix; posteriorly. it passes into the descending horn of the ventricle, and at the bottom of the horn enters a mass of gray matter known as th2 nucleus amygda The velum interpositum is a vascular extension of the pia mater into the interior of the brain through the great transverse fissure. It enters the third ventricle beneath the corpus eallosum and above the optic thalami, corpora quadrige mina, and pineal gland. Anteriorly, it bifurcates.