THE LATERAL VENTRICLE (Ventriculus Lateralis) The hemispheres contain the largest of the six ventricles (Figs. 37, 46, 53, 54 and 96). Situated one on either side of the median line, the ventricles of the hemispheres are very naturally called the lateral ventricles. Each represents a branch of the cavity of the embryonic neural tube (Figs. 17 and 53). In consequence, the lateral ventricles communicate with all others except the fifth. By the interventricular fora men (of Monro), each directly communicates with the third ventricle; and through that, indirectly, with the fourth and sixth. The foramen interventriculare is situated between the front of the thalamus and the calumna of the fornix (Fig. 35). It extends between the anterior extremity of the third ventricle (the aula) and the junction of the anterior horn with the central parr of the lateral ventricle. The lateral ventricles are lined with ependyma, which is a transparent membrane composed of two layers when complete, viz., neuroglia and a covering of columnar ciliated epithelial cells. Over the thala mus (the part seen in the lateral ventricle) and the chorioid plexus, the neurogliar layer is absent.
The ventricles are filled with a displaceable liquid, called the cerebrospinal fluid. This fluid is secreted by the epithelial cells of the chorioid plexuses and constantly flows out into the subarachnoid spaces; it escapes through the medial wall of the inferior horn of the lateral ventricle and the roof of the fourth ventricle. The whole amouut of cerebrospinal fluid is said to average from ioo-130 cc.; but in any individual it varies inversely as the brain-mass; with increased blood supply or hemorrhage or tumor growth in the brain, the amount of fluid is diminished so, that dangerous pressure will not be exerted upon the delicate brain tissues. Varied function is thus made possible and often life is preserved. The ancients considered the ventricles the abode of the soul.
The lateral ventricle may be studied best in four parts: the central part (or body); the anterior horn; the inferior horn; and the posterior horn.
The central part of the lateral ventricle (Figs. 45, 47 and 5o) is the ventricle of the parietal lobe of the cerebrum. The following are its boundaries: Roof—Corpus callosum.
Floor (from before, backward and inward)— Caudate nucleus of the corpus striatum, Vena terminalis and stria terminalis (tmnia semicircularis), Thalamus (covered by epithelium), Lamina chorioidea epithelialis and chorioid plexus, Fornix.
Medial wall—Septum pellucidum.
Lateral wall—Internal capsule.
The corpus callosum forms a complete roof for the central ,part of the lateral ventricle. The roof inclines upward and outward from the septum pellucidum, the inner wall of the ventricle, to the superior lamina of the internal capsule, which forms its outer wall. The floor of the central part of the ventricle is formed by the six parts, as named above, which will now be considered in the order given.
Corpus Striatum (Figs. 37, 38, 39 and 4r).—The striated body is the basal ganglion of the hemisphere. It is an ovoid mass of gray matter imbedded, for the most part, in the cerebral 8 medulla; but it is continuous below with the anterior perforated substance, and extends above to the lateral ventricle. It measures 6.3 cm. (2.5 in.) from before backward, 3.1 cm. (1.25 in.) transversely, and, from above downward, 3.7 cm. (1.5 in.). It is placed anterior and lateral to the thalamus and forms the third of the great divisions of the cerebral hemisphere, viz., the neopallium, the rhinencephalon and the corpus striatum. It is a reddish-gray body, and its streaked appearance is due to the white capsular fibers which pierce it. Embryologically the corpus striatum is built up of several nuclei which appear in man in the order of their philogenetic origin; first, the globus pallidus of the lentiform nucleus, the only part found in fishes; second, the nucleus amygdalce; and, third, the caudate nucleus and putamen of the lentiform nucleus. The last three are first developed in reptiles and birds. In the mature human brain the caudate and lentiform nuclei are easily distinguished, being separated by the internal capsule; but the amygdala has no definite boundary and forms the antero inferior part of the striate body where the capsule does not divide it. This undivided part of the corpus striatum is continuous with the uncus, the anterior perforated substance and the claustrum. The amygdala, which forms a part of the uncus, is a reflex center of the rhinencephalon. The globus pallidus, assisted by the putamen and caudate nucleus, constitutes an autonomous organ to steady the action of the lower motor neurones, preventing hypertonicity, rigidity and tremor (S. A. Kinnier Wilson).