CENTRAL OR VENTRICULAR GRAY MATTER It is located (I) in the floor and walls of the third ventricle, the hypothalamus; (2) in the middle commissure of that ven tricle, the massa intermedia; and (3) around the cerebral aqueduct, the stratum griseum centrale.
1. The Hypothalamus, Pars Optica.—The lamina cinerea terminalis and the tuber cinereum (Figs. 21 and 33) form a sheet of gray substance that connects the inferior and medial sur faces of the cerebral hemispheres. The tuber cinereum, on each side of the median line, contains a small triple nucleus, the supraoptic nucleus of Cajal. It is made up of an anterior, a posterior and a dorsal cell-group. It is not surely known whether any fibers of the superior commissure or inferior com missure rise or terminate in this nucleus; nor is the relation to it of the basal bundle of Wallenberg, the olfacto-mesencephalic fasciculus, understood. Probably a part of this path is relayed in the supraoptic nucleus. The optic chiasma is white matter, and the hypophysis is not composed of nerve tissue at all and, therefore, neither one need be described in this place. From the floor of the third ventricle the gray matter extends laterally beneath the thalamus, and is continuous with the anterior perforated substance. The gray matter of the floor also ex tends up to the sulcus hypothalamicus on the medial surface of the thalamus. The tuber cinereum receives efferent fibers from the corpus striatum of both sides. Some of these fibers form a commissure just above that of Gudden; hence it is called the commissura superior (Meynerti) to distinguish it from the commissura inferior (Guddeni) in the optic chiasma. The fibers of Meynert's commissure cross through the tuber ciner eum anterior to infundibulum.
Hypothalamus, Pars Mammillaris (Figs. 31 and corpora mammillaria (albicantia), though composed of fornix fibers on the surface, contain in the interior two nuclei, the medial and lateral. The medial nucleus is the larger of the two. It receives the end-tufts of the fibers in the columna of the fornix and gives origin to internuncial fibers, connecting it with the lateral nucleus, and to the fasciculus mammillaris princeps.
The latter bifurcates, sending one branch, the mammillo thalamic bundle (of Vicq d'Azyr), up to the anterior nucleus of the thalamus and the other branch, the fasciculus mammillo tegmentalis, backward into the tegmentum. The fasciculus mammillo-thalamicus (Vicq d'Azyri) connects the fornix with the thalamus. The mammillo-tegmental bundle descends to the nucleus tegmenti profundus and central gray matter of the mid-brain; some of its fibers continue along the medial longi tudinal bundle into the reticular formation of the pons and medulla, probably ending in various efferent nuclei.
The small lateral nucleus of the corpus mammillare gives origin to the peduncle of the mammillary body, which accord ing to Flechsig ends in the nucleus tegmenti dorsalis and the substantia grisea centralis of the mid-brain and is thence con nected with the motor nerve-nuclei and the automatic centers of the medulla by the dorsal longitudinal bundle of Schutz. Through the fornix, the stria medullaris thalami, the habenulo peduncular tract and the descending axons of the inter peduncular nucleus; through the fornix, the mammillo-thalamic bundle and the thalamo-spinal fasciculus; through the mam millo-tegmental tract, the peduncle of the mammillary body, the olfacto-mesencephalic bundle and the dorsal longitudinal bundle of Schutz, etc., some of the reflex connections of the olfactory nerves are established.
2. The massa intermedia (the middle commissure, Figs. 33 and 87) joins the medial nuclei of the thalami. It is formed, when present, by the approximation and fusion of the thalami in the second month of embryonic life. It is occasionally ab sent. In the massa intermedia are cell-bodies and transverse fibers. The latter appear to be loops which reach only to the median line; at least many of the fibers do not cross to the oppo site side. It receives internuncial fibers from the thalamus (Sachs). According to Cajal the massa intermedia of the rabbit contains eight distinct nuclei. It appears to be accessory to the medial nucleus. It is not a commissure in the ordinary sense of that term.