Page:The New International Encyclopædia 1st ed. v. 14.djvu/431

* NERVOUS SYSTEM AND BRAIN. 381 NERVOUS SYSTEM AND BRAIN. arc two siiuiU while piotubcraiices each about the size of a jjca just behind the tuber cinereum. They are composeil externally of white matter, internally of gray matter, and are formed by the anterior crura of the fornix. (10) The pos- terior perforated space lies between the corpora albicantia and the anterior fibres of the pons. Its perforations are due to vessels which pass to the optic thalarai. (11) The optic commissure is the crossing of the right and the left optic tracts to form the optic nerves. It is situated just in front of the tulier cinereum. (See also below — optic nerve, xuuler Criiniul Xcrvcs.) (12) The ccreliral peduncles or crura cerebri are two diverging bundles of white fibres which appear to come out from behind the anterior border of the pons. Each peduncle passes upward, for- ward, and outward, and enters the under surface of the hemisphere. (For description of cerebral ])cduncles, see above under M id-Druin.) (13) The under surface of the cerelirum also show's the exit of the third and fourth cranial nerves, the former coming out from the inner margins of the crura in front of the pons, the latter from the outer margins in front of the pons. 4ntcor.latvnl. Sep tuci: ATUpttfor T.scmz. Corp.calL Cozul.Tvuo. Forajifart, METENCEPHALON, MESEXCEPHALON. AND TUALAMENACEPHA- LON FROM THE PORSAb BFRFACE. Ant. COT. lat. vent.^ anterior cornu of later.al ventricle; hth rfn^, fifth ventricle; S^p. luci., eeptum Ivicidnm; Ant. pil.fnr., anterior pillar of fornix; T. semi., taenia semicir- cularis; Ant. com.y anterior commissure; Sul. c/^or., sulcus choroideus; Na.^ n.ites ; Corp. gen. iH^, corpus genicula- tuin internum; L7r. pin., stria pinealis; Peif. con., pedunculus conarii ; Pin. gl., pineal gland; Snl. corp.q. /., sulcus corpus ([uadratus Inngitudinalis ; Test., testis; Fren. vet,, i'rcnalus veli ; Ling, liiigula; Em. ter., eminentia teres; Tuher. acuit., tuberculuTn acusticuni : .^/. ci»/., ala cineroa; Tulier. cnn, tiihcrculnni cuneatuui : Fun. gra., funiculus gracilis; Fun. cun., funiculus cuneatus ; Lat. col., lateral column. Ventricle.s of tiik I'.RAiN. The lateral ventri- cles represent the expanded anterior end of the embryonic neural canal, and are the cavities of the heniis])lieres. They are lined by a single layer of simple cylindrical ppitheli n, the epen- dyma, derived from the epiblastic lining of the neural canal. The two ventricles are separated from each other in the median line by a vertical septum known as the septum lucidum. The shape of the ventricles is irregular, each ven- tricle consisting of a main cavity or body and extending oil' 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 cait}- of the ventricle is roofed in by the uiuIlT surface of the corpus callosum. its lloor is made by the corpus striatum and o])tic thalamus, al- though the ta'uia semicircularis, choroid plexus, corpus fimbriatum, and fornix are also situated in its lloor. The third ventricle is a long nar- row cavity lying between the optic thalami. Above it is bouiuled by the under surface of the velum interpositum, to which is attached the choroid plexus of this ventricle. Its fioor is made by the lamina cinerea, tuber cinereum, in- fundibulum, corpora albicantia, and posterior perfor.ited space. Three commissures cross the cavity of the ventricle, known respectively as the anterior, the middle, and the posterior com- 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. Posteriori}", the aqueduct of Sylvius con- nects the third ventricle with the fourth. A fourth opening in the anterior part of the lloor 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 which has been cut off 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 be 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 stirfaces 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 fibres 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- ventricular 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 cms cerebri. It forms a part of the lateral wall and fioor nf the body of the lateral ventricle. As the middle or de- scending horn of the ventricle descends the thalamus comes to form the roof of this portion of the ventricle. In front is a prominence knovn as the anterior tubercle, while its posterior part