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OLF

Online Encyclopedia
Originally appearing in Volume V04, Page 403 of the 1911 Encyclopedia Britannica.
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OLF. TUBER * From Cal. R.C.S. England. constricted off, while just behind the openings of the foramina of Munro a constriction occurs which divides the prosencephalon into two secondary vesicles, the anterior of which, containing the foramina of Munro, is called the telencephalon, while the posterior is the thalamencephalon or diencephalon. A constriction also occurs in the hind vesicle or rhombencephalon, dividing it into an anterior part, the metencephalon, from which the cerebellum is developed, and a posterior or myelencephalon, the primitive medulla oblongata. At this stage the general resemblance of the brain to that of the lamprey is striking. Before the secondary constrictions occur three vertical flexures begin to form. The first is known as the cephalic, and is caused by the prosencephalon bending sharply downward, below and in front of the mesencephalon. The second is the cervical, and marks the place where the brain ends and the spinal cord begins; the concavity of this flexure is ventral. The third to appear has a ventral convexity and is known as the pontine, since it marks the site of the future pons Varolii; it resembles the permanent flexure in the reptilian brain. It will now be seen that the original neural canal, which is lined by ciliated epithelium, forms the ventricles of the brain, while superficial to this epithelium (ependyma) the grey and white matter is subsequently formed. It has been shown by His that the whole neural tube may be divided into dorsal or alar, and ventral or basal laminae, and, as the cerebral hemispheres bud out from the dorsal part of the anterior primary vesicle, they consist entirely of alar laminae. The most characteristic feature of the human and anthropoid brain is the rapid and great expansion of these hemispheres, especially in a backward direction, so that the mesencephalon and metencephalon are hidden by them from above at the seventh month of intra-uterine life. At first the foramina of Munro form
End of Article: OLF
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