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From Embryology

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Interior of Brain of Human Embryo of Five Weeks

From model by Wilhelm His (1831-1904).

The Diencephalon.—From the alar lamina of the diencephalon, the thalamus, metathalamus, and epithalamus are developed. The thalamus (Figs. 650 to 654) arises as a thickening which involves the anterior two-thirds of the alar lamina. The two thalami are visible, for a time, on the surface of the brain, but are subsequently hidden by the cerebral hemispheres which grow backward over them. The thalami extend medialward and gradually narrow the cavity between them into a slit-like aperture which forms the greater part of the third ventricle; their medial surfaces ultimately adhere, in part, to each other, and the intermediate mass of the ventricle is developed across the area of contact. The metathalamus comprises the geniculate bodies which originate as slight outward bulgings of the alar lamina. In the adult the lateral geniculate body appears as an eminence on the lateral part of the posterior end of the thalamus, while the medial is situated on the lateral aspect of the mid-brain. The epithalamus includes the pineal body, the posterior commissure, and the trigonum habenulæ. The pineal body arises as an upward the evagination of roof-plate immediately in front of the midbrian; this evagination becomes solid with the exception of its proximal part, which persists as the recessus pinealis. In lizards the pineal evagination is elongated into a stalk, and its peripheral extremity is expanded into a vesicle, in which a rudimentary lens and retina are formed; the stalk becomes solid and nerve fibers make their appearance in it, so that in these animals the pineal body forms a rudimentary eye. The posterior commissure is formed by the ingrowth of fibers into the depression behind and below the pineal evagination, and the trigonum habenulæ is developed in front of the pineal recess.

The corpus striatum (Figs. 651 and 653) appears in the fourth week as a triangular thickening of the floor of the telencephalon between the optic recess and the interventricular foramen, and continuous behind with the thalamic part of the diencephalon. It increases in size, and by the second month is seen as a swelling in the floor of the future lateral ventricle; this swelling reaches as far as the posterior end of the primitive hemisphere, and when this part of the hemisphere grows backward and downward to form the temporal lobe, the posterior part of the corpus striatum is carried into the roof of the inferior horn of the ventricle, where it is seen as the tail of the caudate nucleus in the adult brain. During the fourth and fifth months the corpus striatum becomes incompletely subdivided by the fibers of the internal capsule into two masses, an inner, the caudate nucleus, and an outer, the lentiform nucleus. In front, the corpus striatum is continuous with the anterior perforated substance; laterally it is confluent for a time with that portion of the wall of the vesicle which is developed into the insula, but this continuity is subsequently interrupted by the fibers of the external capsule.



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Reference

Gray H. Anatomy of the human body. (1918) Philadelphia: Lea & Febiger.


Cite this page: Hill, M.A. (2024, May 21) Embryology Gray0653.jpg. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/File:Gray0653.jpg

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current08:00, 20 May 2012Thumbnail for version as of 08:00, 20 May 2012698 × 700 (108 KB)Z8600021 (talk | contribs)==Interior of brain of human embryo of five weeks== From model by His. {{Gray Anatomy}} Category:Human Category:Historic Embryology Category:Week 5 Category:Gray's 1918 Anatomy Category:Neural