a narrow, vertically oriented, irregularly quadrilateral cavity in the midplane, extending from the lamina terminalis to the rostral opening of the mesencephalic aqueduct. This ventricle communicates at its rostrodorsal corner with each of the two lateral ventricles through the left and right interventricular foramen of Monro. Its narrow roof is formed by the tela choroidea that is attached on either side to the tenia thalami; its lateral wall is formed by the medial surface of the thalamus and, below the hypothalamic sulcus, by the hypothalamus, which also forms its floor. In lateral profile, the third ventricle exhibits a number of recesses: in its floor, from rostral to caudal, 1) the supraoptic recess in the acute angle between the base of the lamina terminalis and the dorsum of the optic chiasm, 2) the infundibular recess extending ventrally into the infundibulum but (in humans) not into the hypophysial stalk, and 3) the mammillary or inframammillary recess caused by the protrusion of the mammillary bodies into the ventricle. From its posterocaudal corner, the pineal and suprapineal recesses extend caudally in relation to the pineal gland.
Synonym(s): ventriculus tertiusTA, ventricle of diencephalon
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