PARTS DERIVED FROM THE FORE-BRAIN 



909 



pedunculomammillaris in the mid-brain tegmentum; its destination is doubtful. 

 The axones from the lateral nucleus join the latter bundle. 



[NOTE. The posterior perforated substance and the corpora albicantia are 

 generally included under the head of the Pars Mammillaris Hypothalami. ] 

 , Third Ventricle (ventriculus tertius) (Fig. 664). The third ventricle is the 

 adult representative of the cavity of the primary fore-brain vesicle, but only so 

 much of it as is not carried laterad, on either side, in the rapidly growing, eventually 

 huge cerebral hemisphere buds to form the lateral ventricles. It is a narrow, 

 cleft-like interval between the two thalami and hypothalamic gray, limited frontad 

 by the terma, continuous caudad with the aqueduct and laterad, through the 

 foramina of Monro, with the lateral ventricles. Its roof is destitute of nerve 

 tissue and is formed by a delicate, fused ependymal and pial layer, invaginated 

 on either side of the median plane by the plexuses of the lateral ventricle. The 

 pial layer is one of the constituents of the fold known as the velum interpositum. 

 The floor of the ventricle is formed by structures already described on the basal 



Lateral ventricle 



CORPUS CALLOJLVJ.M 



Epithelial lining 



of ventricle 

 Vein of corpus 



stria turn 

 Choroid plexus of 



lateral ventricle 



Velum interpositum 

 Veins of Galen 



Epithelial lining of 

 ventricle 



Choroid plexuses of 

 third ventricle 



Third ventricle 



FIG. 666. Coronal section of lateral and third ventricles. (Diagrammatic.) 



aspect in the intercrural space viz., the tuber cinereum, corpora albicantia, and 

 posterior perforated substance, as well as the optic chiasm and a portion of the 

 tegmentum of the crura cerebri. Much of the floor, it may be noted, is formed 

 by the primitive, undifferentiated central gray; and although the optic vesicle 

 develops from its ventrocephalic portion, the caudal shifting of central optic 

 connections to thalamus and mid-brain has made this portion of the neural tube 

 wall comparatively insignificant. The lateral walls are formed in part by the 

 thalami, in part by the hypothalamic gray ventral extension. The fornix may be 

 seen, shining through a thin lamina of gray substance and the ependyma, coursing 

 caudoventrad to the corpus albicans. A slight furrow, the aulix or sulcus of 

 Monro, may sometimes be traced from the aqueduct to the foramen of Monro, 

 curving ventrad of a bridge-like fusion of the two thalami the middle commissure 

 (medicommissure). (The term commissure is inappropriate, as no commissural 

 fibres appear to pass from one thalamus to the other in this "thalamic fusion;" 

 it is absent in about 10 per cent, of brains examined.) 



The cephalic wall is formed by the lamina terminalis or terma, the rudimentary 

 mediancephalic wall of the neural tube. The terma is attached to the dorsum 

 of the optic chiasm ; dorsally it is reenforced by the anterior commissure. 



As seen in mesal section or as shown by a cast of the ventricle (Fig. 664) it 

 is seen to be of irregular outline. Frontad is the optic recess, dorsad of the 



