OPTIC THALAMI 705 



ridge is formed by the junction of the tuber cinereum and the lamina cinerea 

 above the optic commissure. The floor is formed by the following structures: — 

 l)ehind, by the tegmental portion of the diverging crura cerebri, and between them 

 by the posterior perforated space; in front of this by the corpora albicantia, the 

 tuber cinereum with the infundilnilum and the lamina cinerea. The latter slopes 

 upwards to the anterior commissure, and is relegated by some anatomists to the 

 anterior wall instead of to the floor. 



The central grey matter of the third ventricle is continuous behind with the grey matter of 

 the aciueduct of Sylvius, and [Jiisses in front into the basal grey commissure. _ It also covers the 

 lower parts of the internal surfaces of the o]>tic thalami, and gives a thin covering to the anterior 

 pillars of the fornix and to the bundles of Vicq d'Azyr. 



The optic thalami are a pair of large ganglia placed one on each side of the 

 third ventricle. Each optic thalamus is a short prismatic body whose long axis is 

 directed forwards and inwards. This axis is not straight, but is slightly curved 

 with its convexity upwards. The thalamus presents an anterior and a posterior 

 extremity or tubercle, and four surfaces: a superior and an internal which are free, 

 and an inferior and an external which are imbedded in the cerebral substance. 

 The anterior tubercle is a blunt rounded point Avhich forms the posterior bound- 

 ary of the foramen of Monro. The posterior tubercle, or pulvinar, is broader 

 than the anterior, and is compressed from above downwards. It is directed back- 

 wards and downwards; it is continued externally into the outer geniculate body, 

 and overhangs the inner geniculate body. It is placed above and in front of the 

 descending cornu of the lateral ventricle, but is separated from the cavity of the 

 latter by the choroid plexus. The superior surface shows a slight convexity, 

 more pronounced in the antero-posterior than in the transverse direction. It is 

 covered with a layer of white fibres. It presents a groove, the sulcus choroideus, 

 which underlies the free edge of the fornix, separated from the latter only by the 

 velum interpositum. This groove marks off an antero-external area, wider in front 

 than behind, which ends in the anterior tubercle and which forms part of the floor 

 of the lateral ventricle. Internal to the sulcus choroideus is a larger area, w^hich is 

 covered by the fornix and velum interpositum. This area is limited internally for 

 about its anterior half by the stria pinealis, or peduncle of the pineal body, a 

 white bundle of fibres which runs along the sharp edge which separates the superior 

 from the median surface. The posterior part of this area slopes gradually into the 

 l)ulvinar. Between the superior surface and the posterior part of the'stria pinealis 

 is a small triangular area, the trigonum habenulse, which is sunk below the level 

 of the adjacent part of the thalamus. The internal surface is nearly flat and 

 bounds the cavity of the third ventricle, towards which it presents a slight con- 

 vexity. It is of a pale-grey colour owing to a thick layer of ependyma covering 

 the ganglionic substance, and is united to its fellow of the opposite side by a broad 

 band of grey matter, the middle or grey commissure (commissura mollis). 

 It is crossed at its lower margin by a groove, the sulcus of Monro, which passes 

 from the foramen of Monro to the commencement of the aqueduct of Sylvius. The 

 external surface rests against the fibres of the inner capsule behind the genu, 

 being separated from the nucleus lenticularis by the inner capsule. The inferior 

 surface is placed over the crus cerebri, on the tegmental part of Avhich it rests. 



In coronal sections through the posterior part of the thalamus, the external surface is seen to 

 be separated from the inferior surface by a distinct angle ; but in similar sections through the 

 anterior part the two surfaces pass into one another, the angle being rounded oif. The grey sub- 

 stance of the thalamus is indistinctly marked off into three nuclei : an antero-superior, an external, 

 and an internal. The largest part of the antero-superior nucleus forms the anterior tubercle ; the 

 thinner portion passes backwards above and between the other two nuclei ; its shape has been not 

 inaptly compared to that of the caudate nucleus. A remarkable bundle of white fibres, the ansa 

 lenticularis, passes below the optic thalamus and inner capsule from the lenticular nucleus of the 

 corpus striatum. 



The pineal body (epiphysis cerebri or conarium) is a reddish-grey body about 



the size of a cherry-stone. It has the form of a flattened cone, compressed from 



above downwards. Its long axis is directed forwards and slightly upwards. Its 



base is directed forwards and passes into a bilaminar stalk. Its superior surface is 



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