THE BKAIN IX THE EDENTATA. 355 



gemina form a large flattened area of a quadrilateral shape. The optic thalami are 

 separated tlie one from tlie other by a median slit, which is the third ventricle. A 

 shallow furrow begins on each side near the posterior extremity of the third ventricle 

 and proceeds obliquely outward and backward at an angle of about 60° with the mesial 

 plane ; this furrow on each side serves as a line of demarcation between the optic thalamus, 

 which is placed in front of it, and the corpora quadrigemina behind it. Tlie anterior 

 pair of the corpora quadrigemina form two broad flattened eminences (fig. 27) which 

 rise to a slightly higher level than the optic thalami and posterior corpora quadrigemina. 

 Together they form a sector of about 120 of a circle of 95 mm. radius. In the mesial 

 line they are separated by a shallow furrow. A small body shaped like an arrow-head 

 lies free in tlie anterior part of this mesial furrow. This is the pineal body. It is solid, Ijut 

 is attached anteriorly to the thin walls of a pouch-like diverticulum of the third ventricle. 

 A very deep furrow extends across the mesial line under the corpus pineale (fig. 4<), and 

 from this the furrows which separate the corpora quadrigemina from the optic tlialanius 

 start. If these furrows be traced laterally they will be found to dip down upon the 

 lateral surface of the brain-stem (fig. 27), and in the deep groove thus formed we find 

 the large projecting posterior geniculate body (Hgs. 27 and 2). 



The dorsal surface of the oiitic thalamixs presents a quadrilateral outline. Its anterior 

 and posterior boundaries are two parallel obliquely-placed grooves, both of which begin 

 at the mesial plane and extend outward and backward, the anterior boundary for aljout 

 16 mm., and the posterior boundary (which is the thalamo-mesencephalic groove) for 

 9'5 mm. The postero-lateral extremity of the optic thalamus consists of a large rounded 

 knob, the outline of which resembles, both in extent and degree of curvature, the postero- 

 lateral boundary of the quadrigeminal body. Trom a profile view (fig. 2) we can readily 

 understand hoAV this large lateral knob of the optic tlialamus is related to the ill-defined 

 anterior geniculate body and the optic tract. The oblique antero-lateral boundary of the 

 optic thalamus consists of a deep groove Avhich separates it from the corpus striatum 

 (tig. 27). Upon each side of the posterior part of the third ventricle, and just in front of 

 tlie attachment of the pineal diverticiilum, we find a large pear-shaped area very faintly 

 marked off from the rest of the dorsal surface of the optic thalamus by shallow furrows. 

 This is the ganc/Uon habciiakc. It extends about as far forward as the mid-point of the 

 slit-like third ventricle and then gradually shades away. A very prominent ridge of 

 nerve-fibres is connected with this ganglion, and forms the dorso-mesial lip of the third 

 ventricle in the Avliole of its extent : this is tlie tcenia tliaJami. 



The 2)osterior quadrigeminal bodies form a pair of large rounded projections upon the 

 postero-lateral aspects of the anterior quadrigeminal bodies. They are placed a con 

 siderable distance apart, and are connected across the middle line by a tempering narrow 

 bridge of grey substance (fig. 27). Upon the lateral aspect of the mesencephalon each 

 posterior quadrigeminal body is prolonged into an oblique column of a greyish colour 

 which extends downward and forward toward the prominent, somewhat triangular, 

 posterior geniculate body (fig. 2). This column is called the hruchiaub of the posterior 

 geniculate body. In the angle between it and the crus cerebri we find the irregularly 

 quadrilateral region of the urea tecjmenti. 



