110 THALAMENCEPHALON. 



mesial (internal) with the inferior quadrigeminal bodies, the lateral (external) 

 with the superior quadrigeminal bodies and optic tract. The two are separated by 

 a part of the optic tract which sweeps round the internal geniculate body in passing 

 as the superior brachium, to the grey matter of the corp. quadrig. sup. (fig. 86, p. 117). 

 Both the geniculate bodies are continuous with the posterior part of the thalamus, 

 lying between this and the lateral margin of the crusta. The internal geniculate 

 body receives ventrally and mesially the inferior brachium, which passes forwards 

 to it by the side of the tegmentum from the corp. quadrig. inf., and appears to dip 

 under it. Some of the fibres of the optic tract appear to pass out of it on its latero- 

 ventral aspect, but as the internal geniculate body remains unaffected when the eye 

 is extirpated, and is, moreover, as well as the corp. quadrig. inf., well developed 

 in the mole, it is probable that this connection with the optic fibres is more apparent 

 than real. 



The lateral or external geniculate body is, on the other hand, intimately related 

 to the optic tract and corp. quadrig. sup. as well as to the visual area of the 

 cortex cerebri (occipital region). It is of a somewhat darker colour than the 

 adjacent part of the thalamus with which it is intimately connected, and is formed 

 of several curved layers of grey matter with white lamellas of optic nerve-fibres 

 separating them (fig. 81, c.g.e.} 



Its cells are large and pigmented, and appear chiefly to send their nerve-fibre 

 processes to the cortex of the occipital region, for if this region is removed on one 

 side in young animals the cells of the external geniculate body of the same side 

 undergo atrophy (Monakow.) On the other hand, if the eyes are extirpated, the 

 atrophy which extends along the optic tracts involves the grey matter between the 

 cells, but not the cells themselves. From this it is inferred that the intercellular 

 grey matter of these bodies is largely composed of ramifications (? of collaterals) 

 of the retinal fibres. The same statements apply to the adjacent part of the 

 optic thalamus (pulvinar) which appears intimately associated both in structure 

 and function with the lateral geniculate bodies. 



The optic thalami (fig. 82, Tho), as seen from above after the removal of the 

 corpus callosum, fornix, and velum interpositum, are large oval-shaped masses of grey 

 substance covered dorsally by a thin stratum of white fibres. The dorsal surface is 

 marked by a shallow longitudinal groove, which inclines inwards as it passes forwards, 

 and terminates a short distance from the anterior extremity of the surface. This 

 anterior extremity is raised into a prominence termed the anterior tubercle (fig. 

 82, Ts), and together with the part of the upper surface which is placed outside 

 the groove, projects into the ventricle of the corresponding cerebral hemisphere 

 (fig. 88), and is covered by the lining epithelium of that cavity. This part of the 

 upper surface is limited externally by a white band, the stria terminalis ($/"), which 

 separates it from the part of the corpus striatum which is seen in the lateral 

 ventricle. The longitudinal groove above mentioned corresponds with the thickened 

 margin of the fornix, the edge of which extends over the surface of the thalamus 

 along the line of the groove. The part of the upper surface behind the groove does 

 not appear in either the third or the lateral ventricle, and is therefore not covered with 

 epithelium like the rest. It is limited internally by a sharp edge which separates 

 it from the mesial surface and which is marked by a white stria (taenia fornicis, fig. 

 82, Tfo.} leading to the pineal body. It is along this stria, which is surmounted by 

 a ridge-like extension of ependymal tissue that the thin epithelium which roofs in 

 the ventricle is attached laterally : this epithelium always comes away when the 

 covering of the velum interpositum is removed. At the posterior and inner 

 extremity of the thalamus, there is seen, as in front, a well-marked prominence 

 (posterior tubercle or pulvinar (/V) ) which projects over and partially conceals the 

 brachia of the corpora quadrigemina. Between the pulvinar and the peduncle of the 



