THE CORPORA QUADRIGEMINA. 



105 



c. 



S.TtVV 



cbl 



are broader and longer and also darker in colour, but slightly less prominent than 

 the lower or posterior (c.q.i) 1 . Laterally the corpora quadrigemina are not bounded 

 by a distinct groove, but each appears to be prolonged obliquely upwards and forwards 

 into a prominent white tract, known as the brachium of the corresponding tubercle. 

 The lower (posterior) brachium (fig. 77, br.i.) loses itself underneath an oval 

 prominence which is seen at the side of the upper end of the crus cerebri, and is 

 termed the inner geniculate bod?/ (fig. 77, c.g.i. ; fig. 78, c.g.m.}. The upper (anterior) 

 brachium passes between the same geniculate body, and the prominent posterior 

 extremity of the optic thalamus into the external geniculate body (fig. 78, c.g.i.} and 

 the optic tract, of which it maybe regarded as the main prolongation (see also fig. 86, 

 p. 117). The continuity is much better seen externally in some animals than in man. 

 The connection of the superior quadrigeminal bodies with the optic tract and the 

 sense of sight is far more intimate than that of the inferior. For if in a young 

 animal the eye on the one side be extirpated, the operation is found to be followed 



Fig. 77. VlEW OF THE MEDULLA, PONS, AND 



MESENCEPHALON FROM THE BIGHT SIDE AND 

 BEHIND. (E. A. S. ) 



The cerebellum, the inferior medullary velum, 

 and the right half of the superior medullary 

 velum, have been cut away, so as to display the 

 fourth ventricle. 



c.q.s, c.q.i, superior and inferior quadri- 

 geminal bodies of the left side ; the pineal 

 gland, p, is seen projecting backwards between 

 the superior bodies, and the frsenulum. fr, 

 passes up from the superior medullary velum, 

 s.m.v, to the interval between the posterior 

 quadrigeminal bodies ; th, right thalamus opti- 

 cus ; br.i, brachium of the inferior quadrigeminal 

 "body passing underneath the inner geniculate 

 body, c.g.i. ; /, superficial stratum of fibres of 

 the fillet, covering the tegmentum of the crus 

 cerebri ; c, crusta of the crus cerebri, separated 

 from the tegmentum by the lateral groove, l.g. ; 

 P, upper part of the pons ; III, IV, &c., the 

 corresponding cranial nerves. The rest of this 

 figure will be found described at p. 43. 



after some time by atrophy of the 

 superior quadrigeminal body and of its 

 brachium, whereas the inferior quadri- 

 geminal body and brachium is un- 

 altered (Gudden). Moreover, in the 

 mole the inferior quadrigeminal body 

 is well developed, whereas the superior 

 is atrophied. 



The lower or posterior quadri- 

 geminal bodies are composed almost 



entirely of grey substance (the so-called nucleus of these bodies (fig. 76, c.q.p.) which 

 is separated by a thin layer of the fillet from the central grey matter of the aqueduct 

 and contains numerous small and some larger nerve-cells. The connections of these 

 have not yet been satisfactorily worked out, but they are believed to be closely 

 related to the termination of the fibres of the lower fillet. The nuclei are united 

 across the middle line by a commissural portion of grey matter which is bounded 

 superficially and deeply by transverse white fibres derived from the fillet. 



1 The term nates is often applied to the superior or anterior corpora quadrigemina, and testes to the 

 inferior or posterior. These names were used by Vesalius, but are somewhat misleading, and have fallen 

 into disuse. 



