I430 A MANUAL OF ANATOMY 



acoustic ftUet, and they terminate in arborizations around the cells of the 

 central nucleus. 



The dorsal white layer derives its fibres from the lateral fillet and from the 

 axons of the cells of the central grey nucleus. The fibres pass into the inferior 

 brachium, by which they are conducted to the internal geniculate body. 



The ventral white layer also derives its fibres from the lateral fillet and 

 from the axons of the cells of the central grey nucleus. This layer separates 

 the central nucleus from the subjacent grey matter of the aqueduct of Sylvius. 

 Some of the fibres cross the median plane, and decussate with corresponding 

 fibres of the opposite side, superficial to the roof of the aqueduct of Sylvius. 

 Others enter the tegmentum of the crus cerebri of the same side and also of 

 the opposite side, in which their course is downwards in the lateral fillet. 



The lower quadrigeminal body {colliculus inferior B.), which receives its 

 fibres from the lateral or- acoustic fillet, is associated with the acoustic ap- 

 paratus. 



The upper quadrigeminal body [colliculus superior B.) is composed of the 

 following layers: 



1. Stratum zonale. 3. Stratum opticum. 



2. Stratum cinereum. 4. Stratum lemnisci. 



The stratum zonale is the most superficial layer, and probably consists of 

 retinal fibres which are derived from the outer root of the optic tract. Many 

 of these fibres pass into the stratum cinereum and terminate in connection 

 with its cells. Others cross the median plane and decussate with corre- 

 sponding fibres from the opposite side, superficial to the roof of the aqueduct 

 of Sylvius. 



The stratum cinereum, or second layer, lies beneath the stratum zonale, 

 and consists of a crescentic layer of grey matter, containing many nerve-cells. 

 It represents the grey nucleus of the upper quadrigeminal body, and the 

 axons of its cells pass to the more deeply seated strata. 



The stratum opticum is the third layer, and it consists of grey matter, 

 which contains numerous nerve-cells and nerve-fibres. The fibres are con- 

 ducted to this stratum by the superior brachium, and they are of two kinds : 

 (i ) Many are retinal fibres and are derived from the outer root of the optic 

 tract. (2) Others are coriicifugal fibres, which come from the higher visual 

 centre in the cortex of the occipital lobe and form part of the optic radiation. 

 The fibres pass into the stratum cinereum and terminate in arborizations 

 around its cells. The axons of the cells of the stratum opticum pass into the 

 stratum lemnisci. 



The stratum lemnisci is the deepest layer. Like the stratnm opticum, it 

 consists of grey matter, which contains numerous nerve-cells and nerve- 

 fibres. The fibres are derived from the following sources: (i) Many are 

 derived from the mesial or main fillet; and (2) some are the axons of cells 

 belonging to the stratum opticum and stratum lemnisci. The fillet-fibres 

 terminate in the stratum lemnisci. The fibres formed by the axons of the 

 cells of the stratum opticum and stratum lemnisci cross the median plane 

 and decussate with the corresponding fibres of the opposite side. This decus- 

 sation is known as the fountain-decussation of Meynert. The fibres, after 

 crossing, form the ventral longitudinal bundle, or tectospinal tract, of that 

 side, and this bundle or tract descends through the pons Varolii and medulla 

 oblongata into the corresponding ventral or antei:y)r column of the spinal 

 cord. 



The upper quadrigeminal body, through means of the superior brachium, 

 is one of the three lower visual centres, the other two being the pulvinar and 

 the external geniculate body, both of which belong to the optic thalamus. 



Development of Corpora Quadrigemina. — ^The corpora quadrigemina are 

 developed from the dorsal wall or roof of the mesencephalon. 



Commissure of Meynert. — Tliis commissure or decussation con- 

 .sists of fibres which issue Irom each upper quadrigeminal body and 



