QUADUIGEMINAL BODIES. 107 



1. Stratum zonale ; superficial white layer. This is a relatively thin stratum of 

 transversely coursing fine nerve-fibres, which are derived through the superior 

 brachiura from the optic tract and retina. 1 Many of them dip down and lose 

 themselves in the next layer, but some appear to be continued towards the middle 

 line and to decussate with others from the same stratum of the opposite side. 



2. Stratum cinereum ; grey cap. A layer of grey matter, sernilunar in section, 

 being of considerable thickness opposite the most prominent part of the tubercle, 

 but thinning off at its margins. Its nerve-cells are numerous but vary in size, the 

 smaller ones being near the stratum zonale, the larger in the deeper part of the 

 layer. They mostly send their dendrites or protoplasmic processes outwards, 

 i.e., towards the stratum zonale, while their nerve-processes (axis-cylinder processes) 

 are directed inwards towards the deeper layers. 



3. Stratum albo-cinereum superius ; upper grey-white layer ; stratum opticum. 

 The grey matter of this layer is largely interrupted by nerve-fibres, which are 

 continued like those of the stratum zonale from the optic tract, which enters by the 

 superior brachium at the antero-lateral aspect. The fibres differ in calibre in the 

 different parts of the layer, and it may be subdivided accordingly into (a) a medio- 

 dorsal zone of coarse medullated fibres, (/3) an intermediate zone of fine medullated 

 fibres, and (y) a central zone containing much grey matter interspersed with bundle? 

 of white fibres. 



These are described by Ganser as three distinct layers, and termed the third, or superficial 

 medullated layer, the fourth, or middle medullated, and the fifth, or middle grey layer ; while 

 the fourth layer of Tartuferi, immediately to be mentioned, becomes the sixth and seventh of 

 Ganser. 



The whole stratum is richly beset with large nerve-cells, which send their axis- 

 cylinder processes mostly into the next or fourth stratum. Of the nerve-fibres, those 

 of the intermediate zone (/3) are retinal fibres according to Tartuferi ; those of the 

 central zone (y) are probably derived from the corona radiata of the occipital region 

 of the brain, whilst the coarse fibres of the medio-dorsal zone are perhaps derived 

 from the opposite side. 



4. Stratum albo-cmereum inferius: deep grey-white layer: stratum lemnisci. 

 This, although composed of grey matter, is also traversed by many nerve-fibres which 

 appear to be connected with the upper fillet, and probably end in the layer. Some 

 of the fibres, however, are derived from the large cells of the fourth layer, and others 

 from the nerve-cells of this stratum itself (which contains many cells of large size). 

 It is possible that some fibres are derived (over the aqueduct) from the fillet of 

 the opposite side. 



Ganser subdivides this layer into two, which he terms respectively the deep white (sixth), 

 and the deep grey (seventh) layers. 



Structure of the optic lobes of birds. The relations of the cells and fibres have not 

 been very satisfactorily made out in the corpora quadrigemina of mammals, but in the optic 

 lobes of the bird, which correspond, as we have seen, to the anterior or superior quadrigeminal 

 bodies of the mammal, these relations have recently been considerably elucidated by the 

 investigations of Ramon y Cajal. 



All who have specially worked at this subject distinguish more strata in the optic lobes of 

 birds than in the corresponding bodies of mammals. Thus Bellonci makes the number of 

 layers (exclusive of the central grey matter of the aqueduct) nine, Stieda, twelve, whilst 

 Cajal distinguishes as many as fourteen strata. Of these the most superficial (1.) is a layer 

 of thick medullated fibres coming directly through the optic tract and chiasma from the 

 retina of the opposite side. (In birds all the optic nerve-fibres cross at the chiasma.) They 

 pass in from the side (so that they are cut across in a sagittal section, fig. 79), and after a 

 variable course turn downwards into the deeper layers, where they end at four different levels 

 (as far as the seventh layer) in non-medullated terminal arborisations. Of these terminal 



1 Monakow states that in the dog the anterior brachiura contains some fibres from the occipital cortex. 



