108 



MESENCEPHALON. 



ramifications, that in the seventh layer is flattened horizontally, the others are more extended 

 vertically (fig. 80). Some of these terminal arborisations end in a special circumscribed part 

 of the lobe, which is known as the ganglion of the. roof. This contains a large number of nerve- 

 cells, the protoplasmic processes of which interlace with the optic arborisations, whilst their 

 nerve-fibre processes are directed towards the deeper layers of the lobe. All the layers below 

 the first contain nerve-cells, as well as nerve-fibres, but the seventh is mainly composed of the 



expanded ramifications of the optic fibres just men- 

 tioned, and of similar expansions of the superficially 

 directed nerve-fibre processes of cells from the tenth 

 layer (fig. 79, ./,./). The cells vary in size and shape 

 in the different layers, but on the whole they are 

 smallest nearer the surface and largest in the deeper 

 layers (tenth to thirteenth). With the exception 

 of the cells of the tenth layer just mentioned, the 

 cells send their axis-cylinder processes downwards to 

 pass away as nerve-fibres of the fourteenth layer, 

 which is chiefly formed of large medullated nerve- 

 fibres, although some nerve-cells are even here inter- 

 spersed. But some of the smaller cells of the more 

 superficial layers (fig. 79, d) belong to Golgi's 

 second type of nerve-cell, i.e., their axis-cylinder 

 processes do not pass into nerve-fibres, but break up 

 into a terminal arborisation a short distance from the 

 cell, and interlace amongst the cells of some of 

 the other layers. The third, fifth, seventh, and ninth 

 layers all have relatively few cells and a molecular 

 aspect, due apparently to the fine arborisations of the 

 nerve-fibre or axis- cylinder processes which they con- 

 tain. The protoplasmic processes of the cells are, some 



Fig. 79. ANTEKO-POSTERIOR SECTION OF THE OPTIC LOBE OF A BIRD PREPARED BY GOLGI'S METHOD 



(R. y Cajal). 



a, optic fibres cut across ; b, stellate cell of second layer ; c, fusiform cell of third layer ; d, cell 

 with axis-cylinder ending in a varicose arborisation in the eighth layer ; e. large horizontal cell of fourth 

 layer ; f,ff,k, globular cells of eighth layer ; t, cell with descending axis-cylinder of this layer ; j, cell 

 with axis-cylinder ascending to optic layer ; k, collateral arborisation of this axis-cylinder in the seventh 

 layer : m, large fusiform cells with recurrent axis-cylinders ; n, pyramidal cell with descending axis- 

 cylinder ; o, large cell of tenth layer; p,q,r, cells of the lowest layers, all with axis-cylinders directed 

 towards the deep layer of nerve-fibres, s. 



Fig. 80. TRANSVERSE SECTION OF OPTIC LOBE OF A BIRD, GOLGI'S METHOD (R. y Cajal). 



The superficial part only is represented, as far as the seventh layer. 



a, optic fibres ; b, their arborisation in the second layer ; c, that in the fourth layer ; d, that in the 

 fifth layer ; c, that in the seventh layer. 



of them, very long, and when coursing vertically often extend as far as the layer of optic 

 fibres, on the one hand, and the deep medullary layer on the other. The axis-cylinder 

 processes sometimes come off from the dendrites. sometimes from the body of the cell. Of 

 the fibres which form the deepest layer, although many, as just stated, are derived from the 



