THE POSTERIOR COMMISSURE. 



109 



cells of the optic lobe, and are therefore passing away by that layer (probably to the higher 

 parts of the brain, on the one hand, and to the bulb and spinal cord, on the other), some have 

 a contrary direction and are passing -into the grey matter, where they end in extensive 

 ramifications extending as far towards the surface as the sixth layer. 



The fibres of the deep medullary layer, on leaving- the optic lobe, partly encircle, partly 

 traverse, four groups of nerve-cells, which are known as the optic f/anglia. It is probable 

 that these may in part represent the external or lateral geniculate body of mammals. The 

 fibres give off collaterals, which end in ramifications amongst the cells of these ganglia ; some 

 of tbe fibres appear altogether to terminate in this way. On the other hand, some of the 

 ganglion-cells send their axis-cylinder processes to join the stream of traversing nerve- 

 fibres. It is not known what ultimately becomes of these processes, but from analogy 

 with what obtains in mammals (see diagram, p. 119), it is not improbable that they may pass 

 to the cortex cerebri. 



The posterior commissure (fig. 73, Cop., fig. 78, c.p.\ which overlies the 

 upper end of the aqueduct and appears in the posterior wall of the third ventricle, 

 is generally described with that cavity. It appears, however, to be in part a con- 



c.qr.s 



Fig. 81. SECTION THROUGH THE SUPERIOR PART OF ONE OF THE SUPERIOR CORPORA QUADRIGEMINA 



AND THE ADJACENT PART OF THE OPHC THALAMUS (after Meynert). 



, aqueduct of Sylvius ; gr, grey matter of the aqueduct ; c.q.s, quadrigeminal eminence, consisting 

 of: I, stratum lemnisci ; o, stratum opticum ; c, stratum cinereum ; Tk, thalamus (pulvinar) ; c.g.i, 

 C.g.e, internal and external geniculate bodies ; br.s, br.i, superior and inferior brachia; /, upper fillet ; 

 p.l, posterior longitudinal bundle; r, raphe ; III, third nerve; n.III, its nucleus; l-p.p, posterior 

 perforated space ; s.n, substantia nigra ; above this is the tegmentum with its nucleus, the latter being 

 indicated by the circular area ; cr, crusta ; II, optic tract ; M, medullary centre of the hemisphere ; 

 n.c, nucleus caudatus ; st, stria terminalis. 



tinuation of the commissural fibres of the fillet above mentioned. Its fibres, 

 according to Meynert, coming from the tegmental part of the mesencephalon, cross 

 to the other side, and after passing through the thalamus diverge into the white 

 substance of the cerebral hemispheres. They may in part comprise commissural 

 fibres between the two thalami, and some are connected with the pineal roots. 

 According to Darkschewitsch they are divisible into two categories. Some form a 

 ventral portion of the commissure, being derived from the posterior longitudinal 

 bundle and the nucleus of the third nerve, and pass across to the root of the pineal 

 body of the opposite side, while the rest, which form a dorsal portion of the com- 

 missure, connect the deep white layers of the superior corpora quadrigemina with 

 the corona radiata of the opposite side. 



The geniculate bodies are intimately related to this region of the brain, the 



