'.LEMENTS OF HISTOLOGY. [Chap. XVIII. 



closes again into a small canal (the aqueductus Sylvii) 

 which having passed in front out of the region of the 

 corpora quadrigemina opens out again as the third 

 ventricle. The parts around the aqueductus Sylvii 

 represent the mesencephalon (Fig. 91), developed 

 from the middle brain vesicle in the embryo. They 

 include the wall of the aqueductus Sylvii, the corpora 

 quadrigemina, and the crura cerebri. 



The aqueductus Sylvii is lined with epithelium 

 and a layer of neuroglia continued from the fourth 

 ventricle. The raphe of the medulla and of the pons 

 are continued into the lower wall of the aqueductus. 

 The lining layer of neuroglia is on its antero-inferior 

 aspect surrounded by a layer of grey substance con- 

 tinued from the grey substance of the floor of the fourth 

 ventricle. It contains in a nerve network numerous 

 multipolar ganglion cells grouped into nerve nuclei, 

 connected with the third, fourth, and part of the fifth 

 pair of nerves. In front of this layer is one of con- 

 siderable thickness called the tegmentum, which is the 

 dorsal or posterior portion of the crus cerebri. 



210. The corpora quadrigemina. Each of 

 the two lower lobes consists of a superficial layer of 

 white matter, and a deep grey portion, containing 

 multipolar ganglion cells of various sizes embedded 

 in a fine nerve network. Between this and the 

 grey substance of the wall of the aqueductus Sylvii 

 are tracts of white matter, forming part of the fillet. 

 In each of the two upper lobes there is also a super- 

 ficial layer of white matter, beneath which is a layer 

 of grey matter (stratum cinereum) ; underneath this 

 is the main portion (the stratum opticum) consisting 

 of longitudinal tracts of nerve-fibres, between which 

 are small masses of grey substance. Between this 

 stratum opticum and the grey matter forming the 

 wall of the aqueductus Sylvii is a layer of white 

 matter, part of the fillet. 



