160 ELEMENTS OF HISTOLOGY. [cu p . xvm. 



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 middlfe 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 frontal aspect 

 surrounded by a layer of grey substance continued 

 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 representing the tegmentum, which 

 is the dorsal or posterior portion of the crus cerebri. 



210. The corpora quadrigemina. Each of 

 the two inferior prominences consists of a superficial 

 layer of white matter, and a deep grey portion, con- 

 taining 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 thejillet. 

 In each of the two superior prominences there is also 

 a superficial layer of white matter, beneath which 

 is a layer of grey matter (stratum cinereum) ; under- 

 neath this is the main portion the stratum opticum 

 consisting of longitudinal tracts of nerve-fibres, be- 

 tween 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. 



