506 THE BRAIN 



pulled posteriorly to expose, as far as possible, the corpora quadrigemina, 

 i.e. the four rounded eminences on the dorsal aspect of the mesencephalon. 

 In doing this, care should be taken to secure and preserve the slender 

 trochlear nerve which issues from a lamina, called the anterior medullary 

 velum, immediately below the inferior pair of quadrigeminal bodies, and 

 winds round the lateral side of the pedunculus cerebri. 



THE MESENCEPHALON. 



The mesencephalon is the stalk which occupies the aperture 

 of the tentorium cerebelli, and connects the cerebral hemi- 

 spheres with the parts in the posterior cranial fossa. 1 It is 

 about three-quarters of an inch long, and it consists of 

 a dorsal part, the lamina quadrigemina, and a much larger 

 ventral part, which is formed by the two large pedunculi cerebri 

 In the undissected brain the lamina quadrigemina is com- 

 pletely hidden from view by the splenium of the corpus 

 callosum, which projects posteriorly over it, and also by 

 the superimposed cerebral hemispheres. The pedunculi 

 cerebri, however, can be seen, to some extent, at the base of 

 the brain, where they bound the posterior part of the inter- 

 peduncular fossa. The mesencephalon is tunnelled from 

 below upwards by a narrow passage called the aquseductus 

 cerebri (Sylvius). This channel lies much nearer the dorsal 

 than the ventral surface. 



Lamina Quadrigemina. The dorsal surface of the lamina 

 quadrigemina is raised into four eminences or colliculi, two 

 superior and two inferior, which are called the corpora quadri- 

 gemina. Each colliculus is composed, for the most part, of 

 grey matter, although each has a superficial coating of white 

 fibres. The superior pair are larger and broader than the 

 inferior pair^ but they are not so well denned nor yet so 

 prominent. 



A longitudinal and a transverse groove separate the quadri- 

 geminal bodies from each other. The longitudinal groove 

 occupies the median plane, and extends upwards as far as the 

 posterior commissure. From its lower end a short but well- 

 defined narrow band of white fibres, the frenulum veli, passes 

 to the anterior medullary velum, a lamina placed immediately 

 below the inferior pair of quadrigeminal prominences. The 



1 If the mesencephalon was divided, when the brain was removed, the 

 divided parts must be fixed together with pins while the superficial characters 

 are being studied. 



