THE CEREBRUM 451 



passes backwards, above the posterior commissure and the entrance of the 

 aquaeductus cerebri, into the stalk of the pineal body. The second is 

 placed still higher, and is carried backwards for a greater distance. Its 

 walls are epithelial, and therefore it cannot be seen in an ordinary dissection. 

 It is termed the recessus suprapinealis (Fig. 177). 



Dissection. The further study of the cerebral hemispheres 

 should be postponed until the examination of the mid-brain 

 or mesencephalon is completed. The membranes should be 

 removed from the upper surface of the cerebellum, and the 

 prominent anterior part of that organ may then be pulled back- 

 wards to expose, as far as possible, the corpora quadrigemina, 

 i.e. the four rounded eminences or colliculi on the dorsal aspect 

 of the mesencephalon. As the cerebellum is displaced back- 

 wards, care should be taken to secure and preserve the slender 

 trochlear nerves. They wind round the lateral sides of the 

 pedunculi cerebri, after they have issued from a lamina, called 

 the anterior medullary velum, which lies immediately below 

 the inferior pair of colliculi. 



THE MESENCEPHALON. 



The mesencephalon or mid-brain is the stalk which occupies 

 the aperture of the tentorium cerebelli, and connects the 

 cerebral hemispheres 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 backwards 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 interpeduncular 

 fossa. The mesencephalon is tunnelled from end to end 

 by a narrow passage called the aqu&ductus cerebri (Sylvius). 

 The aqueduct lies much nearer the dorsal than the ventral 

 surface of the mid-brain, and it connects the third ventricle 

 with the fourth ventricle. 



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, 



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, 

 ill 29 a 



