THE PHYSIOLOGY OF THE BRAIN. 



617 



it with the frontal and occipital lobes of the cerebral hemisphere. 

 The tegmentum contains gray masses and many transverse 

 and longitudinal fibres. Many of the fibres, cp, come from the 

 anterior peduncle of the cerebellum ; these cross in the pos- 

 terior part of the tegmenturn ; most of them end in a large 

 mass of gray matter in the front of the tegmentum named the 

 red nucleus ; others run forward to the optic thalamus direct. 

 Other longitudinal fibres are continued from the fillet some of 



FIG. 176. Diagram of cross-section of midhrain in region of posterior corpora 

 quadrigemina : P, pes; Ln, locus riiger; Tg, tegmentum; .S, aqueduct of Sylvius; 

 cq, right corpus quadrigeminum. In the pes on left side are indicated by /r, fibres 

 from frontal lobe of cerebral hemisphere; /, fibres of fillet; py, fibres of pyramidal 

 tract; oc, fibres from occipital lobe of cerebral hemisphere. In tegmental region, 

 /', fillet fibres to anterior corpus quadrigeminum; /", fillet fibres for posterior 

 corpus quadrigeminum; cp. fibres from cerebellar peduncle. Parts containing gray 

 nerve matter are shaded in horizontal lines. 



these,/", end in the posterior corpora quadrigemina; others, 

 /', in the superior corpora quadrigemina and the occipital re- 

 gion of the cerebral hemispheres. The corpora quadrigemina 

 are covered by a layer of medullated fibres, but their main 

 mass is gray matter. The anterior pair are closely connected 

 with the optic tracts, and therefore with the optic nerves and 

 the retinas : to^ikh' outer sides and in front are the external 

 corpora geniculata, gray masses closely associated with the 

 optic tracts. 



The structure of the midbrain shows that it is in great part 

 merely a commissure between the parts in front of and behind 

 it: but its connection with fibres of the optic tract shows 

 that it has a close relation to visual sensations; and the origin 

 in it of the oculo-motor and abducens nerves, that from it the 

 eye muscles, and the iris and ciliary muscle are innervated. 



