FIBRES OF CEREBRUM. 209 



pulled aside, a white band, about a quarter of an inch in width, will be 

 seen to issue from the transverse fibres of the pons, and to bend upwards 

 over the peduncle of the cerebellum to the corpora quadrigemina (fig. 

 65, ). 



This is the upper or commissural piece of the fillet (p. 128), which 

 passes beneath the corpora quadrigemina, and joins with the similar part 

 of the opposite side over the Sylvan aqueduct. 



STRUCTURE OP THE CEREBRUM. In each cerebral hemisphere three 

 principal sets of constituent fibres are recognized, viz., diverging, trans- 

 verse, and longitudinal. The former are in part derived from the spinal 

 cord ; while the two latter, joining distant .pieces of the cerebrum, are con- 

 sidered to be only connecting or commissural in their office. 



Peduncular or diverging fibres (fig. 56). In the crus cerebri two 

 bundles of longitudinal fibres are collected; these are separated, in part, 

 by gray matter, and are derived from the medulla oblongata (p. 191). 



Dissection. A complete systematic view of the diverging fibres can- 

 not be now obtained on the imperfect brain. At this stage the chief pur- 

 pose is to show the passage of the radiating fibres from the crus through 

 the two cerebral ganglia. 



To trace the diverging fibres onwards beyonds the crus cerebri, and 

 through the corpus striatum, the nucleus caudatus of this body should be 

 scraped away (fig. 65) ; and the dissection should be made on the left side 

 on which the striate body and the optic thalamus remain uncut. In this 

 proceeding the pecten of Reil comes into view, viz., gray matter passing 

 between the white fibres in the corpus striatum, and giving the appearance 

 of the teeth of a comb. 



On taking away completely the prolonged part of the nucleus caudatus, 

 others of the same set of fibres will be seen issuing from the outer side of 

 the optic thalamus, and radiating to the posterior and inferior lobes. 



After tracing those fibres, the upper part of the optic thalamus may be 

 taken away at the posterior end, to denude the accessory bundles to the 

 peduncular fibres, from the corpora quadrigemina and the superior pedun- 

 cle of the cerebellum (fig. 65, *) : the last band lies beneath the corpora 

 quadrigemina. 



Their arrangement (fig. 65, 6 ). Some of the diverging fibres radiate 

 from the peduncle of the cerebrum to the surface of the hemisphere, pass- 

 ing in their course through the two cerebral ganglia (optic thalamus ( 6 ) 

 and corpus striatum ( 9 ) ), and they form a conically-shaped bundle, whose 

 apex is below and base above. 



The fibres forming the free or fasciculated part (crust) of the peduncle 

 (fig. 56) pass through the striate body. The fibres on the opposite aspect, 

 which form the tegmentum (fig. 57, rf), are transmitted through the under 

 part of the optic thalamus, and through the corpus striatum, reaching as 

 far forwards as, but much farther back than those of the crust. 



In the thalamus and the corpus striatum the fibres are greatly increased 

 in number. The upper or sensory set receive also accessory bundles from 

 the superior peduncle of the cerebellum (fig. 65, ') in the crus cerebri 

 (p. 191) ; and from the pair of the corpora quadrigemina, and the corpora 

 geniculata of the same side, in the thalamus. 



On escaping from the striate body and the thalamus the fibres decussate 

 with the converging fibres of the corpus callosum, and radiate then into 

 the anterior, middle, and posterior parts of the cerebral hemisphere, form- 

 ing the corona radiata. In the hemisphere the fibres are continued to 

 14 



