ErEyCEPHALOy—TIIE CEEEBELLUM 713 



matter which i;; placeil helow the optio thalamus, and extends backwards into the 

 mesencei>halon below the aqueiluct. It appeal"^ round in coixnial sections (^tig. 423). 



The best marked of the white strands are the superior peduncles of the ceii^ 

 helium, the tillet, and the posterior longitudinal bundle i^tig. 444). The tilaments 

 of origin of the third nerve, passing from its nucleus through the tegmentum and 

 inner part of the substantia nigra, are also conspicuous in stations (tig. 443). 



The superior cerebellar peduncle is readily distinguishable in stvtions through 

 the lowest part of the mesencephalon as a white. crescent-shapiH.1 bundle with its 

 inner or concave side bounded by the grey-coloui-eil formatio reticularis which 

 separates it from the grey matter of the aqueduct. Higher up it takes a deeper 

 course below the lloor of the aqut\iuct, and dtvussates with its fellow of the oppl^site 

 side. It then passes into the subthalamic region, and separates into an inner and 

 an outer division. The inner division }nisses internal to the red nucleus and entei-s 

 the under surface of the optic thalamus. The outer division passes external to the 

 red nucleus and probably enters the globus pallidus of the lenticular nucleus. 

 Fibres have been descrilxxl as passing from the rt\I nucleus to the cerebral cortex. 



The fillet appeai-s at the level of the lower quadrigeminate bodies as a tiat baud 

 of longitudinal fibres in the ventral inirt of the tegmentum, into which it has 

 passed from the pons (fig. 429) and dorsal to the substantia nigra. It consists of 

 three parts, an outer or lateral, internal or mesial, and an intermediate or middle 

 l>art. The lateral part jnisses outwards to the surface and curving round the outer 

 side of the superior cerebellar peduncle it terminates at the lower quadrigeminate 

 body i^fig. 4"2o). It is also known as the lower tillet, and in addition to fibres 

 derived from the ascending antero-lateral tract by the superior medullary velum 

 it contains tiln-es from the o}>}>osite auditory nucleus, and other fibres which 

 ha\e jnissed from the nuclei of the fasciculus gracilis and fasciculus cuneatus of the 

 opi>osite side by the superior pyramidal de<.nissation. The internal or mesial fillet 

 turns downwards into the inner part of the crusta and is continued to tlie sub- 

 thalamic tegmental region, where it enters the ansa lenticularis, by which it is 

 conveyed to the medullary lamimv of the lenticular nucleus. The middle or inter- 

 mt\liate fillet is separable into two parts: an outer part, calkxl the uj^per fillet, 

 which terminates in the white matter of the superior quadrigeminate body, and 

 a remaining ]>ortion which }>asses upwards into the subthalamic tegmental region, 

 and possibly reaches the brain cortex. The fibres of the middle fillet are derivtHl 

 from the antero-lateral cohunn of the cord of the same side, from the nuclei of the 

 funiculus gracilis and the funiculus cuneatus of the opposite side by the suinnior 

 })yramidal decussation, and from the cerebellum. 



The posterior longitudinal bundle is a strand of large white fibres which lies 

 directly beneath the ventral i^art of the central grey matter of the aqueduct in the 

 lower part of the mid-brain, whilst in the up]>er part it gains a more lateral position 

 (fig. 424); it can be traced downwards to the pons (fig. 429) and medulla, and 

 upwards to the posterior connnissure. the pineal body, and the subthalamic region. 

 It is a connnissural strand which connects the cerebellum with the nuclei of the 

 cranial nerves, and it also associates the various cranial miclei together. It is par- 

 ticularly connected with the third and sixth cranial nuclei of opposite sides, some 

 of the fibres decussating through the raphe, and it possibly contains fibres which 

 have passed upwards from the antero-lateral colunms of the spinal cord. 



ThK ErENCEFHALON 



Dissection.— The i>la niator should l>o i-ouiovtxi fi-om the superior and inferior snrtiuvs of tlie 

 cinobolhmi ; earo should Ik> taken, however, in removiiisr it from the infoiior vernntorm pixxvss. 

 In disstvting the latter region, the uiembranos should W elinped close with .<«.'i!v<^>rs rather than 

 pulled out, as by the latter method the choroid plexuses of the fourth ventricle would be removtHl 

 and tlie fniirile posterior medullary velum would be torn. The student should examine tii-st the 

 upper and then the lower siutaee of the cerebellum, but he should defer the study of the anterior 

 parts of the inferior vermiform process (pyramid and nodule) luitil the fourth ventricle is opened. 



The CEREBELLUM consists of two lateral masses, the hemispheres, which are 

 united by a narrower median ]>ortion. the vermiform ]n'ocess. This is seen in 

 sagittal section in fig. 427. This median portion is but slightly raised above the 



