THE MESENCEPHALON. 1115 



The Crusta. — The crusta, or pes pedunculi, appears in transverse sections 

 (Fig. 963) as a bold sickle-shaped field that occupies the most ventral portion of the 

 mid-brain. It consists chiefly of longitudinally coursing fibres which, having traversed 

 the internal capsule, are passing from various parts of the cerebral cortex to lower 

 levels in the brain-stem and the spinal cord. The longitudinal fibres are separated 

 into bundles by the invasion of numerous strands from the fibre-complex, known as 

 the stratum intermedium, which lies along the ventral border of the substantia 

 nigra. The fibres of the crusta comprise three general sets: the cortico-pontile , the 

 cortico-bulbar, and the cortico-spinal. 



The cortica-pontile fibres include those passing from the cells of the cerebral 

 cortex to the cells of the pontile nucleus as links in the cortico-cerebellar paths. 

 They are represented by the froyito-p07itile and the temporo-occipito-poyitile tracts^ 

 which occupy approximately the median and lateral fifths of the crusta respectively. 

 The cortico-bulbar-fibres include the efferent strands which pass from the motor 

 areas of the frontal lobe to the nuclei of the motor fibres originating in the bulbar portion 

 of the brain-stem (trigeminal, abducent, facial, glosso-pharyngeal, vagus and hypo- 

 glossal nerves). These tracts occupy something less than the fifth of the crusta 

 lying next the fronto-pontine tract. The cortico-spinal fibres include the great 

 motor strands which, as the pyramidal tracts, are so conspicuous at lower levels. 

 These tracts share with the fronto-bulbar paths the middle three-fifths of the crusta, 

 appropriating approximately the lateral three-quarters of this area (Fig. 1012). 



The Median Fillet. — Repeated reference has been made to the median fillet 

 (lemniscus medialis) in the preceding descriptions of the brain-stem ; a general con- 

 sideration of this important sensory tract may here be given. It begins at the lower 

 part of the medulla, about on a level corresponding with the upper limit of the 

 pyramidal decussation, as axones of the cells within the nucleus gracilis. These 

 sweep ventro-medially as the deep arcuate fibres, for the most part cross the raphe, 

 and bend sharply brainward. Succeeding the condensation of the fillet-fibres into 

 the sensory decussation (Fig. 922) which marks the lowest limit of the tract, the 

 fillet receives continuous additions of arcuate fibres from the gracile and cuneate 

 nuclei so long as these collections are present. On reaching the inferior olivary 

 nuclei in its journey brainward, the fillet forms a laterally compressed tract, the 

 interolivary stratum, lying immediately dorsal to the pyramids (Fig. 928). 

 Towards the upper end of the pons, the fillet gradually exchanges its sagittal plane 

 and median position for an obliquely horizontal disposition, with an increasing 

 tendency to migrate laterally. The fibres arising from the nucleus cuneatus, which 

 below occupied the ventral part of the fillet, now constitute the lateral part of the 

 tract, whilst those from the nucleus gracilis form its medial portion. Within the 

 mid-brain the median and the lateral fillets form a continuous crescentic tract which, 

 within the upper part of the tegmentum and after the disappearance of the acoustic 

 paths, is represented chiefly by the superficial and laterally placed tract which the 

 median fillet has now become. A considerable part of its fibres end around the cells 

 of the deeper gray stratum of the superior coUiculus, some passing over the aque- 

 duct to the coUiculus of the opposite side. The remaining fibres continue upward 

 through the tegmentum, lateral and dorsal to the red nucleus, and the subthalamic 

 region, to terminate chiefly in relation with the cells within the ventral part of the 

 optic thalamus. After such interruption the impulses are carried by fibres arising 

 within the thalamus to various parts of the cerebral cortex. Whether fillet-fibres 

 gain the cortical gray matter without interruption within the thalamus is uncertain. 

 Other fibres, said to be derived from the cuneate nucleus, end in the corpus subtha- 

 lamicum, and the lenticular nucleus (globus pallidus), from whose cells a certain num- 

 ber of fibres proceed by way of a strand placed above the optic chiasm, the com- 

 missure of Meynert, to the globus pallidus of the opposite side. Still other fibres 

 are traceable into the posterior commissure of the brain and into the mammillary 

 body. 



The constituents of the median fillet, however, are by no means restricted to 

 the fibres arising from the gracile and cuneate nuclei of the posterior columns, but 

 include numerous important accessions from the reception-nuclei of all the sensory 

 cranial nerves connected with the brain-stem. From the cells within the more 



