1 1 12 • HUMAN ANATOMY. 



of these fibres is downward through the brain-stem and into the anterior column of the cord 

 (tractus lecto-spinalis medialis). Whether these fibres are interrupted in small secondary nuclei 

 within the tegmentum, or pass unbrokeniy from the coliicular cells to the cord is undetermined. 

 It is probable that, as constituents of a spino-tectal path, fibres also ascend from the spinal cord 

 to the quadrigeminal bodies. According to Kolliker, some of the radial fibres are traceable 

 through the tegmentum, passing to the outer side of the red nucleus and piercing the tract of 

 the median fillet, and into the substantia nigra, whose cells they probably join as axones. The 

 commissure of the superior colliculi is formed by fibres that cross the mid-line to the opposite 

 quadrigeminal body and probably includes, in addition to the axones of cells within the colliculi 

 themselves, fibres from the fillet and optic tracts. 



The most important connections of the superior colliculus, as may be anticipated from the 

 foregoing description of its structure, are : 



I. With the optic tract, without interruption in the lateral geniculate body, by way of the 

 superior brachium. Such fibres serve a special purpose, namely, to carry stimuli which excite 

 pupillary reflexes, by transference to the oculomotor nucleus. 2. With the posterior sensory 

 columns of the spinal cord, indirectly by way of the median fillet. 3. With the cochlear nuclei 

 by way of the lateral fillet, thus establishing a path for audito-visual reflexes. 4. With nuclei 

 of the third, fourth and sixth cranial nerves, controlling the eye-muscles, especially the oculo- 

 motor, by w^y of the posterior longitudinal fasciculus. 5. With the lower levels of the brain- 

 stem and the spinal cord by way of the tecto-bulbar and tecto-spinal tracts. 



The lateral geniculate body belongs to the diencephalon and may be regarded as a special- 

 ized part of the optic thalamus ; the consideration of its structure therefore, properly falls with 

 that of the metathalamus (page 11 26). 



The Tegmentum. — The tegmental region of the mid-brain includes, as seen in 

 transverse sections (Fig. 961), the U-shaped area extending from the quadri- 

 geminal bodies behind to. the crescents of the substantia nigra in front. In the vicin- 

 ity of the central gray matter that surrounds the Sylvian aqueduct, the tegmentum 

 consists chiefiy of a foundation resembling the formatio reticularis seen at lower 

 levels. This substance is produced by the intermingling of transverse or arcuate and 

 longitudinal fibres and a meagre amount of gray matter with irregularly distributed 

 nerve-cells, that fills the interstices between the strands of nerve-fibres. The more 

 lateral and ventral parts of the tegmentum are to a large extent occupied by the 

 prominent fibre-tracts belonging to the fillets and to the superior cerebellar peduncles, 

 or by collections of gray matter, as the red nuclei. Special groups of nerve-cells 

 and of nerve-fibres mark the origin and course of the oculomotor and trochlear 

 nerves. 



The details of the tegmentum vary with the level of the plane of section. Thus, at the lower 

 end of the mid-brain the tracts of the cerebellar peduncles approach the mid-line as they ascend 

 and those of the fillets assume a more lateral position ; whilst at higher levels these tracts, which 

 lower in the mid-brain are so conspicuous, either terminate to a large extent, or become so 

 broken up as to no longer form impressive bundles. 



In sections passing through the lower pole of the inferior quadrigeminal bodies (Fig. 960), 

 the zone overlying the substantia nigra is occupied to a great extent by the median fillet, which 

 here appears as a broad but thin crescentic or comma-shaped field, whose outer and thicker 

 end lies at the periphery and abuts against the base of the dorsally arching tract of the lateral 

 fillet. At the inner end of the median fillet, near the mid-line, an isolated group of obliquely 

 cut fibres sometimes indicates the position of the lemnisco-crustal bundle that appears ventrally 

 among the robust strands of the crusta. Taken together, the two fillets form a compact tract, 

 the outer contour of which, at the level now considered, resembles a horizontally placed Gothic 

 arch, the summit of the curve lying at the surface and the lower and upper limits of the arch 

 being the median and lateral fillets respectively. The lateral fillet continues the sweep of the 

 fillet-stratum along the periphery of the tegmentum until it embraces the lower pole of the 

 inferior colliculas in the manner previously described (page mo). 



Dorsal to the tract of the median fillet, and separated from the latter by a thin layer of com- 

 pact foundation-substance, the ventral tegmental field, lies the broad curved band formed by 

 the blending of the two superior cerebellar peduncles. At lower levels (Fig. 936) these stalks 

 are separate and appear as laterally placed and conspicuous crescentic areas of transversely cut 

 fibres ; but opposite the lower limit of the inferior quadrigeminal bodies the ventral ends of these 

 crescents meet at the mid-line and interlace to form the decussation of the cerebellar peduncles. 

 At a slightly higher level, after their decussation has been almost completed (Fig. 961), the 

 cerebellar peduncles appear as prominent rectangular fields, with rounded corners, on each 

 side of and close to the mid-line. These fields of transversely cut fibres represent the peduncles 



