THE TEGMENTUM AND THE PEDUNCLE OF THE MIDBRAIN. 399 



the pons. In about the middle third of the pes, beneath these bundles, lies 

 also the traetus cortico-spinalis, the pyramidal tract, the only bundle of the 

 pes which extends farther than the pons. 



Dorsal from the pes pedunculi lies the stratum intermedium (see Fig. 

 191), with fibers from the corpus striatum; then comes the substantia 

 nigra, an aggregation of fine nerve-fibrils and ganglion-cells, the significance 

 of which is still quite unknown. 



In the tegmentum one notices at once the two large, round, reddish- 

 gray bodies; they are the red nuclei of the tegmentum: nuclei ruhri teg- 

 menti, or nuclei iegmenti. The corpus subthalamicum which lies near them 

 (see Fig. 179) does not appear in the plane of this section. 



The red nucleus, into which fibers pass from the thalamus and from 

 the cerebral cortex, is at this point rich in medullated fibers. These pass 

 ventral to the posterior quadrigeminal bodies, and, for the most part, decus- 

 sate there with those of the opposite side. They belong to the anterior 

 cerebellar peduncle and the decussation is called the decussation of the 

 anterior (superior) cerebellar peduncles. This decussation is very promi- 

 nent in a frontal section through this region. Farther posterior these tracts 

 — Traetus tegmento-cerebellares — form thick bundles which lie external 

 to the red nuclei, and then pass farther and farther to the side and finally 

 reach the outer surface, whence they pass backward to the cerebellum. 

 Since, after injury of the cerebellum, the superior peduncle degenerates as 

 far as the tegmental nucleus, its origin must be in the cerebellum and its 

 end in the red nucleus (Mahaim, et al). 



A nearly horizontal section through the thalamus, the corpora quadri- 

 gemina, and the cerebellum, following the plane of the superior peduncles, 

 gives the relations between the thalamus, the red nticleus, the tegmental 

 radiation, superior peduncles, and cerebellum, as shown in Fig. 193. 



In the cerebellum the superior peduncle enters the corpus dentatum. 



Exterior to the tegmental nucleus lies a thick bundle of obliquely-cut 

 fibers (see Fig. 192), which appear to emerge from under the corpora quadri- 

 gemina. They pass downward in the region dorsal to the substantia nigra. 

 The fibers arise mostly from the ganglia of the corpora quadrigemina and 

 are called the inferior lemniscus, or lower fillet. The upper fillet from the 

 thalamus lies, at the level now being considered (see Figs. 191 et seq.), 

 somewhat external to and below the red nucleus, and appears as a separate 

 bundle of transversely divided fibers. To the outside of it lie the fibers of 

 the lower fillet. It thus happens that there is a broad band of transversely 

 divided fibers just above the substantia nigra, which is called the layer of 

 the fillet, or stratum lemnisci (see also Fig. 194). 



The greater part of the stratum leranisci can be traced posteriorly as far as the 

 nuclei of the sensory nerves and of the posterior columns. Meynert first demon- 



