802 NEUROLOGY 





is named the tegmentum; the ventral, the base or crusta; the two bases are separated 

 from each other, but the tegmenta are joined in the median plane by a forward 

 prolongation of the raphe of the pons. Laterally, the tegmenta are free; dorsally, 

 they blend with the corpora quadrigemina. 



The base (basis pedunculi; crusta or pes} is semilunar on transverse section, and 

 consists almost entirely of longitudinal bundles of efferent fibers, which arise from 

 the cells of the cerebral cortex and are grouped into three principal sets, viz., 

 cerebrospinal, frontopontine, and temporopontine (Fig. 710). The cerebrospinal 

 fibers, derived from the cells of the motor area of the cerebral cortex, occupy 

 the middle three-fifths of the base; they are continued partly to the nuclei of the 

 motor cranial nerves, but mainly into the pyramids of the medulla oblongata. 

 The frontopontine fibers are situated in the medial fifth of the base; they arise from 

 the cells of the frontal lobe and end in the nuclei of the pons. The temporopontine 

 fibers are lateral to the cerebrospinal fibers; they originate in the temporal lobe 

 and end in the nuclei pontis. 1 



The substantia nigra (intercalatum) is a layer of gray substance containing 

 numerous deeply pigmented, multipolar nerve cells. It is semilunar on transverse 

 section, its concavity being directed toward the tegmentum; from its convexity, 

 prolongations extend between the fibers of the base of the peduncle. Thicker 

 medially than laterally, it reaches from the oculomotor sulcus to the lateral sulcus, 

 and extends from the upper surface of the pons to the subthalamic region; its 

 medial part is traversed by the fibers of the oculomotor nerve as these stream for- 

 ward to reach the oculomotor sulcus. The connections of the cells of the substantia 

 nigra have not been definitely established. It receives collaterals from the medial 

 lemniscus and the pyramidal bundles. Bechterew is of the opinion that the fibers 

 from the motor area of the cerebral cortex form synapses w r ith cells whose axons 

 pass to the motor nucleus of the trigeminal nerve and serve for the coordination 

 of the muscles of mastication. 



The tegmentum is continuous below with the reticular formation of the pons, 

 and, like it, consists of longitudinal and transverse fibers, together with a consider- 

 able amount of gray substance. The principal gray masses of the tegmentum 

 are the red nucleus and the interpeduncular ganglion; of its fibers the chief longi- 

 tudinal tracts are the superior peduncle, the medial longitudinal fasciculus, and 

 the lemniscus. 



GRAY SUBSTANCE. The red nucleus is situated in the anterior part of the teg- 

 mentum, and is continued upward into the posterior part of the subthalamic region. 

 In sections at the level of the superior colliculus it appears as a circular mass 

 which is traversed by the fibers of the oculomotor nerve. It receives many terminals 

 and collaterals from the superior cerebellar peduncle also collaterals from the 

 ventral longitudinal bundle, from Gudden's bundle and the median lemniscus. 

 The axons of its larger cells cross the middle line and are continued downward 

 into the lateral funiculus of the medulla spinalis as the rubrospinal tract (page 761) ; 

 those of its smaller cells end mainly in the thalamus. The rubrospinal tract forms 

 an important part of the pathway from the cerebellum to the lower motor centers. 



The interpeduncular ganglion is a median collection of nerve cells situated in 

 the ventral part of the tegmentum. The fibers of the fasciculus retroflexus of 

 Meynert, w T hich have their origin in the cells of the ganglion habenulse (page 812), 

 end in it. 



Besides the two nuclei mentioned, there are small collections of cells which 

 form the dorsal and ventral nuclei and the central nucleus or nucleus of the raphe. 



1 A band of fibers, the tractus peduncularis transverSus, is sometimes seen emerging from in front of the superior collic- 

 ulus; it passes around the ventral aspect of the peduncle about midway between the pons and the optic tract, and 

 dips into the oculomotor sulcus. This band is a constant structure in many mammals, but is only present in about 

 30 per cent, of human brains. Since it undergoes atrophy after enucleation of the eyeballs, it may be considered aa 

 forming a path for visual sensations. 



