THE CEREBRAL PEDUNCLES. 



293 



and the crescents of the substantia nigra in front. In the vicinity of the 

 central gray matter, the tegmentum consists chiefly of a reticular foundation 

 resembling the formatio reticularis seen at lower levels. This substance is 

 produced by the intermingling of transverse or arcuate and longitudinal 

 fibres, together with a meagre amount of gray matter, with irregularly dis- 

 tributed cells, that fills the interstices between the strands of fibres. The 

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

 extent by the prominent fibre-tracts belonging to the mesial and lateral fillets 

 and to the superior cerebellar peduncles, or by collections of gray matter, the 

 red nuclei. 



The nucleus ruber, or nucleus tegmenti (Fig. 340), is of ovoid form 

 and reddish tint, when fresh, and consists of a complex of gray matter and 

 nerve-fibres. The latter preponderate below, where the red nucleus receives 



FIG. 340. Section across brain-stem (mid-brain) at level F, Fig. 329. i, 2, root-fibres of oculomotor 

 nerve; 3, interpeduncular space; 4, red nucleus; 5, substantia nigra; 6, crusta of cerebral peduncle; 7, 

 stratum intermedium ; 8, lateral geniculate body ; 9, fillet fibres; 10, superior brachium ; n, median genic- 

 ulate body and (12) its nucleus; 13, optic fibres; 14, pulvinar of optic thalamus; 15, stratum zonale of 

 (16) superior colliculus ; 17, Sylvian aqueduct; 18, sup. colliculus; 19, central gray substance; 20, 22, 

 nucleus of oculomotor nerve; 21, posterior longitudinal fasciculus; 23, red nucleus. X K- (Preparation 

 by Professor Spiller. ) 



the fibres of the superior cerebellar peduncle, but are much less numerous 

 above, since many fibres come to an end around the rubral neurones. 

 These elements are very variable in size (20-60 //) and shape, but are mostly 

 irregularly triangular or stellate. The red nuclei not only constitute im- 

 portant stations in the path connecting the cerebellum with the spinal cord 

 {cerebello-rubro- spinal fibres') , but also probably contribute links in the 

 chain uniting the cerebral cortex with the cord {cerebro-rubro-spinal fibres). 

 While some of the constituents of the superior cerebellar peduncle pass 

 around the red nucleus and continue, as cerebello-thalamic fibres, uninter- 

 ruptedly to the optic thalamus, the majority of the fibres of the peduncle 

 end around the neurones of the nucleus, from which then proceed axones 

 as the rubro-thalamic fibres. It is important to remember that, in a 

 general way, the ventral part of the brain-stem transmits the great efferent 

 or motor paths, while within the dorsal or tegmental part ascend the chief 

 afferent or sensory tracts. The posterior longitudinal fasciculus serves as an 

 association-path linking together the nuclei of the cranial nerves. 



