iio8 



HUMAN ANATOMY. 



marks the line along which the root-fibres of the third cranial nerve emerge. On 

 transverse section (Fig. 963) these furrows are seen to correspond with the edges of a 

 crescentic field of deeply pigmented gray matter, the substantia nigra, by which 

 each peduncle is subdivided into a dorsal portion, the tegmentum, and a ventral 

 part, the crusta (basis pedunculi). The latter lies ventral to the superficial lateral 

 and median furrows, and contributes largely to the bulk of the free part of the 

 peduncle. When traced upward it is found to enter the cerebral hemisphere and 

 become continuous with the internal capsule. It contains the great motor tracts and 

 is the chief pathway by which efferent cortical impulses are transmitted to the lower 

 lying" centres. The tegmentum, on the contrary, in a general way is associated 

 with the sensory tracts, and, above, enters the subthalamic region (page 1127). 



The dorso-lateral surface of the mid-brain, just where it passes into that of 

 the superior cerebellar peduncle, shares with the latter a triangular area, the trigo- 



FiG. 959. 



Emerging fibres of fourth nerve 

 Fourth tieri'e c"^ 



Decussation of fourth nerve 

 Sylvian aqueduct 



Central grav snbstance 

 \ ^ Mesencephalic root of trigeminus 



Substantia ferruginea 



Superior cerebellar 



peduncle 



Decussation of cerebellar 



peduncle 



Transverse fibres 



Pyramidal tracts 



Transverse section of brain-stem at level L (Fig. 919), junction of pons and mid-brain ; superior cerebellar pedun- 

 cles are beginning to decussate ; trochlear decussation seen above Sylvian aqueduct. Weigert-Pal staining. X 3- 

 Preparation by Professor Spiller. 



num lemnisci, which, as implied by its name, is related to the underlying and 

 here superficially placed tract of the fillet (lemniscus). Above, this area extends 

 as far as the inferior brachium and is limited in front by the sulcus mesencephali 

 lateralis, whilst behind it is defined from the superior cerebellar peduncle by a slight 

 furrow (sulcus limitans posterior). When closely examined the triangular field is seen 

 to be subdivided by a faint groove into an upper and a lower area, which correspond 

 with the underlying fibres of the lateral and of the mesial fillet respectively. A 

 superficial strand of fibres, the tractus peduncularis transversus, is sometimes seen 

 crossing the lateral surface of the mid-brain. It appears on the dorsal aspect of the 

 latter, between the inferior brachium and the median geniculate body, winds around 

 the latero-ventral surface of the peduncle and disappears in the vicinity of the 

 mammillary body. According to Marburg, the strand establishes a connection 

 between the cells of the retina and a nucleus in the floor of the third ventricle and 

 represents, in a rudimentary condition, the basic optic root found in many animals. 

 The Sylvian aqueduct (aquaeductus cerebri) represents the cavity of the middle 

 brain-vesicle and; therefore, is lined with an ependymal layer continuous above and 

 below with that clothing the interior of the third and fourth ventricles. As seen in 



