THE NERVOUS SYSTEM 1437 



Central Tract. — ^The fibres of this tract descend from the optic thalamus, 

 or, it may be, from the nucleus lenticularis of the corpus striatum, through 

 the pons Varolii into the medulla oblongata. Its fibres are regarded as 

 terminating in connection with the inferior ohvjiry nucleus. 



Bundle ol Manzer. — The fibres of this tract descend from the lower quadri- 

 geminal body to the formatio reticularis of the lateral part of the pons \'arolii. 



Spino-thalamlc Tract. — The fibres of this tract, as stated in connection 

 with the tracts of the spinal cord, arise from the cells of the dorsal grey comu 

 of the opposite side. Having crossed in the ventral or white commissure, 

 they enter the antero-lateral or indirect cerebellar tract (tract of Gowers) 

 in which they ascend through the medulla oblongata, pons Varolii, and 

 tegmentum of the cms cerebri to the optic thalamus of the side to which 

 they have crossed. 



Subthalamic Tegmental Region. — ^This region represents the up- 

 ward prolongation of the tegmentum of the cms cerebri beneath 

 the posterior part of the inferior or ventral surface of the optic 

 thalamus. The prolongation contains (i) an upward extension 

 of the red nucleus of the tegmentum, (2) the fibres of the superior 

 peduncle of the cerebellum, and (3) the main or mesial fillet (chief 

 sensory tract). 



The upnard extension of the red nucleus ceases about the level of 

 the corresponding corpus mammillare. Some of the fibres of the 

 superior peduncle of the cerebellum terminate, as stated, in the red 

 nucleus, and others encapsule it, as they do in the tegmentum. 

 Many fibres issue from the cells of the red nucleus, and these, along 

 with the investing fibres of the superior cerebellar peduncle, enter 

 the inferior or ventral surface of the optic thalamus. 



The main fillet (chief sensory tract), which lies on the dorso- 

 lateral aspect of the red nucleus, also enters the inferior or ventral 

 siurface of the optic thalamus. 



Development ol the Crura Cerebri. — ^The crura cerebri are de- 

 veloped from the ventral wall of the mesencephalon. 



Crusta or Pes. — The crusta or pes is, as stated, the ventral portion 

 of the cms cerebri, and it is separated from the tegmentum of the 

 cms by a mass of dark grey matter, called the substantia nigra, which 

 is situated in the interior. Externally the separation is indicated 

 on the outer aspect by the lateral sulcus, and on the inner aspect 

 by the oculo-motor sulcus, through which the fasciculi of the oculo- 

 motor nerve emerge. The crusta is continuoiis with the internal 

 capsvde of the corpus striatum, and it consists of longitudinal centri- 

 fugal fibres, which arise in the cells of the cerebral cortex. These 

 fibres are arranged in two sets, namely, pyramidal and cortico- 

 pontine. The pyramidal fibres form the motor tract from the 

 Rolandic region of the cerebral cortex; and the cortico-pontine 

 fibres are arranged in two strands, namely, fronto-pontine and 

 temporo-pontine. 



The pyramidal fibres form the motor tract from the Rolandic 

 region of the cerebral cortex. They traverse the lenticular portion 

 of the posterior limb of the internal capsule, and then occupy the 

 middle three-fifths of the crusta. Thereafter they descend through 

 the ventral portion of the pons Varolii and the pyramid of the 



