THE NERVOUS SYSTEM 1431 



cross to the opposite posterior longitudinal bimdle, in which they 

 descend towards the pons Varolii. 



Crura Cerebri. — The crura or pedunculi cerebri are two large 

 strands which are situated above the pons Varolii. They He at 

 first near each other, being separated by the interpeduncular fossa, 

 but afterwards diverge as they pass upwards and outwards to the 

 cerebral hemispheres. The inner surface of each cms looks towards 

 the interpeduncular region, and it presents a furrow, called the 

 ocido-motor sulcus, through which the funiculi of the third cranial, 

 or oculo-motor, nerve emerge, near the pons. The outer surface 

 looks towards the temporal lobe of the brain, which to a large 

 extent overlaps the cms, and this surface also presents a furrow, 

 called the sulcus lateralis. The slender fourth cranial nerve Ues 

 upon this surface. Close to the cerebral hemisphere the ventral 

 and outer aspects of the cms are embraced by the optic tract of the 

 corresponding side. 



Each cms is composed of two parts — ventral and dorsal. The 

 ventral part is variously known a^ the crosta, basis, or pes, and the 

 dorsal part is called the tegmentum. The separation between these 

 two parts is indicated superficially by the sulcus lateralis on the 

 outer surface, and the oculo- 

 motor sulcus on the inner SUr- ^^g^"^**^ Corpora Quadrigemina 



face. Within the cms the two /X\_^^T^ 



parts are separated by a mass (...l.-tA ) ^. tegmentum 



of dark grey matter, called the /^ ? A Lateral Sulcus 



substantia nigra. J ^ K 



The crusta is continuous su- / \ j\ 



periorly ^vith the internal cap- ( V yT ^""^'^ 



sule of the corpus striatum, and V Nv^/\..x^ --^— Crusta 

 interiorly its fibres enter the ven- ^ — ^^^ ^^r;- — ^ 

 tral part of the pons VaroUi. ^"tjcuio-Motor suicm 



The upward prolongation of the p^^ 602 -Topogr.^phy of the Cros 

 tegmentum constitutes the sub- Cerebri (after Poirier). 



thalamic tegmental region. 



Structure of the Crusta, Basis, or Pes. — ^The crusta, as seen in 

 transverse section, presents a crescentic outline, the concavity of 

 the crescent being occupied by the convexity of the substantia 

 nigra. It consists of longitudinal corticifugal fibres, which arise 

 in the cells of the cerebral cortex. These fibres form two groups — • 

 pyramidal and cortico-pontine. 



The pyramidal fibres, on either side, constitute the motor tract 

 from the pre-Rolandic or motor region of the cortex of the frontal 

 lobe, and they arise, for the most part, from the cells of that region, 

 which control the muscles of the upper and lower limbs, as weU as 

 the muscles of the face, though indirectly. 



Tegmentum.— The tegmentum is continuous interiorly with the 

 formatio reticularis of the dorsal portion of the pons Varolii, which 

 in turn, is continuous interiorly with the formatio reticularis of the 

 medulla oblongata. The two tegmenta, right and left, are separated 



