286 THE RHOMBENCEPHALON. 



SURFACES. 



The medulla oblongata presents four surfaces: The anterior, 

 posterior and two lateral, separated by the anterior lateral and 

 posterior lateral grooves. In the upper medulla, the surfaces are 

 clearly denned; but they become less distinct as they descend to 

 the inferior and nearly circular extremity (Figs. 81, 85 and 86). 



The anterior lateral sulcus (s. lateralis anterior} separates 

 the anterior from the lateral surface, and is in line with the ex- 

 its of the anterior roots of the spinal nerves. No corresponding 

 groove exists in the cord. From the anterior lateral groove issue 

 the roots of the hypoglossal nerve and the anterior root of the 

 first cervical nerve. The abducent (or sixth) nerve rises nearly 

 in line with it from the transverse groove between the pons and 

 the medulla (Fig. 86). 



Posterior Lateral Sulcus. (S. lateralis posterior}. The pos- 

 terior lateral sulcus of the medulla separates the lateral from the 

 posterior surface (Figs. 86 and 87). It descends between the olive 

 and the restiform body and is continued through the spinal cord. 

 Through this sulcus into the cord run the posterior roots of the 

 spinal nerves, and, likewise, the sensory roots of the vagus and 

 glossopharyngeal nerves run through it into the medulla; while 

 the motor roots of the ninth and tenth and the cerebral root of 

 the eleventh nerve emerge from the medulla through the pos- 

 terior lateral sulcus. The roots of the seventh, eighth and inter- 

 mediate nerves are found at the superior end of the sulcus in the 

 transverse groove between the medulla and pons. The posterior 

 lateral sulcus is not parallel with the axis of the medulla, but bends 

 outward and forward as it ascends. Inferiorly, it is obliterated 

 for a short distance by the crossing of the fasciculus cerebello- 

 spinalis (direct cerebellar tract) from the lateral to the posterior 

 surface. 



The anterior surface (fades anterior} of the medulla, bounded 

 on either side by the anterior lateral sulcus, extends from the trans- 

 verse sulcus below the pons down to the spinal cord (Fig. 85). 

 It is made up of symmetrical halves united, below, by the decus- 

 sation of the lateral (crossed) pyramidal tracts; but separated, 

 above, by the anterior median fissure, which terminates at the 



