ANATOMY OF THE MEDULLA OBLONGATA 119 



less parallel longitudinal grooves. These are the anterior and posterior median 

 fissures, and a pair each of anterior lateral and posterior lateral sulci (Figs. 86, 

 89). By means of the fissures it is divided symmetrically into right and left 

 halves; while these, in turn, are marked off by the sulci into ventral, lateral, and 

 dorsal areas, which as seen from the surface appear to be the direct upward con- 

 tinuation of the anterior, lateral, and posterior funiculi of the spinal cord. 

 But, as we shall see in the following chapter, this continuity is not as perfect 

 as it appears from the surface; because the tracts of the cord undergo a rear- 

 rangement as they enter the medulla oblongata. The posterior median fissure 

 does not extend beyond the middle of the oblongata, at which point its lips 

 separate to form the lateral boundaries of the caudal portion of the fourth ven- 

 tricle. The caudal half of the medulla oblongata contains a canal, the direct 

 continuation of the central canal of the spinal cord, and is known as the closed 

 portion of the medulla oblongata (Fig. 85). This canal opens out into the fourth 

 ventricle in the rostral half, which helps to form the ventricular floor, and which 

 is often spoken of as the open part of the medulla oblongata.* 



Fissures and Sulci. The posterior median fissure represents the continua- 

 tion of the posterior median sulcus of the spinal cord and, as noted above, ends 

 near the middle of the medulla oblongata. The anterior median fissure is con- 

 tinued from the spinal cord to the border of the pons, where it ends abruptly 

 in a pit known as the for amen ccecum. Near the caudal extremity of the medulla 

 oblongata this fissure is interrupted by interdigitating bundles of fibers which 

 pass obliquely across the median plane. These are the fibers of the lateral 

 corticospinal tract, which undergo a decussation on passing from the medulla 

 oblongata into the spinal cord, known as the decussation of the pyramids. The 

 anterior lateral sulcus also extends throughout the length of the medulla ob- 

 longata and represents the upward continuation of a much more indefinite groove 

 bearing the same name in the spinal cord. From it emerge the root filaments 

 of the'hypoglossal nerve. From the posterior lateral sulcus emerge the rootlets 

 of the glossopharyngeal, vagus, and accessory nerves (Figs. 86, 88, 89). 



The ventral area of the medulla oblongata is included between the anterior 

 median fissure and the anterior lateral sulcus, and has the false appearance of 

 being a direct continuation of the anterior funiculus of the spinal cord. On 

 either side of the anterior median fissure there is an elongated eminence, taper- 

 ing toward the spinal cord, and known as the pyramid (pyramis Fig. 86). It 

 is formed by the fibers of the corticospinal or pyramidal tract. Just before the 

 fibers of this tract enter the spinal cord they undergo a more or less complete 



