THE RHOMBENCEPHALON 773 



THE RHOMBENCEPHALON 

 The Medulla Oblongata 



The medulla oblongata (Figs. 629, 632, 635) lies on the basilar part of the 

 occipital bone. It is quadrilateral in outline, but much wider in front than behind, 

 and coiTipressed dorso-ventrally. Its length, measured from the root of the first 

 cervical nerve to the pons, is about two inches (ca. 5 cm.). 



Its ventral svirface is convex in the transverse direction, and presents a ventral 

 median fissure (Fissura mediana ventralis), which is continuous behind with the 

 similar fissure of the spinal cord. The posterior part of the fissure is faintly marked, 

 but in front it becomes deeper and ends in a small depression (Foramen caecum) 

 behind the central part of a transverse band, the corpus trapezoideum. On either 

 side of the fissure is a rounded tract, the pyramid (Pyramis), which is bounded 

 laterally by a faint groove (Sulcus intermedins ventralis). The pyramids join 

 the pons in front; behind they become narrower and disappear into the substance 

 of the medulla, in which their fibers intercross, forming the decussation of the 

 pyramids (Decussatio pyramidum).i The superficial origin of the oculomotor 

 nerve (N. oculomotorius) is just lateral to the anterior end of the pyramid. The 

 corpus trapezoideum is a transverse band which extends across the surface imme- 

 diately behind the pons. It is crossed by the pyramids, which cut off a small 

 central part. The lateral part extends out to the roots of the seventh and eighth 

 nerves on either side. Behind the lateral part of the corpus trapezoideum there 

 is a variably developed rounded eminence, the tuberculimi faciale. The root- 

 fibers of the hypoglossal nerve form an oblique linear series lateral to the posterior 

 part of the pyramid. 



The dorsal surface is largely concealed by the cerebellum and forms the greater 

 part of the floor of the fourth ventricle. The dorsal median fissure (Fissura 

 mediana dorsalis), the direct continuation of the corresponding groove of the spinal 

 cord, extends forward to about the middle of the surface. Here the restiform 

 bodies, which constitute the lips of the fissure, diverge to form the lateral boun- 

 daries of a triangular depression; this is the posterior part of the rhomboid fossa or 

 fioor of the fourth ventricle of the brain. The dorso-lateral groove winds outward 

 and forward to the lateral aspect of the medulla, where it presents the roots of the 

 glosso-pharyngeal, vagus, and accessory nerves. Lateral to it is a distinct oval 

 eminence on the anterior part of the lateral column, termed the tuberculum cin- 

 ereimi. The central canal of the cord is continued in the posterior part of the 

 medulla, inclines dorsally, and opens in the posterior angle of the fourth ventricle. 

 Hence it is customary to distinguish a closed and an open part of the medulla. 

 The dorsal aspect of the latter, which is concealed at present, will be considered 

 later in the description of the fourth ventricle. 



The lateral surface is narrow behind, wider and rounded in front. From it 

 the root-fibers of the ninth, tenth, and eleventh cranial nerves arise in a linear 

 series, and alongside of it the spinal part of the eleventh nerve passes forward to 

 join the medullary root. Close inspection reveals the presence of strias which 

 curve ventrally and backward from the surface of the restiform body toward the 

 hypoglossal root-fibers; these are the external arcuate fibers (Fibrae arcuatje 

 externae). The recess between the lateral aspect of the medulla and the cerebellum 

 is occupied by an irregular mass of villous projections of the pia mater, containing 

 tufts of vessels; this is the chorioid plexus of the fourth ventricle, and is the lateral 

 edge of the tela chorioidea of the ventricle. On raising the chorioid plexus, it is 

 seen that the tela chorioidea is attached to the dorsal aspect of the medulla, and 

 reinforces here the wall of the fourth ventricle; also that the restiform body ter- 



^The decussation varies superficially in different specimens. In some thereis a distinct 

 superficial crossing of fibers so that the median fissure is practically effaced at this point. 



