THE NERVOUS SYSTEM 223 



The Medulla Oblongata. — The medulla oblongata, or myel- 

 encephalon, is the stem of the true brain and extends from 

 the point of origin of the first spinal nerve to the pons Varolii. 

 The cerebellum must be removed in order to study carefully the 

 features of the medulla, and as it -is cut away the student should 

 notice just ventrad to it the very thin roof of the fourth ven- 

 tricle. Its caudal portion, composed of a reflection of the pia 

 mater lined with epithelium, takes the name of inferior medul- 

 lary velmn, and the cranial part, formed of pia and a thin layer 

 of nervous matter, is called the anterior medullary velum 

 (Fig. 106). 



The fourth ventricle (Figs. 106 and 107) is an enlargement of the 

 canalis centralis of the cord. Two bands of fibers called clavce, 

 the three peduncles or crura of the cerebellum, and the posterior 

 corpora quadrigemina form its lateral boundaries. Its cavity is 

 shallow and pointed at either extremity. The floor, colled fossa 

 rhomboidea, is formed by the continuation of the gray matter 

 of the spinal cord. Where the ventricle is widest a tract of 

 fibers, the stria medullaris, arises from the midline and proceeds 

 laterad to help form the auditory nerve. 



Laterad of the clava a larger band of fibers, the funiculus 

 cuneatus, runs parallel with a more lateral lying bundle, the 

 funiculus cuneatus lateralis, or funiculus of Rolando. The band 

 of fibers on either side uniting the cerebellum to the medulla 

 oblongata is the restiform body, or inferior peduncle of the cere- 

 bellum. On the ventral aspect of the medulla may be seen the 

 anterior pyramids, composed of a band of fibers lying on either 

 side of the median line and apparently emerging from the pons. 

 They form a section of the crossed pyramidal or motor tract 

 connecting the brain with the cord. The olivary eminence lies 

 just laterad of the pyramid. The fact that seven posterior 

 cranial nerves originate from the medulla shows the importance 

 of this part of the brain. The destruction of the respiratory 

 centers, or vital knot, lying ventrad to the caudal end of the 

 fourth ventricle causes instant death. 



