SPINAL CORD AND COMMENCEMENT OF MEDULLA OBLONG ATA. 187 



diverge somewhat in the region of the oblongata, so that the 

 central gray matter lies exposed on the posterior surface of the 

 spinal cord. Only a thin membrane of gray matter divides the 

 central canal from the free suriace. This canal expands along 

 with the diverging posterior columns, and from now on is called 

 the fourth ventricle. The thin membrane which covers it is 

 called the velum medullare posticum, and is continuous anteriorly 

 with the cerebellum. In the sagittal section, Fig. 56, you can 

 see how the hind-brain roof is formed by the velum medullare 

 posticum, the cerebellum, and the velum medullare anticum. 

 Just at the commencement of the fourth ventricle is an opening- 

 through the velum medullare posticum, which leads from the 

 outside into the ventricular cavity. It is the previously-described 

 foramen of Magendi. Through this the fluid in the ventricles 

 communicates with that which lies outside between the pia and 

 the medulla, and, in the cavity of the arachnoid, bathes the 

 whole central nervous svstem. 



V 



In Fig. 112 this whole roof is removed, so that we are 

 looking: directly into the fourth ventricle. Its floor is bounded 



O * 



behind by the diverging posterior columns and in front by the 

 anterior cerebellar peduncles, which converge toward the corpora 

 quadrigemina. This condition of things has given it the peculiar 

 shape from which it has received its name, fossa rhomboidalis. 



The posterior view of the oblongata (Fig. 112) shows that 

 the posterior columns disappear above, and that the posterior 

 cerebellar peduncles, the corpora restiformia (see below) take 

 their place. The enlargement in the upper portion of the inner 

 posterior column is called the clava, and is caused by the nucleus 

 funiculi gracilis. 



A front (ventral) view of the medulla oblongata (Fig. 113) 

 shows first the thick pyramidal columns ascending from the 

 spinal cord. External to them, imbedded in the prolongations 

 of the lateral columns, are two good-sized enlargements, the 

 olivary bodies. Not far above them the thick masses of fibres 

 which constitute the pons pass across in front of the pyramids. 



