202 TOPOGRAPHICAL ANATOMY OF THE 



belluni, nor in the neiglilio\irl>ood of tiie choroid plexuses of the fourth 

 ventricle, vascular fringes that project from the groove between the 

 medulla oblongata and the cerebellum immediately behind the pons. 



The HIND-BRAIN (Rhombencephalon). — The hind-brain consists of 

 the medulla oblongata, the pons and the cerebellum. 



Medulla oblongata. — The spinal cord and the medulla oblongata 

 are directly continuous with each other, and when the brain and cord 

 have been removed from the cranium and the vertebral canal, it is 

 impossible to set a precise limit to the two structures. For descriptive 

 purposes, however, the medulla is held to extend from the level of the 

 foramen magnum to the pons. At first it has the same general form as 

 the spinal cord, but soon it becomes dorso-ventral]y flattened and 

 expanded laterally ; a change of form associated with the opening out 

 of the central canal into the fourth ventricle. 



The bilateral character of the medulla oblongata is made manifest 

 on the surface by the presence of ventral and dorsal median fissures, 

 continuous with corresponding landmarks on the spinal cord. While 

 the ventral media7i fissure (fissura mediana ventralis) can be traced as 

 far as the pons, where it ends abruptly at the foramen ccecwm, its depth 

 is not uniform throughout. Near the posterior end of the medulla 

 numerous fibres pass from one side to the other (the decussation of the 

 pyramids) and cause the fissure to be very shallow in this region. The 

 dorsal median fissure (fissura mediana dorsalis) is not so extensive. It 

 is continued only to the extremity of the fourth ventricle, where it 

 apparently widens out in conformity with the lateral divergence of the 

 two halves of the dorsal portion of the medulla oblongata. 



Along each side of the ventral median fissure is a very definite 

 strand of nerve fibres known as the pyramid of the medulla oblongata 

 (pyramis medullce oblongatse). Followed towards the cerebrum, the 

 pyramid disappears beneath the pons. The fibres composing it are 

 derived from nerve cells in the grey matter of that part of the cerebral 

 cortex that lies in neighbourhood of the cruciate sulcus. From their 

 origin these fibres, which are motor in function, pass by way of the 

 corona radiata and internal capsule into the basis pedunculi and so 

 through the pons into the medulla oblongata, where, close to the foramen 

 magnum, they cross over from one side to the other (decussatio 

 pyramidum) to travel along the crossed pyramidal tract of the opposite 

 half of the spinal cord. 



Lateral to the pyramid is a groove, of no great depth, from which 

 the rootlets of the abducent and hypoglossal nerves arise. The groove 

 separates the pyramid from an elevation known as the facial tubercle 

 (tuberculum faciale). In the adult the tubercle is not very conspicuous, 



