522 THE BRAIN 



where it forms the posterior wall or roof of the lower part 

 of the fourth ventricle. 



The surface of each lateral half of the medulla oblongata 

 should now be studied. It is well, however, to defer the 

 examination of the medullary part of the floor of the fourth 

 ventricle till a later period. The dissector has already noticed 

 two linear rows of nerve fila issuing from and entering the 

 medulla oblongata on each side. The anterior row consists 

 of the roots of the hypoglossal and the uppermost part of the 

 anterior root of the first cervical nerve. They continue up- 

 wards on the medulla oblongata in the line of the anterior 

 nerve roots of the spinal medulla, and they emerge along the 

 bottom of a more or less distinct groove. The posterior row is 

 formed of the nerve fila of the accessory, vagus, and 

 glosso-pharyngeal nerves, and they lie in series with the 

 posterior roots of the spinal nerves. 



By these two rows of nerve fila, each side of the 

 medulla oblongata is divided into three districts, viz., an 

 anterior, a lateral, and a posterior, similar to the surface areas 

 of the three funiculi on the side of the medulla spinalis. At 

 first sight, indeed, they appear to be direct continuations 

 upwards of those portions of the spinal medulla ; it is easily 

 demonstrated, however, that that is not the case, and that the 

 fibres in the three funiculi of the medulla spinalis undergo a 

 rearrangement as they are traced into the medulla oblongata. 



Anterior Area of the Medulla Oblongata Pyramis. The 

 district between the antero-median furrow and the row of 

 hypoglossal nerve fila issuing from the medulla receives the 

 name of the pyramid. An inspection of the surface is almost 

 sufficient to show that this is formed by a compact mass of 

 longitudinally directed fibres. It expands somewhat, and 

 assumes a more prominent appearance as it passes upwards, 

 and, finally, reaching the lower border of the pons, it becomes 

 slightly constricted and disappears from view by plunging into 

 that structure. The pyramids are the great motor strands of 

 the medulla oblongata. 



Although the pyramid, at first sight, appears to be the 

 continuation upwards of the anterior funiculus of the spinal 

 medulla, it contains within itself only a very small proportion 

 of fibres which occupy that funiculus. This will be at once 

 manifest if the decussation of the pyramids is examined. For 

 this purpose introduce the back of the knife-blade into the 



