476 THE BRAIN 



the epithelium of the posterior wall of the central canal 

 appears on the surface, and forms the posterior wall or roof 

 of the lower part of the fourth ventricle in the triangular 

 interval between the diverging posterior funiculi of the 

 medulla oblongata. 



The surface of each half of the medulla oblongata should 

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

 tion 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 

 fila of the hypoglossal nerve and the uppermost fila of the 

 anterior root of the first cervical nerve. They continue along 

 the side 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. As they enter 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 of each half of the medulla spinalis. 

 At first sight, indeed, they appear to be direct continuations 

 upwards of the funiculi 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 fissure 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 the pyramid 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, as it reaches the lower border of the 

 pons, it becomes slightly constricted and disappears from 

 view by plunging into the pons. The pyramids are the great 

 motor strands of the medulla oblongata. 



The pyramid, at first sight, appears to be the continua- 

 tion upwards of the anterior funiculus of the spinal medulla, 

 but it contains also a large number of fibres which, at a lower 



