THE MEDULLA OBLONGATA. 



545 



ottom of these furrows, the surface of the medulla oblongata on each side is 

 ivided into three districts, viz., an anterior, a lateral, and a posterior, similar to 

 le surface areas of the three funiculi on the side of the spinal medulla. Indeed, 

 ;, first sight, they appear to be direct continuations upwards of these three portions 

 [ the spinal medulla ; this, however, is not the case, because the fibres of the 

 iree funiculi of the spinal medulla undergo a rearrangement as they proceed 

 pwards into the medulla oblongata. 



Anterior Area of the Medulla Oblongata Pyramis. The district between 

 ie anterior median fissure, and the sulcus lateralis anterior, along the bottom 

 ; f which the root-fila of the hypoglossal nerve issue from the medulla oblongata, 

 '3ceives the name of the pyramid. An inspection of the surface is sufficient to 

 ! aow that the pyramid is composed of a compact strand of longitudinally directed 

 erve-fibres. It represents, in fact, the portion of the great cerebro-spinal fasciculus 

 'hich is destined to carry fibres from the cerebral hemisphere to all the motor 

 uclei on the other side of the medulla oblongata and medulla spinalis. Somewhat 

 onstricted at the place where it emerges from the pons (Fig. 478) it swells 

 nmediately to form a prominent rounded column, which passes vertically down- 

 ward, separated from the pyramid of the other side by the fissura mediana anterior, 

 'owards the lower part of the medulla oblongata it gradually tapers. 



Although the pyramid at first sight appears to be continuous with the anterior 



uniculus of the medulla spinalis, only a very small proportion of the fibres 



ontained in the latter are derived from the pyramid. This at once becomes 



Manifest when the lips of the anterior median fissure are thrust apart at the place 



f junction between the medulla oblongata and spinal medulla. The pyramid is 



hen seen to divide at this level into two parts, viz., a small portion composed of 



variable number of the most lateral fibres of the pyramid, termed the fasciculus 



erebrospinalis anterior (O.T.. direct pyramidal tract), and a much larger portion, 



ituated next the median fissure, called the 



asciculus cerebrospinalis lateralis (O.T. crossed 



tyramidal tract). The anterior cerebro-spinal 



; asciculus is continued down into the anterior 



uniculus of the medulla spinalis, and in this 



t takes up a medial position next the median 



issure. The lateral cerebro-spinal fasciculus 



s broken up into three or more coarsel)undles, 



vhich sink backwards and at the same time 



;ross the median plane, to take up a position 



n the posterior part of the opposite lateral 



uniculus of the spinal medulla. The term 



iecussatio pyramidum (decussation of the pyra- 



nids) is applied to the intercrossing of the 



corresponding bundles of the lateral cerebro- 



' spinal fasciculi of opposite sides. 



The anterior cerebro-spinal fasciculus is, 

 therefore, the only part of the pyramid which 

 'las a place in the anterior f uniculus of the 

 spinal medulla. The much larger part of this 

 hmiculus, termed the fasciculus anterior pro- 

 'prius, as it is traced up into the medulla 

 oblongata, is seen to be thrust aside by the 

 'decussating bundles of the lateral cerebro- 

 spinal fasciculus. It thus comes to occupy 

 i deep position in the substance of the medulla FlG . 4 80. -DIAGRAM OF THE DECUSSATION OF 



oblongata, behind and to the lateral side of THE PYRAMIDS (modified from van Gehuchten). 

 ' the pyramid NH, Nucleus hypoglossi ; NV, Vago-glosso- 



Lateral Area of the Medulla Oblongata. 5^C^S ; FS> TraCtUS S mariUS; NA ' 

 This is the district on the surface of the 



dulla oblongata which is included between the two rows of nerve-roots, viz., the 

 hypoglossal roots in front, and the root-bundles of the accessory, the vagus, and the 



36 



HYPOCLOSSAL 



LATERAL CEREBRO- 

 SPINAL FASCICULUS 

 ANTERIOR CEREBRO- 

 SPINAL FASCICULUS 



