THE MEDULLA OBLONGATA. 



307 



In sections of the upper cervical region the spinal accessory 

 roots may be made use of, remembering, however, that they 

 pursue a course obliquely upward through the lateral columns. 



The application of these rules to the medulla will be pointed 

 out later on. 



NOTE. To demonstrate the obliquity of the anterior rootlets, find, by a trans- 

 verse section, the exact direction of the anterior rootlets, and then make longi- 

 tudinal sections through the anterior column and horn on this line. 



THE MEDULLA OBLONGATA. 



In the upper part of the cervical region changes take place 

 in the arrangement of the elements of the cord transforming it 

 into the medulla oblongata. The changes are as follows : be- 

 fore the external signs 

 of decussation ap- 

 pear, it is seen that 

 the fibres of the later- 

 al columns change 

 their vertical course 

 and bend forward and 

 inward. This fact is 

 demonstrated by the 

 oblique sections of 

 bundles and fibres. A 

 little higher these 

 bundles and fibres 

 can be traced across 

 the gray matter be- 

 hind the anterior horn 

 into the opposite an- 

 terior column, which is to become by this addition the anterior 

 pyramid. The decussating fibres take the place of the ante- 

 rior commissure lower down, and the fibres pass upward and 

 forward across the median line. The fibres of the anterior 

 columns do not decussate at all, but give way to and mingle 

 with the fibres from the lateral columns. 



The shape and structure of the anterior horns are about the 

 same as lower down. The posterior horn expands suddenly at 



FIG. 132. Diagram of the medulla, pons, etc., natural size, to 

 show the direction of sections for displaying the different nuclei 

 and roota : ll/, line of section to show the early decnssation of the 

 lateral columns and spinal accessory tract; 11, line of section to 

 show the spinal accessory tract and decussation of the pyramids ; 

 11 & 12, region of the spinal accessory and hypoglossal ; 10, pneu- 

 mogastric; 9, glosso-pharyngeal ; 8, acoustic; 6 & 7. abducens 

 and facial ; 5. trigeminus ; 4, patheticus ; 3, motor oculi ; c. g., 

 corpora quadrigemina ; c. c., crua cerebri. 



