PYRAMID TRACT IN SQUIRREL AND CHIPMUNK ■ 155 



In the chipmunk, as in the squu-rel, the decussation appears to 

 be complete. 



Transverse section through inedulla oblongata just caudal to 

 pyramidal decussation {fig. 24)- All the fibers have now crossed 

 and are found in the posterior column cut transversely, where 

 they form the dorsal pyramid tract. ]t is larger than in the 

 previous sections and has moved nearer to the middle line. At 

 its inner angle two distinct bundles are seen in this section. 



Transverse section through cephalic portion of first cervical seg- 

 ment of the spinal cord {fig. 25) . This section, passing through the 

 proximal part of the first cervical segment, shows the crossed 

 dorsal tract lying in the posterior column. It is wedge-shaped 

 in outline, with the base lying against the posterior commissure 

 and the apex directed dorsalward. The inner angle now reaches 

 the posterior median septum. 



Transverse section through caudal portion of first cervical seg- 

 ment {fig. 26). In the first cervical segment the tract undergoes 

 rapid changes in size and form. Its area in the caudal portion 

 of the segment is somewhat reduced and the degenerated fibers 

 which it contains are not so closely packed as in the last section. 

 It is still triangular in shape but it is narrowed laterally and al- 

 most the whole extent of its mesial border lies in contact with 

 the posterior median septum. 



Transverse section through second cervical segment {fig. 27). 

 The tract now lies in the angle formed by the median septum 

 with the border of the posterior horn. It is still triangular in 

 outHne but the base is directed backwards and the apex forwards. 



Transverse section through fourth, sixth and eighth cervical seg- 

 ments {figs. 28, 29, 30). At these three levels there is Httle change 

 except in the number of degenerated fibers, which decrease as 

 the thoracic region is approached. The area occupied by the 

 tract in each of these segments is shown in the corresponding 

 figure. 



Transverse section through second thoracic segment {fig. 31). The 

 tract is considerably reduced in size, but shows the same general 

 outline and occupies the same position as in the cervical region. 



