GENERAL CUTANEOUS SUBDIVISION. II3 



section as a loop. They are therefore spoken of as arcuate fibers. 

 The greater number of them run deeply imbedded in the wall of 

 the cord (or brain) and are known as internal arcuate fibers to 

 distinguish them from the remainder which have a part of their 

 course on the surface and are known as external arcuate fibers. 

 The internal arcuate fibers after reaching the opposite side of the 

 cord or brain bend forward and help to form the ventro-lateral 



Fig. 55. — Cells in the dorsal horn of the cord of a chick embryo of nineteen 

 days incubation. From Cajal (Te.xtura, etc.). A, large marginal cell; B, giant cell 

 of the center of the horn; C, cell of the interstitial nucleus; a, b, c, neurites. 



fiber tracts. Continuing forward these fibers collect into a bundle 

 which is more definitely Hmited in higher vertebrates than in fishes. 

 In man this bundle is known as the ascending lemniscus. In fishes 

 the fibers go to the nuclei in the tectum mesencephali and form 

 what is known as the tractus hulho-tectalis. Its relations in 

 mammals are more complex and will be treated later (Chap. XVI). 



