INTERNAL ANATOMY. 



93 



to the opposite side of the canal, where 

 they continue their course in the longitu- 

 dinal direction (Fig. 47). The giant-fibre 

 belonging to the most anterior giant-cell 

 differs in several respects from the other 

 giant-fibres. It is much larger than the 

 others, and, whereas the latter lie on either 

 side of the nerve-cord, the fibre in question 

 lies in the middle line immediately below 

 the central canal (Figs. 43 and 47). 



These giant-fibres traverse the spinal 

 cord almost throughout its entire length, 

 stopping short at some distance from its 

 anterior and posterior ends. The giant- 

 cells are arranged one after the other in 

 two groups, one group lying in the anterior 

 third of the spinal cord, the fibres from 

 which run backwards, and the other group 

 occupying the posterior third of the cord, 

 the fibres from which run forwards (Fig. 

 48). 



The giant-fibres are in no direct con- 

 nexion with the outgoing nerves, but the 

 giant-cells usually occur opposite a sensory 

 (i.e. dorsal) root (Fig. 49). 



In the spinal cord of Petromyzon giant- 

 fibres are present in considerable numbers, 



Fig. 48. Scheme illustrating the course of the giant- 

 fibres and their origin from the giant-cells A-7. in the spinal 

 cord of Amphioxus. (After ROHDE.) Fl &- 4 8 - 



A-L. Giant ganglion-cells whose giant processes traverse the spinal cord from 

 before backwards. A is about at the level of the sixth sensory root, counting from 

 the first cranial nerve. M-7.. Giant ganglion-cells whose giant processes traverse 

 the spinal cord from behind forwards. M is about at the level of the fortieth sen- 

 sorv root. 



