94 



AJVATOMY OF AMPHIOXUS. 



while in the higher Fishes and tailed Amphibia, as well 

 as in the tadpoles of the anourous Amphibia, the giant- 

 fibres are represented by the so-called fibres of Manthner* 



They are not found in the spinal cord of adult tailless 

 Amphibia, Birds, and Mammals. ^^ 



Their occurrence in such large numbers in Amphioxus 

 is therefore the symbol of an archaic organisation. 



Giant-fibres form a very striking feature in the ventral 

 nerve-cord of many Invertebrates. Here, however, they 



Fig. 49. — Part of spinal cord seen from above ; from a preparation stained 

 witli methylene-blue. (After Retzius.) 



g.c. Giant ganglion-cell lying across central canal, mo. Motor root. s. Sensory 

 root. 



appear often to lose their nervous function, and serve 

 rather as elastic supporting rods for the nerve-cord. They 

 are enclosed in thick sheaths of connective tissue, and 

 have been found to originate in giant ganglion-cells. 

 When the enclosed nerve degenerates, they become hol- 

 low tubes containing a coagulable fluid. (EisiG.) 



With regard to the internal origin of the nerves which 

 pass out from the spinal cord, our knowledge only extends 

 to the dorsal roots. At the base of the ventral roots the 



* Also known as Miillerian fibres. 



