INTERNAL ANATOMY. 



95 



fibres appear to stop, and in their place a peculiar granular 

 structure of unknown significance is found (Fig. 49). 



The fibres which constitute a dorsal root are derived 

 from two sources. Part of them are continuations or 

 branches of the longitudinal fibres on the same side of the 

 nerve-cord, on which a given dorsal root may be, while the 

 other moiety appears to arise largely from groups of small 

 bipolar ganglion-cells in the neighbourhood of the central 



Fig- 50- — Diagram illustrating the internal origin of the nerve-fibres of a sen- 

 sory root. (Combination of two figures of RETZIUS.) 



The cells giving rise to the processes lying on the same side as a sensory root 

 .5", which divide into a T at the base of the root, are naturally in contiguity with 

 the central canal, but are displaced for the purpose of the diagram, m.l. Middle 

 line. 



canal, which send one process each in the direction of the 

 dorsal root, and another process from the opposite pole of 

 the cell to join in with the longitudinal fibres of the other 

 side of the spinal cord (Fig. 50). ^^ 



We will now compare, or rather contrast, the central 

 nervous system of Ampbioxus with that of an Annelid 

 such as the common earthworm. The type of nervous 

 system presented by the latter is common to a vast propor- 

 tion of the Invertebrates. It consists essentially of three 



