NERVOUS SYSTEM 



141 



the physiological properties of the connections existing between the 

 various components of the reflex arc, apart from the giant fibre 

 "startle" response. From Fig. 43, however, it can be seen that 

 sensory axons, the cell bodies of which are in the epidermis, pass to 

 the ventral nerve cord, there to make synaptic connection either 

 directly or via an association neurone, with motor fibres. As a result of 

 sensory stimulation in one segment a motor response may occur via 

 fibres in the same segmental nerve, the contralateral nerve of the 

 pair, the nerve before or after that stimulated, or even the segment 



Fig. 43. The anatomical relations of motor axons in the ventral 



nerve cord of Eisenia. fgv = ventral giant fibres; nsa = anterior 



segmental nerve ; nsm = median segmental nerve ; psn = posterior 



segmental nerve (from Grasse, 1949). 



before or behind. Connections via association neurones extend for 

 up to three segments from the one stimulated (Stephenson, 1930, 

 Hanstrom, 1928). Alternatively if the stimulus is strong enough the 

 association neurones may be by-passed and the giant fibre rapid 

 response brought into action. 



Not only can responses be spread from segment to segment by 

 neurones within the ventral nerve cord, but the sensory fields of the 

 nerves supplying the body wall also cover more than one segment. 

 In the course of an electrophysiological investigation Prosser 



