MUSCLE, NERVE AND LOCOMOTION 



75 



4. Co-ordination 



Various experiments have been conducted on Hirudo and 

 Haemopis to elucidate the role of the nervous system in locomotion. 

 Rhythmical electrical activity is found in the ventral nerve cord 

 of a swimming leech, but this becomes irregular if the nerve cord 

 is isolated from peripheral stimulation by cutting all segmental 

 nerves. If the cord is isolated from the supra- and sub-oesophageal 

 ganglia by transection just behind the head the electrical activity 

 disappears. If the body of a leech is cut transversely leaving only 

 the ventral nerve cord intact the two halves will show co-ordinated 

 locomotory activity (Fig. 41), but if the nerve cord only is tran- 



Posterior 



Anterior 



sec 



Fig. 41 . Record, taken from a cinematograph film, showing 

 co-ordinated swimming movements of the anterior and posterior 

 regions of a leech which were connected by nerve cord only. 

 From Gvdiy etal, 1938. 



sected the activity on either side of the cut is unco-ordinated. 

 Anterior to the cut the body becomes rounded in cross section as 

 for crawling, and there is an increase in tonus in the circular 

 muscles. Posterior to the cut the body becomes flattened and may 

 perform swimming movements, while there is a loss of tonus in 

 the circular muscles. It thus appears that the ventral nerve cord 

 plays a vital role in the co-ordination of locomotion but it only 



