6 C. A. G. WIERSMA 



Other body half. Furthermore, not only do the Mauthner's cells make this 

 collateral connexion on the axon hillock, but presynaptic excitatory fibers 

 of the eighth nerve make similar connexions at the other side. This may well be 

 the outstanding example of the importance of the location of the synapses 

 on the postsynaptic element. However, the spiral structure shown around the 

 axon hillock is almost certainly not of common occurrence, and thus this may 

 be one of those cases in which a special mechanism was developed, which is 

 not generally present, and serves here the special purpose of preventing that 

 these two cells ever fire at about the same time. 



In conclusion I would like to make the following remarks. Specific in- 

 hibitory neurons are certainly present in nervous systems of different forms 

 including mammals. It is likely from the evidence at hand that they have 

 developed in primitive animals. It is, at present, still debatable how much of 

 the inhibitory phenomena shown in nervous systems can be explained by their 

 action, but tliis question is now more and more opening up to experimental 

 approach. My own inchnation is to predict that a good deal of this type of 

 connexion will be found to be responsible for those inhibitions which are 

 almost momentarily established as well as for some of the long-lasting effects, 

 influenced by tonic inhibitory discharges. Undoubtedly cessation of activity 

 can, even under physiological conditions, be brought about by other means, 

 such as changes of the firing level by hormone influences. By untangling these 

 possibilities and by attacking the problem on different levels of organization, 

 a good deal of important information about and a better understanding of 

 nervous integration appears within reach. 



REFERENCES 



Becht, G. (1959) Studies on insect muscles. Bijdragen tot de Dieikimde 29 : 5-40. 

 BiEDERMANN, W. (1887) Zur Kenntniss der Nerven und Nervenendigungen in der quer- 



gestreiften Muskeln der Wirbellosen. Sitzber. Akad. Wiss. Wien Math.-natiiiw. Kl. 



96 : 8-39. 

 BiEDERMANN, W. (1888) tJber die Innervation der Krebsschere. Sitzber. Akad. Wiss. Wien. 



Math.-mtiirw. Kl. 97 : 49-82. 

 Desmedt, J. E. and Mechelse, K. (1958) Suppression of acoustic input by thalamic stimu- 

 lation. Proc. Soc. Exptl. Biol. Med. 99 : 111-115. 

 Eccles, J. C. (1957) The Physiology of Nerve Cells. Johns Hopkins Press, Baltimore. 

 EcKERT, R. O. (1960) Feedback in the crayfish stretch receptor system. Aitat. Rec. 137 : 



351-352. 

 Florey, E. and Biederman, M. A. (1960) Studies on the distribution of Factor I and 



acetylcholine in crustacean peripheral nerve. J. Gen. Physiol. 43 : 509-522. 

 Foster, M. (1880) A Text Book of Physiology (3rd Ed) Macmillan, New York. 

 Frouch, F. W. (1907) Die Analyse der an der Krebsschere auftretenden Hemmungen. 



Z. allgein. Physiol. 7 : 393-443. 

 Hoffmann, P. (1914) Uber die doppelte Innervation der Krebsmuskeln. Zugleich ein 



Beitrag zur Kenntnis nervoser Hemmungen. Z. Biol. 63 : 411^42. 

 Hughes, G. M. and Wiersma, C. A. G. (1960) The co-ordination of swimmeret movements 



in the crayfish. /. Exptl. Biol. 37 (Vol 4) 



