824 



Comparative Animal Physiology 



complete deafferentation but the rhythm can be transmitted across a deaf- 

 ferented region of the cord. About 25 continuous intact segments in any 

 part of the cord are needed to maintain the rhythm. Swimming in elasmo- 

 branchs apparently depends on reflex excitation, and peripheral connections 

 are essential, but the cord is capable of transmitting the rhythm through a 

 region where the nerve roots have been cut. Stimuli which accelerate the 

 swimming rhythm in normal fish inhibit it in spinal or fatigued fish. Spinal 

 dogfish show a variety of reflexes, particularly of fins; a single segment iso- 

 lated from the rest of the cord can mediate homolateral and crossed re- 

 sponses.^^®- ^^^ 



Spinal teleosts also show fin reflexes elicited from localized sensory areas,"*^* 

 some of them enhanced, that is, released from cephalic inhibition by the 

 spinal section. ^^^ There appears to be much difference in autonomy of the 

 cord from genus to genus. In fish which swim by undulatory waves, as the 



Fig. 308. Rhythmic responses of posterior abdomen of goldfish (Carassius) to stream 

 of water appHed at side of body, after transection of cord, a, at about vertebra 16; h, at 

 vertebra 7; c, between vertebra 7 and medulla; d and e, at anterior meduUa, ahead of 

 vagus root. From Holst.^ 



eel, section of the cord interferes very little with swimming. An eel still 

 swims when the skin is removed and when the muscles of a quarter of the 

 body are removed. It is as if the undulatory rhythm were inherent in the cord 

 and normally released by the medulla or by peripheral stimulation. ^'^•^ Fish 

 which swim with fins show little or no swimming when the cord is tran- 

 sected anteriorly.-"^' ~^^ Simple reflex responses of the fins and tail region 

 are easily elicited and the sensitivity of- the skin to a stream of water may 

 actually be enhanced after spinal section, as demonstrated in studies on 

 Carassius.-^^- ""^ When spinal section is made close to the medulla the reflex 

 response becomes prolonged, and when the cut is at the level of the vagal 



