Nervous Systems 



789 



of the mantle cavity, thus sending the animal backward or forward, accord- 

 ing to the funnel angle; this escape or predatory reaction is, according to 

 Young, twice as fast by virtue of the giant fiber system as it would be with- 

 out it. 



Young*^^ has described an interesting series among cephalopods (Fig. 

 296). In the decapod Sepia the cells contributing to the large giant fibers 

 are scattered through the stellate ganglion. In Loligo these cells are confined 

 to one posterior lobe, whereas in the octopods Eledone and Octopus the lobe 

 is present and contains cells which may resemble neurones but which lack 

 true processes. This lobe, the epistellar body, receives the secondary giant 

 fibers from the visceral ganglion; its possible neurosecretory function is in- 

 dicated by an atonia of the mantle after removal of the epistellar body. 



m.c 



Fig. 296. Comparison of stellate ganglion of. A, Sepia, B, Loligo, and C, Eledone. 

 m.c, Mantle connective (from brain mass); Tp.g.f-, preganglionic fiber; ep. epistellar lobe. 

 From Young.^" 



Giant nerve fibers have evolved many times. They are absent from ani- 

 mals closely related to animals which have them. For example, giant fibers 

 are found in only a few polychaetes, are absent from crabs but present in 

 shrimps and prawns, absent from octopods but present in decapods, absent 

 from adult anurans but present in urodeles. Wherever they are present they 

 seem to function in rapid escape reactions; they are efficient, since one nerve 

 impulse in a giant fiber can activate muscles of a wide body area. A single 

 nervous unit carries out a complex fast response. 



Interneuronic Transmission, Continuity versus Contiguity. In all nerve 

 centers the axons branch extensively and terminate on the dendrites and 

 cell body of the receiving neurone— rarely also on its axon. The pattern of 

 dendrites differs greatly according to the nerve center, and for many years 

 histologists disagreed as to whether there is continuity or contiguity be- 

 tween neurones. With the introduction of cytological fixation methods and 

 degeneration experiments the argument seems well settled in favor of con- 

 tiguity. The fine axon branches often terminate in enlargements, boutons; 

 in numerous preparations two separate membranes can be seen— Mauthner 

 cells of urodeles^s. 47. 48 (^ig. 297, A), oculomotor nucleus of fish (Fig. 297, 



