v. 17 



SYMPATHETIC OF ELASMOBRANCHS 



173 



17. Autonomic nervous system 



The sympathetic system of elasmobranchs consists of an irregular 

 series of ganglia, approximately segmental, lying dorsal to the pos- 

 terior cardinal sinus and ex- 

 tending back above the kidneys. 

 These ganglia contain motor 

 nerve-cells (post - ganglionic 

 cells) whose ascons end in the 

 smooth muscles either of the 

 arterial walls or of the viscera. 

 The cells themselves are con- 

 trolled by pre-ganglionic nerve- 

 fibres whose cell bodies lie in 

 the spinal cord and whose pro- 

 cesses run out in the ventral 

 spinal roots and rami com- 

 municantes (Fig. 109). In 

 higher animals the sympa- 

 thetic ganglia send postgangli- 

 onic fibres back to the spinal 

 nerves for distribution to the 

 skin ('grey rami communi- 

 cantes') but these are absent in 

 elasmobranchs and correspon- 

 dingly there is no evidence of 

 sympathetic control of skin 

 functions (e.g. chromato- 

 phores); a very different con- 

 dition is found in bony fishes 

 (p. 222). Another peculiarity 

 of the sympathetic system of 

 elasmobranchs is that it does 

 not extend into the head. This 

 condition is unique among 

 vertebrates, but it is not clear 

 whether it is primary or the 

 result of a secondary loss. 



In mammals it is usual to 

 recognize a parasympathetic 

 system acting in antagonism to 



symp 



Fig. i 10. Diagram of the autonomic nervous 



system of the dogfish. 

 art. artery; card.n. cardiac nerve; cil.g. ciliary gang- 

 lion; ft. heart; in. intestine; k. mesonephros; ov. 

 oviduct; ph. pharynx; pr. profundus nerve; py. 

 pylorus; st. stomach; symp. sympathetic ganglion 

 (with suprarenal near it); u.s. urinogenital sinus; 

 III, V, VII, IX, X, cranial nerves. (From Young, 



Quart. J. Micr. Sci. 75.) 



