i 7 4 ORGANIZATION OF THE HEAD v. 17 



the sympathetic, but this is not easy to define in the elasmobranchs 

 (Fig. no). The vagus, it is true, is well developed, with branches 

 to the heart and gut, but little is known of autonomic fibres in the 

 other cranial nerves, or of a special 'sacral' parasympathetic system. 

 Stimulation of either the vagus or the sympathetic nerves causes 

 contraction of the stomach. A ciliary ganglion connected with the 

 oculomotor nerve is present as in other animals, but there is no 

 sense in which it can be called antagonistic to the sympathetic 

 system, since the latter does not extend into the head. The post- 

 branchial plexus is a network of fibres and cells connected with the 

 vagus but stretching back above the posterior cardinal sinus 

 (Fig. 104). Receptors in this plexus and in the afferent branchials 

 (Fig. 109) may be concerned with vascular reflexes (p. 161). 



