156 MORPHOLOGICAL TYPES 



passing the artery, the vagus gives off a posterior or recurrent 

 laryngeal nerve which loops round the artery, passes dorsal to 

 it, and runs forwards again along the side of the trachea. 

 On the left side the loop of the recurrent laryngeal passes 

 behind the ductus arteriosus. This peculiar course of the 

 recurrent laryngeal nerve is easily understood on referring to 

 the nervous system of Scyllium. The anterior laryngeal nerve 

 corresponds to part of the first branch of the vagus which runs 

 in the 4th visceral (2nd branchial) arch. The posterior or 

 recurrent laryngeal nerve corresponds to part of the 4th branch 

 of the vagus which runs in the 7th visceral (5th branchial) arch. 

 Now the aortic arch, and its representative on the right the 

 innominate artery, are the blood-vessels of the 4th visceral 

 (2nd branchial) arch ; and the ductus arteriosus is the vessel 

 of the 6th visceral (4th branchial) arch. In development 

 these arches are displaced backwards to a considerable extent. 

 But this backward movement of these arteries necessarily pulls 

 back the nerves of the next posterior visceral arch, and this is 

 why the recurrent laryngeal nerves have to loop round the 

 arteries before they can reach their destination. The main 

 branch of the vagus continues backwards to the heart, stomach, 

 and intestine and corresponds to the visceral branch of the 

 vagus of Scyllium. It transmits fibres which belong to the 

 parasympathetic (autonomic) nervous system. 



Parallel with the vagus in the neck is the trunk of the 

 sympathetic nervous system. It swells into the anterior 

 cervical ganglion, on a level with the ganglion of the vagus, 

 and continues forwards into the head accompanying the internal 

 carotid artery. Farther back, the sympathetic trunks have a 

 posterior cervical ganglion, and run backwards accompanying 

 the aorta, swelling into ganglia in most of the segments of the 

 thorax and abdomen. From some of these ganglia, fibres 

 run to the anterior mesenteric ganglion on the root of the 

 anterior mesenteric artery, and to the posterior mesenteric 

 ganglion, which is situated near the root of the posterior 

 mesenteric artery. From these ganglia, fibres are distributed 

 to the smooth muscles of the gut, bladder, and other viscera. 

 It may be repeated that the feature which distinguishes the 



