522 COMPARATIVE ANATOMY. 



from the vagus ; when a larynx and laryngeal muscles are developed, 

 some of these nerves give rise to constant branches. The cardiac 

 relations of the vagus are retained, and, as the intestinal terminal 

 area of the vagus becomes gradually removed from the region of the 

 head, this branch is converted into a long nerve-trunk. 



The hinder portion of the roots which belong to the vagus in the 

 Selachii are, in the Amniota, united into a small nerve-trunk — the 

 accessorius Willisii — which is partly connected with the vagus, and is 

 partly distributed to the muscles of the shoulder-girdle. The root- 

 fibres which form the nerves arise from parts which are placed much 

 farther back than the medulla, especially in the Mammalia, where 

 they are found between the origins of the upper and lower roots of 

 the spinal nerves ; in Man, indeed, they extend as far back as the 

 sixth or seventh pair. 



Lastly, the inferior roots of the area of the vagus form a special 

 nerve-trunk in the Amniota ; this is the hypoglossus, which supplies 

 the muscles of the tongue. It retains its primitive characters in 

 so far as that it is made up of several root-fibres, which moreover 

 pass out separately from the skull, and are arranged in pairs in 

 the Mammalia. 



The posterior complex of nerves given off from the myelence- 

 phalon is therefore the most variable in character. Probably derived 

 from as many separate nerves as there were primitive branchial arches, 

 it is found in its most indifferent condition in the Selachii ; it sepa- 

 rates off a hinder portion in the Teleostei, which forms a special nerve; 

 and in the Amniota forms three different nerves — vagus, accessorius, 

 and hypoglossus. 



Geoenbaur, C, Ueber die Kopfiierven von Hexanchus unci ihr Verhaltniss zur 

 Wirbeltheorie cles Schaclels. Jen. Zeitschr. Bel. VI. 



c) Visceral Nervous System. 



§ 393. 



After the visceral branches are given off from the cerebro-spinal 

 nerves, they become connected together, each uniting itself to the one 

 next behind it ; they thus form a commissure, which runs along the 

 vertebral column, and is continued to the basis cranii; this is the 

 subvertebral chord of the visceral nervous system, or sympathetic. 

 There are ganglia at the points where the rami viscerales of the 

 cerebro-spinal nerves are connected with the chord; the ganglia of 

 the sympathetic chord. From these ganglia nerves, which are 

 made up of fibres of the sympathetic and of cerebro-spinal fibres, 

 pass to their proper areas of distribution. The various nerves, whether 

 passing directly to the viscera, or first traversing the sympathetic 

 chord, are generally collected into trunks set apart for the chief 

 divisions of the viscera, and are known as cardiac, splanchnic, or 



