76 CLASS PISCES. 



Five kinds of nerve fibres, characterised by their structure, function 

 and distribution, may be distinguished. 



1. The system of the somatic sensory (afferent) fibres. These include 

 the largest heavily meduUated fibres which terminate in the skin and 

 myotome muscles. They pass out by the dorsal roots in the cord, and by 

 the roots of the trigeminal in the brain ; * their ganglia being the posterior 

 root ganglia (spinal), and the gasserian. 



2. The somatic motor (efferent) system. The fibres of this system are 

 also large and heavily meduUated : they terminate in the myotome 

 (somatic) striated muscles ; i.e. the muscles derived from the muscle- 

 plates, including those derived from the dorsal part of the mandibular 

 and from the preraandibular somites. They pass out by the anterior 

 roots in the cord, and by the third, fourth, and sixth cranial nerves, and 

 are without peripheral ganglia.. 



3. The visceral sensory (afferent) system {communis system). The 

 fibres of this system are smaller, and they are distributed to the internal 

 mucous surfaces. They leave the cord by the posterior roots, their ganglia 

 here being posterior root ganglia. The cranial fibres of this system are 

 present in the roots of the fifth, seventh, ninth and tenth nerves ; the 

 ganglia being the gasserian (in part), geniculate, glossopharyngeal and 

 jugular ganglia. Their destination is mainly the mucous surfaces of the 

 anterior part of the alimentary canal. 



4. The visceral motor (efferent) system. The fibres of this system may 

 be subdivided into (a) those which innervate the striated voluntary 

 muscles (mesenchymatous) of the anterior part of the alimentary canal 

 (mandibular, branchial, and facial muscles), are fairly large, and are non- 

 ganglionated, and (6) those which supply the unstriped muscles through- 

 out the body (blood-vessels, gut-wall, skin, etc.). The latter are small 

 fibres, all of which pass through peripheral ganglia. The true motor- 

 fibres to the muscles of the small intestine and anterior part of the alimen- 

 tary canal and its appendages (lungs, etc. ) are derived from cranial nerves, 

 whereas the fibres to blood vessels, skin, walls of Miillerian and kidney 

 ducts come from the spinal cord by anterior roots. The cranial nerves 

 contain no vaso-motor fibres. 



5. The acustico -lateral system (see p. 75). This system consists of 

 large fibres and passes out exclusively in the roots of the seventh, eighth, 

 tenth and possibly ninth cranial nerve. It is absent in the trtmk and in 

 higher Vertebrates, except in the auditory nerve, and is distributed only 

 to the membranous lab5Tinth and the lateral line sense organs (lateral 

 line, ampullae and pit organs). 



From this account it will be gathered that in the head the visceral 

 motor (efferent) fibres travel out with the visceral sensory fibres and in 

 the case of the fifth with the somatic sensory fibres as well, the somatic 

 motor fibres being distinct ; whereas in the trunk they leave the cord 

 with the somatic motor fibres. 



vate the visceral and vascular systems." Journal of Physiology, 7, 1886; 

 and "On the cranial nerves" in Journal of Physiology, 10, 1889. O. S. 

 Strong, " The cranial nerves of Amphibia," Journal of Morphology, 10, 1895; 

 C. J. Herrick, " The cranial and first spinal nerves of Amphibia," Journal of 

 Gomp. Neurology, 9, 1899, p. 157. 



* The third nerve appears to contain fibres belonging to this system 

 (muscular sense), and it is possible that a few of them may be contained 

 in the vagus (Arnold's nerve). 



