524 COMPARATIVE ANATOMY 



5. General visceral efferent nerves supply smooth muscles of the 

 viscera, heart muscle and digestive glands. They are relatively small 

 medullated fibers which connect by way of the white rami communicantes 

 with the sympathetic. Their ganglion cells, located in the lateral motor 

 column of the brain and cord, are small. Of the cranial nerves the oculo- 

 motor, facial, glossopharyngeal and vagus nerves contain these fibers. 

 General visceral efferent nerves form the preganglionic fibers of the 

 sympathetic system. Among peripheral nerves they are peculiar in 

 relaying their impulses to a second set of neurones, the cells of which are 

 located in sympathetic ganglia. In amniotes they leave the spinal cord 

 by way of ventral roots only, but in Anamnia some of them are contained 

 in the dorsal roots. (Fig. 442) 



6. Special visceral efferent components are limited to the head where 

 they innervate special striped muscles which develop from the mesoderm 

 of the visceral arches. Their motor centers lie in the ventro-lateral 

 motor column of the medulla between the somatic motor column and 

 the general visceral efferent column. Unlike the general visceral efferent 

 fibers they have no connexion with sympathetic ganglia. They are 

 among the constitutents of the trigeminal, facial, glossopharyngeal, 

 vagus and accessory nerves. 



7. General visceral afferent nerves carry impulses from the mucous 

 lining of the alimentary canal, larynx and trachea. They use the sympa- 

 thetic nerves as pathways but do not have nervous connexion with the 

 cells of the sympathetic ganglia. Some of the fibers reach the medulla 

 oblongata by way of the facial, glossopharyngeal and vagus nerves. 

 Their sensory centers lie in the extended nucleus of the tractus solitarius 

 within the medulla. 



8. Special visceral afferent fibers are found only in cranial nerves in 

 connexion with the senses of smell and taste. The olfactory nerve, and 

 with it possibly the terminal nerve, has been regarded by some as a highly 

 specialized visceral afferent nerve. Gustatory fibers pass to the medulla 

 by way of the facial, glossopharyngeal, and vagus nerves. Like general 

 visceral afferent components their fibers are connected with the nucleus 

 of the tractus solitarius. The centers in the cerebral hemispheres associ- 

 ated with sensations of smell and taste are at present uncertain, but 

 are usually ascribed to the gyrus cinguli. 



EVOLUTION OF THE CRAOTAL NERVES 

 The two anterior pairs of nerves of amphioxus are purely sensory, 

 while all posterior dorsal nerves are mixed. It is customary to compare 

 the first pair of nerves of amphioxus with the olfactory nerves, or with the 

 "terminal" nerves of vertebrates. Since amphioxus has no eyes, no 

 nerves are present comparable with the optic nerves of vertebrates, and 



