CRANIAL NERVES. 167 



and their homologues in the higher vertebrates, arising from the lateral 

 plate region of the embryo. 



3. The visceral sensory nerves are connected inside the brain 

 with the communis tract of the medulla (fascicularis solitarius of human 

 anatomy), while they terminate in special taste organs, usually within 

 the mouth, but in many teleostomes distributed on the sides of the body 

 as well. 



4. The general cutaneous sensory nerves, corresponding to the 

 somatic sensory of the trunk. Internally they are connected with the 

 dorsal horns of the spinal cord and the homologous parts of the myelen- 

 cephalon, while distally they terminate either as free nerves or in special 

 tactile organs in the skin. 



5. The acustico-lateralis nerves, the centre of which is in the 

 cerebellum and in the tuberculum acusticum of the myelencephalon. 

 Distally the fibres terminate in peculiar collections of sense cells known 

 as sense hillocks or neuromasts occurring in the inner ear and in the 

 lateral line organs of the ichthyopsida. 



6. The nerves of special sense (olfactory and optic). 



The first four of these groups occur in the spinal nerves; the last two 

 are confined to the head. While each spinal nerve contains all four 

 components, the same is not true of most of the cranial nerves, some 

 having but a single kind of fibre. On this and other accounts it is 

 necessary to review each nerve in some detail. In the lower verte- 

 brates (ichthyopsida) there are ten of these cranial nerves; in the am- 

 niotes there are twelve. These are known by both name and number. 

 I. The Olfactory Nerve differs considerably in the various groups 

 of vertebrates. The term strictly includes only the fibres extending 

 between the olfactory lobe of the brain and the olfactory epithelium, 

 the fibres terminating in the rhinencephalon by dendrites w r hich, in- 

 terlacing with dendrites of cerebral neurons, form oval bodies, the 

 glomeruli. The olfactory nerve differs from all others in that it arises 

 from cells of the epidermis. In some vertebrates (elasmobranchs, 

 some teleosts, ganoids, snakes, some lizards, fig. 168, A} , the nerve proper 

 is very short, while the olfactory lobe is developed into an elongate 

 structure in which separate regions may be distinguished, a part of the 

 lobe remaining in connexion with the cerebrum, next a narrower stalk, 

 the tractus, and lastly a larger bulbus olfactorius, containing the 

 glomeruli, close to the nasal organ. In other vertebrates (some 

 teleosts, amphibia, some lizards, turtles, fig. 168, B} the nerve is more 



