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 which, 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 
