THE CRANI AL NERVES. The cranial or ence phalic nerves arise from the under surface of the encephalon or brain and leave the cranial cavity through foramina in its floor. According to the older classification of Willis. the cranial nerves were divided into nine pairs. The later and 110W more generally used classification of Sommerring recognizes twelve pairs of cranial nerves. The names of these nerves, taken in order from before backward, are, according to these classifications, as follows: From the physiological standpoint these nerves fall into three groups, according to their func tions. (I) nerves: (a) oculo-motor; (h) pathetic: (c) abducens; (d) (c) poglossal. (2) Sensory nerves: (a) olfactory; (Is) optic; (c) auditory. (3) :Mixed nerves: (a) trifacial: (b) glosso-pharyngeal; (e) pneumogas trie: (4) spinal accessory. Of these the olfac tory. optic, auditory, and parts of the glosso pharyngeal and trifacial are sometimes classified by themselves as nerves of special sense.
The cranial nerves are connected with the sur face of the brain at various points. These points are known as the apparent or superficial origin of the nerves. From these points their fibres can he traced into the substance of the brain to their real or deep arigins in the gray matter.
The cranial nerves, with the exception of the first (olfactory) and the second (optic). are analogous. both embryologically and anatomic ally, to the spinal nerves.
The motor root fibres of the cranial nerves are the axones of neurones whose cell bodies are situated in the gray matter of the medulla and parts above (motor nuclei of the cranial nerves), just as the motor root fibres of the spinal nerves are the axons of neurones whose cell bodies are situated in the gray matter of the cord (anterior horns). These motor nuclei are the nuclei of origin for these nerves. They arc nuclei of ter mination for neurones of higher systems which serve to bring the peripheral neurone under tile control of higher centres.
The neurones which constitute the sensory por tions of the cranial nerves have their cell bodies situated in ganglia outside the central nervous system. These ganglia correspond to the pos terior root ganglia of the spinal nerves. The outwardly directed processes of these cells pass to their peripheral terminations, as do those of the spinal ganglia cells. The central axones of
these neurones enter the medulla and form longi tudinal tracts of fibres in a manner quite anal ogous to the formation of the posterior columns by the central axones of the spinal ganglia cells. The sensory root fibres of the cranial nerves. however, do not ascend, as do those of the spinal nerves, but turn spineward, forming descending roots. These fibres terminate in the gray matter of the medulla (terminal nuclei of the cranial nerves) in the same manner as do the spinal sensory root fibres in the gray matter of the cord and medulla.
The first. or olfactory nerve, is properly not a nerve, but a slender process of the brain which terminates in an enlargement, the olfactory bulb, from which the olfactory nerves themselves are given off. The size and development of this olfactory process and bulb vary greatly in differ ent animals, being in man quite rudimentary as compared with animals noted far their acute sense of smell. The olfactory process is con nected with the rest of the brain by three roots called respectively the internal, or short root. the !piddle, or gray root, and the external. or long root. The olfactm•y nerves proper. that is the bundles of filaments extending from the under surface of the olfactory bulb through the fora mina in the criltriform plate of the ethmoid to the mucous membrane of the nose. are made up of axones of neurones whose cell bodies are located in the nasal mucous membrane. In this respect the peripheral olfactory neurones differ from. all other Imman peripheral sensory neurones. The short, hair-like processes of these cells wthiell pass outward toward the surface are the den drites. The centrally directed processes are the axones. They are non•medullated. and, passing through the eribriform plate in bundles (olfac tory nerves), enter the olfactory bulb, where they terminate in end-arborizations in the olfactory glomeruli. In the glomenili they come into re lation with the dendrites of the mitral cells of the olfactory lobe. The axones of these mitral cells are medullated and form the main mass of fibres which pass back to the brain through the olfactory process or tract and its roots.