From a structural standpoint, the nerve system consists of cell elements peculiarly differentiated from all other tissue cells in that their protoplasm is extended in the form of processes, often to great distances from the nuclear region. The cell elements are held in place by supporting tissue and receive an abundant blood supply; they are partly of ectodermal and partly of mesodermal origin. The cell element of the nerve system, called a _neurone_, is the developmental, structural, and functional unit of the nervous system. It is a single cell presenting unusual structural modifications. It comprises not only the nerve cell body with its numerous protoplasmic processes, or dendrites, but also the axone, which may vary in length from a fraction of a millimeter to fully half the bird’s length. The bulk of the axone is many times the bulk of the cell body. Certain non-medullated axones are surrounded by a delicate, homogeneous, nucleated sheath, called the neurilemma, or sheath of Schwann. THE CRANIAL NERVES The cranial nerves have their origin in the brain and leave the cranial cavity in pairs. They are numbered numerically from before backward, there being twelve pairs in all. The following is a tabulation according to their number, name, and function: No. Name Functional Nature I. Olfactory Smell-sense II. Optic Visual-sense III. Oculomotor Motor to muscles of eyeball and orbit IV. Pathetici Motor to superior oblique muscle of eyeball V. Trifacial Mixed: Sensor to face and tongue. Motor to face VI. Abducentes Motor to External rectus of eyeball VII. Facial Motor to muscles of head and face VIII. Auditory Hearing-sense IX. Glosso-pharyngeal Mixed: Tongue, pharynx and muscles of throat X. Vagus Mixed: Sensori-motor to respiratory tract and part of alimentary tract XI. Spinal accessory Motor to muscles of pharynx, neck and heart XII. Hypoglossal Motor to muscles of the tongue =Olfactorius.=—Nervus olfactorius (Fig. 75, _C_, 16). This is the nerve of smell, one of the nerves of special sense. The organ of smell consists of five layers as follows: First, a layer of olfactory fibers extending in different directions and consisting of a dense plexiform arrangement of the axones of the olfactory cells. From this layer the fibers pass into the layer of olfactory glomeruli where their terminal ramifications mingle with the dendritic terminals of cells lying in the more dorsal layers, to form distinctly outlined spheroidal or oval nerve fiber nests, the _olfactory glomeruli_. Second, a fine granular layer of basic substance containing round cellular structures, the _stratum granulosum_.