CEREBRAL NERVES. 97 



one above the other. The lower ganglia occur near the epidermis of the 

 embryo in positions said to correspond with the epibranchial sense organs 

 of fishes. These organs do not develop in man, but the ganglia are 

 permanent structures. Closely united with the geniculate ganglion of 

 the intermedius is the ganglion of the acoustic nerve. The latter is a 

 purely sensory nerve to the ear. By some comparative anatomists it is 

 considered a part of the intermedius. In the trigeminus it is to be noted 

 that the lateral root joins the mandibular division only. The peculiar 

 optic and olfactory nerves will be considered with the sense organs. 



The sympathetic system in the head supplies the smooth muscles of 

 the blood vessels and iris, together with parts of the pharyngeal mucous 

 membranes and the salivary glands; it sends fibers into the periosteum. 

 The plexuses around the large blood vessels are continuous with the 

 sympathetic plexuses of the neck. Although the cerebral nerves do not 

 have any regularly arranged sympathetic or visceral rami, all of them, 

 except the olfactory, optic, and acoustic, are said to communicate with 

 the sympathetic system. In the head there are four sympathetic ganglia, 

 the ciliary, sphenopalatine, otic and submaxillary, all of which are connected 

 with the trigeminal nerve. They develop later than the semilunar ganglion 

 from which their cells may migrate. The sphenopalatine, otic, and sub- 

 maxillary ganglia are also connected with the intermedius and may receive 

 cells from the geniculate ganglion. The otic further receives the continua- 

 tion of the tympanic branch of the glossopharyngeus. 



STRUCTURE OF NERVE TISSUE. 



In the following sections the structure of nerve fibers and of nerves 

 will be considered first; then the sensory and the motor endings; next 

 the ganglia, spinal and sympathetic; and finally the spinal cord as illus- 

 trating the tissue of the central nervous system. 



NERVE FIBERS. The peripheral processes of nerve cells generally 

 appear as slender homogeneous strands varying in diameter. The smallest 

 are found in connection with the sympathetic system and near the termina- 

 tions of the spinal nerves; the largest fibers are the portions near the cord 

 of those which have the longest course. There is no characteristic differ- 

 ence in diameter between sensory and motor fibers. 



With special methods it has been clearly shown that the nerve fiber 

 consists of longitudinal fibrils imbedded in a protoplasmic neuroplasm. 

 The fibrils begin in the cell body. At the origin of the neuraxon they may 

 appear as if gathered into one coarse stiff fibril which distally is resolved 

 into a bundle. The fibrils are supposed to divide but presumably they 

 do not form network. When the fiber branches the fibrils separate into 

 7 



