



COMPOSITION AND CENTRAL CONNECTIONS OF CRANIAL NERVES 855 



COMPOSITION AND CENTRAL CONNECTIONS OF THE CRANIAL NERVES. 



The cranial nerves are more varied in their composition than the spinal nerves. 

 Some, for example, contain somatic motor fibers only, others contain the various 

 types of fibers found in the spinal nerves, namely, somatic motor, sympathetic 

 efferent, somatic sensory and sympathetic sensory. In addition there are included 

 the nerves of the special senses, namely, the nerves of smell, sight, hearing, equili- 

 bration and taste. 



The Hypoglossal Nerve ( XII cranial) consists of somatic motor fibers only and 

 supplies the muscles .of the tongue. Its axons arise from cells in the hypoglossal 

 nucleus and pass forward between the white reticular formation and the gray 

 reticular formation to emerge from the antero-lateral sulcus of the medulla. The 

 hypoglossal nuclei of the two sides are connected by many commissural fibers and 

 also by dendrites of motor cells which extend across the midline to the opposite 

 nucleus. The hypoglossal nucleus receives either directly or indirectly numerous 

 collaterals and terminals from the opposite pyramidal tract (cortico-bulbar or cerebro- 

 bulbar fibers) which convey voluntary motor impulses from the cerebral cortex. 

 Many reflex collaterals enter the nucleus from the secondary sensory paths of the 

 trigeminal and vagus and probably also from the nervus intermedius and the glosso- 

 pharyngeal. Collaterals from the posterior longitudinal bundle and the ventral 

 longitudinal bundle are said to pass to the nucleus. 



The Accessory Nerve ( XI cranial) contains somatic motor fibers. The spinal part 

 arises from lateral cell groups in the anterior column near its dorso-lateral margin in 

 the upper five or six segments of the cord, its roots pass through the lateral f uniculus 

 to the lateral surface of the cord. It supplies the Trapezius and Sternocleido- 

 mastoideus. The cranial part arises from the nucleus ambiguus, the continuation in 

 the medulla obloiigata of the lateral cell groups of the anterior column of the spinal 

 cord from which the spinal part has origin. The upper part of the nucleus ambiguus 

 gives motor fibers to the vagus and glossopharyngeal nerves. The cranial part 

 sends it fibers through the vagus to the laryngeal nerves to supply the muscles of 

 the larynx. The root fibers of the cranial part of the accessory nerve pass anterior 

 to the spinal tract of the trigeminal while those of the vagus pass through or dorsal 

 to the trigeminal root, and emerge in the line of the postero-lateral sulcus. The 

 nucleus of origin of the spinal part undoubtedly receives either directly or indirectly 

 terminals and collaterals controlling voluntary movements from the pyramidal 

 tracts. It is probable that terminals and collaterals reach the nucleus either directly 

 or indirectly from the rubrospinal and the vestibulospinal tracts. It is also con- 

 nected indirectly with the spinal somatic sensory nerves by association fibers of the 

 proper fasciculi. The cranial part receives indirectly or directly terminals and col- 

 laterals from the opposite pyramidal tract and form the terminal sensory nuclei of 

 the cranial nerves. A few fibers of the cranial part are said to arise in the dorsal 

 nucleus of the vagus and are thus sympathetic efferent. They are said to join the 

 vagus nerve. 



The Vagus Nerve (A' cranial) contains somatic sensory, sympathetic afferent, 

 somatic motor, sympathetic efferent and (taste fibers?) . The afferent fibers (somatic 

 sensory, sympathetic, and taste) have their cells of origin in the jugular ganglion 

 and in the nodosal ganglion (ganglion of the trunk) and on entering the medulla 

 divide into ascending and descending branches as do the sensory fibers of the pos- 

 terior roots of the spinal nerves after they enter the spinal cord. 



(1) The somatic sensory fibers are few in number, convey impulses from a limited 

 area of the skin on the back of the ear and posterior part of the external auditory 

 meatus, and probably join the spinal tract of the trigeminal nerve to terminate in 

 its nucleus. Connections are probably established through the central path of 

 the trigeminal with the thalamus and somatic sensory area of the cortex for the 



