246 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



the so-called higher psychical activities. But the unconscious and 

 involuntary regulation of the organs of circulation and respiration 

 and to a certain extent of the other visceral organs has been cen- 

 tralized, as it were, in the medulla. In addition to the control 

 of the respiration and circulation other important reflex activities 

 are effected through the medulla by means of the vagus nerve, 

 which has its nucleus of origin in this part of the brain. Such, for 

 instance, are the reflex control of the heart through the cardio- 

 inhibitory center and of the motions and secretions of the alimentary 

 canal. 



The Nuclei of Origin and the Functions of the Cranial 

 Nerves. The origin, course, anatomical and physiological relations 

 of the first or olfactory, second or optic, and eighth or auditory 

 nerves have been referred to in the preceding pages. For the 

 sake of completeness the origin and functions of the other cranial 

 nerves may be summarized briefly in this connection. 



The Third Cranial Nerve (N. Oculomotorius) . This nerve arises 

 from the base of the brain on the median side of the corresponding 

 pedunculus cerebri. It is, so far as is known, only a motor nerve, 

 supplying fibers to four of the extrinsic muscles of the eyeballs 

 namely, the internal rectus, the superior rectus, the inferior rectus, 

 and the inferior oblique and to the levator palpebrse. It inner- 

 vates also two important intrinsic muscles of the eyeball, the ciliary 

 muscle used in accommodating the eye in near vision, and the 

 sphincter of the iris, which controls in part the size of the pupil. 

 These two latter muscles belong to the type of plain muscle, 

 and the fibers of the third nerve which innervate them terminate 

 in the ciliary ganglion, whence the path is continued by sym- 

 pathetic nerve fibers (postganglionic fibers) to the muscles. In 

 the interior of the brain the fibers of the third nerve arise from a 

 conspicuous nucleus or collection of nuclei situated in the cen- 

 tral gray matter of the midbrain at the level of the superior col- 

 liculus. The fibers for the ciliary muscle and sphincter pupillae 

 arise more anteriorly than those for the extrinsic muscles. His- 

 tologically three parts at least may be distinguished, as shown in 

 Fig. 107, namely, the lateral (or principal) nucleus, which gives 

 origin chiefly to the fibers innervating the extrinsic muscles; the 

 median nucleus ; and the nucleus of Edinger- Westphal. According 

 to Bernheimer* the large median nucleus gives rise to the fibers 

 that innervate the ciliary muscles, while the Edinger- Westphal 

 nuclei (accessory nuclei) control the movements of the sphincter 

 muscle of the iris. Some of the fibers, particularly those from 

 the lateral nucleus to the inferior rectus, the internal rectus, and 



* Bernheimer, in "Graefe-Saemisch's Handbuch der ges. Augenheilkunde," 

 2d ed., I, 41. 



