226 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



nections have been determined by physiological experiments, but 

 so far it has not been possible to mark out histologically the exact 

 groups of cells concerned. The position and physiological properties 

 of these centers are described in the sections on respiration and 

 circulation. These centers are of especial importance because of 

 their wide connections with the body, their essentially independent 

 activity in reference to the higher parts of the brain, and the abso- 

 lutely necessary character of the regulations they effect. In the 

 development of the brain the functions originally mediated by the 

 lower parts have been transferred more and more to the higher 

 parts, especially in regard to conscious sensation and motion, and 

 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 

 cms cerebri. It is 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 palpebrae. It innervates 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 sympathetic 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 central gray matter of the midbrain at the level of the superior 

 colliculus. The fibers for the ciliary muscle and sphincter pupillse 

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

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



