242 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



merits are performed rhythmically, and the blood-vessels retain 

 their tone so as to maintain an approximately normal blood-pressure. 

 On the contrary, destruction of the medulla, or severance of its 

 connections with the underlying parts, is followed by a cessation 

 of respiration and a loss of tone in the arteries, either of which 

 results in the rapid death of the organism as a whole. The portions 

 of the medulla which exercise these important functions are desig- 

 nated, respectively, as the respiratory and the vasomotor or vaso- 

 constrictor centers. Their location and to some extent their con- 

 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 

 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. 



