FUNCTIONS OF THE NERVOUS SYSTEM 305 



(3) Still other fibers regulate the peristaltic action of 

 stomach and intestines, and so influence digestion, while 

 others influence, to some extent, the process of respiration. 



The sympathetic nerves affect directly only involuntary 

 processes. 



444. Functions of the Medulla Oblongata. The whole of 

 the brain above the medulla may be removed from an 

 animal without killing it. Regular and rhythmical respi- 

 ration will go on, the heart will still beat, and the blood 

 will circulate, while reflex action will continue, and many 

 muscles will contract when the sensory nerves connected 

 with them are irritated. Still more complex movements 

 may take place. If food is placed on the back of the 

 tongue, the numerous muscles concerned in swallowing 

 will be excited and will act with their usual accuracy. 

 It is plain that all these reflex movements must be coor- 

 dinated in the medulla and the lower centers. An animal 

 living thus, by the action of the nervous centers connected 

 only with the spinal cord and the medulla, will exist by 

 means of purely reflex mechanism without sensation and 

 without intelligence. 



But let the medulla be destroyed ; respiration will 

 cease (except in the case of the frog, which continues to 

 breathe by cutaneous respiration), and death will almost 

 instantly follow. The nervous center to which is com- 

 mitted the coordination of respiratory movement has 

 been located at the angle of the wall of the fourth ven- 

 tricle, and that point has been called the vital knot. The 

 medulla may be divided above the respiratory center, and 

 the action of the muscles of the chest and diaphragm will 

 be unaffected. Though the heart, as has been stated, 

 will continue its rhythmic beat for a time after all con- 

 nection with cerebro-spinal centers has been severed, still 



