xvi THE NERVOUS SYSTEM 315 



hack in the medulla. Destruction of the root of this nerve 

 prevents the extrusion of the tongue, but the snapping of the 

 jaws may still occur. The two parts of the reflex of seizing 

 food may therefore take place independently, although they 

 are normally almost always associated. 



The swallowing reflexes persist after destruction of the 

 center for snapping. They are dependent upon the vagus 

 group of nerves, and disappear only when the region of the 

 medulla is injured from which the roots of the vagus group 

 arise. Normal breathing movements still occur when the 

 medulla is cut across just behind the cerebellum and the parts 

 in front of the cut entirely removed. They are not destroyed 

 if the medulla is also cut across at the tip of the calamus 

 scriptorius, but if the region between these two cuts is re- 

 moved, respiratory movements entirely cease. This region 

 includes, therefore, the centers for the movements of 

 respiration. 



The posterior region of the medulla contains those parts 

 of the brain most essential for the maintenance of life. So 

 long as this region remains, the animal may live for a long 

 time. Removal of the medulla farther back than the pars 

 commissuralis results in making locomotion more difficult, 

 although it is still possible when the brain is removed as far 

 back as the tip of the calamus scriptorius. The restless- 

 ness of the animal disappears and the body tends to assume 

 an abnormal attitude. 



There is no center for coordinated locomotion in the 

 medulla. Disturbances of locomotion begin with the fore 

 limbs. If the medulla is cut across at the tip of the calamus 

 scriptorius, the animal sinks on its breast, and the fore limbs 

 are for a considerable time helpless, although the hind limbs 

 are capable of performing vigorous coordinated movements. 

 The reason for this is that the injury lies so near the 



