302 ELEMENTS OF PHYSIOLOGY 



through activity of the cerebrum; the control of the 

 muscles while walking is the function of the cerebellum. 



The cerebrum and cerebellum, like the spinal cord, are 

 divided by deep clefts into hemispheres. The cerebral 

 ganglia are in the lower central region of the cavity (see 

 Plate VII). 



560, The Medulla Oblongata. This, the most important 

 of the cerebral ganglia, is sometimes called the spinal 

 bulb. It may be looked upon as the part of the spinal 

 cord within the skull. It is just within the foramen mag- 

 num, and is intermediate in position and function between 

 the brain and spinal cord. The spinal cord acts reflexly ; 

 the brain acts consciously. Conscious actions are those 

 which are influenced by mental images or ideas. The 

 medulla contains reflex centers and the centers of auto- 

 matic action. This kind of action should not be confused 

 with acquired reflexes, such as walking, to which the term 

 automatic is sometimes applied. Automatic centers are 

 those which are controlled by the condition of the blood. 

 They are stimulated by an increase of carbon dioxid in 

 the blood. The chief of these centers is the respiratory 

 center of the medulla. If it is injured, death ensues by 

 suffocation. It is sometimes called the vital knot, because, 

 although the cerebellum or even the cerebrum may be 

 injured or removed from the lower animals without causing 

 death, the smallest injury to the respiratory center kills 

 the animal immediately. In cases of hanging, it is injury 

 to this center that causes death. Another very important 

 center in the medulla, controlled by automatic action, that 

 is, by the varying condition of the blood, is the center for 

 vasomotor nerves, which regulate the size of the blood 

 vessels. It was previously learned that the vasomotor 

 fibers belong to the sympathetic system. There are also 

 important reflex centers in the medulla, viz., the centers 

 for the secretion of saliva, for swallowing, for vomiting. 



