322 ANATOMY AND PHYSIOLOGY 



In the spinal cord and Us nerves we find the apparatus for reflex 

 action which appears in so many phases as muscle contraction, 

 muscle tone, vessel tone, etc. The spinal cord, then, is a great 

 reflex center, a conducting pathway, and an organ of coordination of 

 skeletal muscles. 



Included in the medulla are centers for still more important 

 re/Zexes: the respiratory center; the cardiovascular center or center 

 for heart-action and vessel tone combined; the heat regulating 

 center; deglutition center, and others. Certain functions which 

 these centers control may be modified by the will; for example, 

 the respiratory act we may take a long full breath or a short and 

 shallow one; breathe rapidly or slowly, at will. Deglutition is still 

 nearer to the realm of voluntary movements only when food 

 reaches the esophagus, is the act of deglutition purely reflex. (Here 

 is the first appearance of unstriped muscle in the digestive tract.) 



Going higher we find the cerebellum presiding over the coordi- 

 nation of conscious and voluntary movements, through its connec- 

 tion with the cortex of the cerebrum on one hand, and the pons, 

 medulla and cord on the other. Also upon the cerebellum depends 

 the maintenance of body equilibrium. For this it is necessary that 

 the semicircular canals of the internal ear should be normal and in 

 perfect connection with the cerebellum. Other sensory connec- 

 tions also contribute to the exercise of this function; for example, 

 to walk unaided without vision is possible, but not in a straight 

 line; or, to walk with feet benumbed is difficult, more so to stand 

 motionless; showing that the cerebellum is stimulated to the 

 coordination by which equilibrium is maintained, by more than 

 one sort of stimulus, probably by many. 



We balance the body in equilibrium without conscious sensa- 

 tion unless we voluntarily direct our attention to the subject. It 

 is the disturbance of equilibrium which we feel. 



Going still higher, we find in the cerebrum the perfecting of 

 the plan for bringing the whole sentient and moving organism into 

 the domain of consciousness and the will. This is by means of 

 the connections of the cerebrum through the pons, medulla, and cord 

 and their nerves, with every part of the body from which afferent 

 impulses come, and to which efferent impulses may be transmitted. 



The importance of these connecting fibers cannot be over- 

 estimated; without them the body would be a disjointed affair. 



