THE CONTROL OF THE RESPIRATION 



345 



The subsidiary centers are entirely dependent upon the master center 

 for their harmonious action, as is shown by the fact that if the phrenic 

 motor neuron which is situated in the cervical spinal cord between the 

 fourth and sixth -spinal segments is isolated from the medulla by a 

 lateral hemisection of the cord just above the fourth segment and by 

 mesial section of the cord opposite the center, the corresponding half of 

 the diaphragm no longer participates in the respiratory act (see Fig. 118), 



The chief center on either side of the midline of the medulla is con- 

 nected with the motor neurons of both sides of the spinal cord, as is 

 proved by the following experiment. When the central end of the vagus 

 nerve is stimulated, the respiratory center becomes excited and the respi- 

 rations more pronounced, the participation of the muscles on both sides 

 of the body being equal in extent. If now we bisect the medulla down the 





Fig. 118. Diagram to show cuts required for isolation of the phrenic center. 



midline and repeat the stimulation of one vagus, the muscles on both sides 

 will still participate in the increased respiration, which they will likewise do 

 if the cervical cord is bisected or hemisected but the medulla left intact 

 (Fig. 119). The simplest interpretation of these results is that commis- 

 sural fibers connect both halves of the respiratory center in the medulla 

 and that each half is also connected with the motor neurons of both sides 

 of the spinal cord. Often, especially in young animals, a hemisection of 

 the cord causes cessation of the movements of the diaphragm on the same 

 side; but this paralyzed side at once begins to contract again when the 

 phrenic of the opposite side is cut, probably because the respiratory 

 impulse descending from the chief center, on finding its way along the 

 motor center of the same side of the cord blocked, is forced to follow the 

 crossed path. The crossing in the cord is believed to take place at the 

 same level as that at which the subsidiary center is located (W. T. 

 Porter 12 ). 



