608 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



the blood pressure be simultaneously made, the respiratory undulations, 

 although in a modified form, will still be present. In this case the con- 

 ditions are exactly reversed, for in inspiration air is forced into the lungs 

 instead of being aspirated, so that the pressure in the thorax is increased 

 during inspiration instead of being decreased, as in normal inspiration. 

 So that we are compelled to attribute the greatest influence in the pro- 

 duction of these respiratory undulations to varying changes in the vaso- 

 motor centres rather than to the mechanical causes producing changes in 

 the intra-thoracic pressure. 



Another cause which may be concerned in the production of these 

 respiratory undulations is also, to a certain extent, mechanical, and 

 depends upon varying pressures within the abdomen. As the diaphragm 

 descends in inspiration, the abdominal contents, including the abdominal 

 vessels, are pressed upon, and, as a consequence, the blood pressure must 

 be increased. The reverse naturally obtains during expiration, and it is 

 found that section of both phrenic nerves and opening of the abdominal 

 cavity causes an almost entire disappearance of the respiratory undu- 

 lations ; so that, therefore, the mechanical changes in the abdominal 

 pressure are likewise, to a certain extent, concerned in the production of 

 the respiratory undulations. 



