RESPIRATORY CENTRE 133 



is favored more than the outflow through the arteries is hindered. 

 The pulmonary circulation is favored also by the increased 

 negative pressure during inspiration, since the lung fibers 

 pull with greater effect on the pulmonary veins than on the 

 pulmonary artery, which produces a greater difference in 

 pressure in the two vessels, so accelerating the flow of blood. 

 During expiration there is not only a return to the former 

 condition, but the intrapulmonary capillaries are actively 

 squeezed between the air in the lungs and the thoracic walls. 

 This again aids the flow in its passage from the right to the left 

 side of the heart. The pressure which inspiration through the 

 descent of the diaphragm exerts on the abdominal organs 

 will force the blood along the arteries toward the limbs and 

 check somewhat its exit from the thorax. In like manner 

 the pressure on the veins will force the blood into the thorax 

 and momentarily check its flow from the lower extremities. 

 Finally, the heart rate is increased distinctly during inspira- 

 tion. The combined action of the above factors is to draw 

 an increased amount of blood into the thorax and to the left 

 ventricle, which, having more blood to pump, raises the blood 

 pressure. It requires some time for the increased amount 

 of blood to reach the left ventricle, and, as a result, the true 

 effect of inspiration on the blood pressure in the aorta is not 

 felt at the very beginning of the act, but comes a little tardily. 

 This delay may be so great in some animals as to produce, 

 apparently, an inversion of the time relations. 



Respiratory Centre. Respiration may continue after destruc- 

 tion of the entire brain except the bulb, where the centre 

 controlling respiration is situated. The area described by 

 Flourens as the nceud vital is located at the level of the calamus 

 scriptorius. The centre is bilateral, one-half being situated 

 on each side of the median line and corresponding in location 

 to the position of the tractus solitarius. The two parts are 

 intimately connected by commissural fibers, but may be sepa- 

 rated by a section in the median line, after which respiration 

 may continue in a normal manner. But each half is connected 

 with the lung and muscles of respiration of the corresponding 

 side, so that if destroyed the movements on its own side cease. 



The respiratory centre is automatic in its activity, but it 

 is influenced by the sensory fibers of perhaps all of the cranial 



