RESPIRATION. 353 



In. studying the action of the respiratory movements on blood- 

 pressure, we must remember that in the descent of the diaphragm 

 in inspiration it presses upon the viscera of the abdomen, and forces 

 at first a quantity of blood along the vena cava inferior, but sub- 

 sequently retards the ascent of the blood from the abdomen and the 

 inferior extremities. In normal expiration, the diaphragm ascends 

 and the viscera, not being so forcibly pressed upon, deliver less blood 

 by the inferior vena cava to the heart. 



During inspiration, the heart's frequency is greater than during 

 expiration and the pulse-curve is somewhat different. If the vagi 

 are divided, there is no difference in the pulse-rate during inspiration 

 and expiration. Now, Hering has shown that distension of the 

 lungs irritates the afferent nerves of the vagus center whose impulses 

 inhibit the cardio-inhibitory centers and allow the heart to run faster. 

 Another cause of the increased rapidity of the heart in inspiration 

 is the spreading of impulses from the respiratory center to the vagus 

 center, inhibiting its activity, and at the same time these radiations 

 of impulses from the respiratory center are inhibiting the vasomotor 

 center. The respiratory center, the vagus center, and the vasomotor 

 center are connected by association fibers, and impulses can spread 

 from the respiratory center over to the others. 



Artificial Respiration. When in artificial respiration air is 

 driven into the lungs through a tracheal cannula with sufficient pres- 

 sure, then the pulmonary circulation is arrested. Hence there is an 

 opposite effect produced between artificial respiration and natural 

 respiration as regards their influence upon the course of the blood. 



The aspirating action of the thorax may suck air into a vein in 

 surgical operations, which, on being transported to the right side of 

 the heart, may block the pulmonary capillaries and cause a sudden 

 death. 



THE FUNCTION OF THE UNSTRIPED MUSCLE OF THE 

 BRONCHIAL SYSTEM. 



If a dog is curarized, the interior of a small bronchus is con- 

 nected with a recording instrument (the chest having been opened), 

 and if a vagas is divided, there will be a marked expansion of the 

 bronchi. If the peripheral end of the vagus be stimulated, then a 

 strong contraction of the bronchi will ensue. It is evident here that 

 the smooth muscles of the bronchi are under the influence of the 

 pneumogastrics. Broncho-dilator as well as broncho-constrictor fibers 

 exist in the vagi going to the circular muscles of the bronchi. These 



