556 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



intrathoracic negative pressure is increased, it is easy to understand how the 

 action which causes inflation of the lungs must affect in like manner such 

 hollow elastic structures as the heart and the great blood-vessels, and thus 

 influence the circulation. It is obvious, however, that this influence must make 

 itself felt to a more marked degree upon the vessels than upon the heart, and 

 upon the flaccid walls of the veins than upon the comparatively rigid walls of 

 the arteries. Moreover, the effects upon the flow of blood through the vessels 

 entering and leaving the thoracic cavity must be different : the inflow through 

 the veins must be favored, and the outflow through the arteries hindered ; but 

 it is upon the flaccid veins chiefly that the mechanical influences of inspiration 

 are exerted. If the thoracic cavity be freely opened, movements of inspiration 

 no longer cause an expansion of the lungs, nor is there a tendency to distend 

 the heart and the large blood-vessels ; if, however, in an intact animal the out- 

 let of the thorax be restricted, as by pressure upon the trachea, the force of the 

 inspiratory movement would make itself felt chiefly upon the heart and the 

 vessels, and it is under such circumstances that the maximal influences of in- 

 spiration upon the circulation are observed. The lungs on the one hand and 

 the heart and its large vessels on the other may be regarded as two sacs placed 

 within a closed expansible cavity, the former having an outlet communicating 

 with the external air, and the latter having inlets and outlets communicating 

 with the extrathoracic blood-vessels, both being dilated when the thorax ex- 

 pands and constricted when it contracts. Moreover, the blood-vessels in the 

 lungs may be compared to a system of delicate tubes placed within a closed 

 distensible bag and communicating with tubes outside of the bag, simulating 

 the communication of the venaB cava? and the aorta with the extrathoracic 

 vessels. When such a bag is distended the tubes also must be distended 

 and their lumina in consequence be enlarged. The lungs in the same way, 

 when expanded by the act of inspiration, are accompanied by a simultaneous 

 dilatation of the intrapulmonary vessels, increasing their capacity, with the 

 natural physical result of lessened resistance to the flow of blood. 



During expiration negative intrathoracic pressure becomes less, so that there 

 is a gradual return of the expanded intrathoracic vessels to that condition 

 which existed at the beginning of inspiration ; at the same time the intrapul- 

 monary vessels are not only subjected to the passive influence of the declining 

 intrathoracic pressure, but are actively squeezed, as it were, between the air in 

 the lungs on one side and the expiratory forces expelling the air on the other. 

 Thus we have during expiration passive and active agents combining to bring 

 about constriction of the intrapulmonary vessels. 



The mechanical effects of the movements of respiration upon blood-pressure 

 may be demonstrated by means of Hering's device (Fig. 141). The chamber 

 A represents the thorax ; the rubber bottom B, the diaphragm ; c, the opening 

 of the trachea ; E D, a tube leading from the thoracic cavity to the manometer 

 I, by means of which intrathoracic pressure is measured ; G is a vessel contain- 

 ing water, colored blue in imitation of venous blood, communicating by means 

 of a tube with an oblong flaccid bag F, in imitation of the heart and the intra- 



