110 CIRCULATION OF THE BLOOD. 



when the heart is acting with undue vigor, it is quite as important to relieve the lungs 

 by a certain amount of regurgitation from the pulmonary artery as by insufficiency of 

 the tricuspid valves. This insufficiency is important, both at the auriculo-ventricular 

 and the pulmonic orifices, in protecting the delicate structure of the lungs from the varia- 

 tions in force to which the action of both ventricles is constantly liable. 



On microscopical examination of the circulation in the lower animals, as the frog, the 

 movement of blood in the capillaries of the lungs does not present any differences from 

 the capillary circulation in other parts, except that the vessels seem more crowded with 

 corpuscles and there is no " still layer " next their walls. 



There are no forces of any importance which are superadded to the action of the 

 right ventricle in the production of the arterial, capillary, or venous circulation in the 

 lungs ; but there are certain conditions which may obstruct the flow of blood through 

 these parts. "We have already noted the effect of introduction of air into the veins in 

 blocking up the capillaries of the lungs and preventing the passage of blood. It is a view 

 pretty generally entertained that in asphyxia the non-aeration of the blood obstructs the 

 pulmonary circulation. We have already considered this subject rather fully in treating 

 of the general effects of arrest of respiration on the circulation. The celebrated experi- 

 ments of Bichat demonstrated the passage of black blood through the lungs in asphyxia 

 and its presence in the arterial system. The experiments of Dalton and others have 

 shown that, in this condition, the obstruction to the circulation occurs first in the sys- 

 temic capillaries, and the distention is propagated backward through the great vessels 

 and the left cavities of the heart to the right side. When the heart is exposed in a living 

 animal and artificial respiration is kept up, temporary arrest of the respiration produces 

 engorgement and labored action of both ventricles. There are no observations which show 

 that increase of pressure in the pulmonary artery is the first and the immediate result 

 of asphyxia. It is true that, after death, the right side of the heart is engorged ; but it 

 is well known, from observations after death and experiments on living animals, that the 

 tonic contraction of the arteries is competent to empty the blood into the veins ; and 

 the facts just stated regarding the insufficiency of the pulmonic semilunar valves explain 

 how the right side of the heart may become engorged as the result of obstruction to the 

 blood-current in the left side. Established facts seem to show that asphyxia does not 

 primarily affect the pulmonary circulation, but that it is possible for venous blood to 

 pass through the lungs without undergoing arterialization. 



Circulation in the Walls of the Heart. The fact that the contractions of the muscu- 

 lar walls of the heart, by which the blood is discharged from the ventricles into the 

 great arteries, necessarily compress the vessels in the substance of the heart itself 

 would lead us to expect certain peculiarities in the cardiac circulation. This question has 

 been lately studied by Lannelongue, who has arrived at the following results : 



During the ventricular systole, which discharges the blood into the aorta, the vessels 

 in the substance of the ventricles are nearly empty, and the vessels in the auricular walls 

 are filled. During the auricular systole, the auricular vessels are empty and the ventricu- 

 lar vessels are filled. We can readily understand this when we reflect that the vessels 

 in the substance of the heart must be compressed when the muscular fibres contract. 



General Rapidity of the Circulation. 



Several questions of considerable physiological interest arise in connection with the 

 general rapidity of the circulation : 



1. It would be interesting to determine, if possible, what length of time is occupied 

 by the blood in its passage through the entire circuit of both the lesser and the greater 

 circulation. 



2. What is the time required for the passage of the entire mass of blood through the 

 heart ? 



