96 CIRCULATION OF THE BLOOD IN THE VESSELS. 



vessels, the current is simply diverted into another channel ; expiratory 

 efforts; the contractions of the right side of the heart; and the force of 

 gravity, which operates, in the erect posture, on the current in all excepting 

 the veins of the head, neck and parts of the trunk above the heart. 



Influence of Muscular Contraction. That the action of muscles has con- 

 siderable influence on the current of blood in the veins situated between 

 them and in their substance, has long been recognized ; and this action is so 

 marked, that the parts of the venous system which are situated in the sub- 

 stance of muscles have been compared by Chassaignao to a sponge full of 

 liquid, vigorously pressed by the hand. It must always be remembered, how- 

 ever, that although the muscles are capable of acting on the blood contained 

 in veins in their substance with great vigor, the heart is fully competent to 

 carry on the venous circulation without their aid ; a fact which is exemplified 

 in the venous circulation in paralyzed parts. 



It has been shown by actual observations with the haemadynamometer, that 

 muscular action is capable of increasing the pressure in certain veins. Ber- 

 nard found that the pressure in the jugular of a horse, in repose, was l - 4 

 inch (31-8 mm.) ; but the action of the muscles in raising the head increased 

 it to a little more than five inches (127 mm.), or nearly four times. Such ob- 

 servations show at once the great variations in the current and the impor- 

 tant influence of muscular contraction on the venous circulation. 



In order that contractions of muscles shall assist the venous circulation, 

 two conditions are necessary : 



1. The contraction must be intermittent. This is always the case in the 

 voluntary muscles. It is a view entertained by many physiologists that each 

 muscular fibre relaxes immediately after its contraction, which is instantane- 

 ous, and that a certain period of repose is necessary before it can contract 

 again. However this may be, it is well known that all active muscular con- 

 traction, as distinguished from the efforts necessary to maintain the body in 

 certain ordinary positions, is intermittent and not very prolonged. Thus 

 the veins, which are partly emptied by the compression, are filled again 

 during the repose of the muscle. 



2. There should be no possibility of a retrograde movement of the blood. 

 This condition is fulfilled by the action of the valves. Anatomical researches 

 have shown, also, that these valves are most abundant in veins situated in the 

 substance of or between the muscles, and they do not exist in the veins of 

 the cavities, which are not subject to the same kind of compression. 



Force of Aspiration from the Thorax. During the act of inspiration, the 

 enlargement of the thorax, by depression of the diaphragm and elevation of 

 the ribs, affects the movements of fluids in all the tubes in its vicinity. The 

 air enters by the trachea and expands the lungs so that they follow the move- 

 ments of the thoracic walls. The flow of blood into the great arteries is 

 somewhat retarded, as is indicated by a diminution in the arterial pressure ; 

 and finally, the blood in the great veins passes to the heart with greater 

 facility and in increased quantity. This last-mentioned phenomenon can be 

 readily observed, when the veins are prominent, in profound or violent inspi- 



