184 DURATION OF THE CIRCULATION. 



on the contraction and dilatation of the heart and on the blood-stream ( 60), it 

 is clear that respiration favours the. blood-stream, so does artificial respiration. 

 When artificial respiration is interrupted, the blood-stream becomes slower (Dogiel). 

 If the suspension of respiration lasts somewhat longer, the current is again acceler- 

 ated on account of the dyspnoeic stimulation of the vaso-motor centre (Heidenhain) 

 (see Vaso-motor centre, vol. ii.) 



(9.) Conditions Affecting Velocity in the Veins. Many circumstances 

 affect the velocity of the blood in the veins. (1) There are regular 

 variations in the large veins near the heart (Valsalva) due to the 

 respiration and the movements of the heart (50, and 60). (2) Irregular 

 variations due to pressure e.g., from contracting muscles ( 87), friction 

 on the skin in the direction or against the direction of the venous 

 current, the position of a limb or of the body. The pump-like action 

 of the veins of the groin during walking has been referred to ( 87). 

 When the lower limb is extended and rotated outwards, the femoral 

 vein in the iliac fossa collapses, owing to an internal negative pressure; 

 when the thigh is flexed and raised, it fills under a positive pressure 

 (Braune). A similar condition obtains in walking. 



91. Estimation of the Capacity of the Ventricles. 



Vierordt calculated the capacity of the left ventricle from the velo- 

 city of the blood-stream, and the amount of blood discharged per 

 second by the right carotid, right subclavian, the two coronary arteries, 

 and the aorta below the origin of the innominate artery. He estimated 

 that with every systole of the heart, 172 cubic centimetres (equal to 

 182 grammes) of blood were discharged into the aorta; this, therefore, 

 must be the capacity of the left ventricle (compare 83). 



92. The Duration of the Circulation. 



The question as to how long the blood takes to make a complete 

 circuit through the course of the circulation was first answered by 

 Hering (1829) in the case of the horse. He injected a 2 per cent, 

 solution of potassium ferrocyanide into a special vein, and ascertained 

 (by means of ferric chloride) when this substance appeared in the 

 blood taken from the corresponding vein on the opposite side of the 

 body. The ferrocyanide may also be injected into the central or cardiac 

 end of the jugular vein, and the time noted at which its presence is 

 detected in the blood of the peripheral end of the same vein]. 

 Vierordt (1858) improved this method by placing under the cor- 

 responding vein of the opposite side a rotating disc, in which was 

 fixed a, number of cups at regular intervals. The first appearance of 



