680 SPECIAL PHYSIOLOGY. 



ready been stated that during, and almost to the end of expiration, 

 the blood pressure in the systemic arteries is increased ; the effects of 

 this increased pressure have even been recognized beyond the capil- 

 laries, in the veins ; for the flow of blood from a divided vein becomes 

 stronger at each expiration. But expiration, though it aids the arte- 

 rial current, must, when the continuity of the veins is perfect, retard 

 the venous current; for the chest-walls must then compress the con- 

 tents of that cavity, including the right auricle and great venous 

 trunks, and so hinder or check the flow of blood into them. This is 

 shown by the accumulation of blood in those veins, by the congestion 

 of the face, and by the distension of the veins of the neck and fore- 

 head, during expiratory efforts, such as coughing or sneezing, and 

 holding the breath, with or without some other accompanying effort. 

 The expiratory thoracic movements cannot, therefore, be regarded as 

 contributing to the venous circulation, the effect on the blood in the 

 arteries being more or less counterbalanced by that on the blood in 

 the veins. The inspiratory movements increase the arterial blood 

 pressure without otherwise affecting the blood-current in the arteries ; 

 because the semilunar valves prevent regurgitation towards the tho- 

 racic space. But the absence of similar valves at the entrance of the 

 venae cavse into the right auricle, so far permits the influence of in- 

 spiration on the blood in the great veins, as to facilitate its entrance 

 into the thorax, i. e., into the great venous trunks and the right auri- 

 cle of the heart. 



If a bent tube be inserted into the jugular vein of an animal, and its 

 lower end be dipped into fluid, the latter will be found to rise within 

 the tube, at each inspiration, sinking again, even a little below its 

 original level, during expiration. (Sir D. Barry.) The blood pressure, 

 as measured by the hsemadynamometer, has also been shown to be 

 less, by from 3 to 7| inches, in the veins during inspiration, especially 

 in those near to the chest ; in the sciatic vein, on the other hand, it is 

 no longer observed. If the veins had rigid walls, the effect of inspira- 

 tion in drawing the venous blood into the thorax, would be consider- 

 able ; but the collapsible character of their coats, and their yielding 

 on pressure, prevent this exhausting process. At the root of the 

 neck, the great veins are more or less supported by, or attached to, 

 the bones or other parts, and so may be partially maintained in a per- 

 vious condition. The effect of inspiration is, indeed, limited to the 

 large veins close to the thorax ; for, as we have seen, the blood pres- 

 sure in the more distant veins of the limbs is not increased during in- 

 spiration. It is this suction force towards the chest, during inspira- 

 tion, which has been named, in regard to its effect on the circulation, 

 the vis afronte, in contradistinction to the vi* a tergo, derived mainly 

 from the heart, modified by the arteries, possibly aided by the nutri- 

 tive and respiratory work accomplished through the capillaries, and 

 certainly assisted by muscular pressure. The existence of this suc- 

 tion force towards the thorax, and its influence on the venous blood- 

 current, are illustrated by the accidents which have sometimes oc- 

 curred in surgical operations in the region of the neck, when air has 

 been drawn in through wounded and patulous veins, and has occasion- 



