606 PHYSIOLOGY OF THE DOMESTIC AKIMALS. 



The large vessels of the thoracic cavity vary in capacity according 

 to the degree of pressure to which their walls are subjected. Naturally, 

 the veins, possessing thinner and less rigid walls, are more influenced by 

 external pressure than are the arteries. When the thorax becomes ex- 

 panded in inspiration, naturally a negative pressure is likewise exerted 

 on the walls of the veins, and they consequently tend to expand. As a 

 consequence the flow of blood toward the heart is accelerated, and, as 

 has been already indicated, the pressure within the veins in the neighbor- 

 hood of the great vessels at the base of the heart becomes negative dur- 

 ing inspiration. If an opening be made in a vein, as, for example, at the 

 root of the neck, at each inspiratory movement there is a tendency for 

 air to enter the venous trunks, forming one of the serious sources of 

 danger in wounds of the large veins. As a consequence of the greater 

 increase in the venous blood-current during inspiration a larger quantity 

 of blood enters the right auricle. 



Further, if an opening be made in the skull of an animal, a distinct 

 rate of pulsation, which is synchronous with the respiratory movements, 

 may be noticed, in addition to that due to the arterial pulsation. At 

 each inspiration the brain sinks, and at each expiration it rises. The 

 ■production of this phenomenon is evidently due, in the first place, to 

 facilitation of the flow of blood from the brain in inspiration and, in the 

 second place, to retardation in expiration, since it disappears on ligature 

 of the large arteries going to the brain or when a free opening is made 

 in the venous sinuses. During inspiration, therefore, a larger quantity 

 of blood enters the right side of the heart, and, as a consequence, a larger 

 quantity escapes from the left ventricle, more blood escapes into the 

 aorta, and the blood pressure is increased. 



During expiration opposite conditions obtain. The pressure on the 

 walls of the great veins of the thorax is now increased by the reduction 

 in the volume of the thorax, and entrance of blood into the thorax is 

 hindered, less blood enters the heart, less is thrown out by the left ven- 

 tricle, and, as a consequence, arterial pressure falls. 



This state of affairs is, however, not quite so simple as this would 

 seem to indicate. In the first place, the effect of the respiratory move- 

 ments on the arteries must be different from that on the veins, for 

 while inspiration tends to facilitate the flow of blood in the veins toward 

 the thorax, it likewise tends to hinder the escape of blood in the arteries 

 from the thorax ; while at the same time a negative pressure is exerted 

 not only on the veins but on the arteries, and, therefore, tends to produce 

 expansion of the arterial trunks in the same way as expansion of the 

 veins is produced. Therefore, the effect of inspiration on the great arte- 

 rial trunks would be to reduce blood pressure. But from the fact that, as 

 already indicated, the walls of the arteries are less yielding than those, 



