94 



THE MECHANISM OE THE CIRCULATION. 



The respiratory mechanism can be thrown out of gear by itself, and with- 

 out damage to the vasomotor mechanism by the injection of curari. The 

 power of the heart is then sufficient by itself to maintain the circulation 

 in the feet-down position, so long as the capacity of the abdominal 

 vessels is kept under control by the vasomotor nerves. 



The effect on the circulation of rendering the intrathoracic pressure 

 positive, is dealt with in the section relating to the diastolic rilling of 

 the heart. Here it may be briefly stated that the blood stagnates in 

 the abdomen, and the circulation ceases, whenever, by any means, the 

 thoracic pressure is rendered sufficiently positive to overcome the 

 venous pressure produced by the driving power of the heart. Owing to 



the influence of gravity, this state of affairs 

 is brought about more easily in the vertical 

 feet-down position than in the horizontal 

 posture. Compensation for the positive 

 intrathoracic pressure is supplied by firm 

 compression of the abdomen, the heart then 

 fills, and the arterial pressure regains its 

 normal level. 



The arterial pressure depends on the 

 energy of the heart, and on the peripheral 

 resistance. It is clear that the arterial 

 pressure likewise depends on the hydrostatic, 

 effect of gravity. If a normal man be placed 

 in the vertical feet-down position, the pressure 

 in the femoral artery will be higher than the 

 pressure in the common carotid artery, by the 

 height of the column of blood which reaches 

 from the carotid to the femoral artery. In 

 a man 6 ft. in stature this difference will 

 amount to about 45 mm. Hg. On the as- 

 sumption of the vertical feet-up or head- 

 down posture the pressure in the common 

 carotid will rise to a like extent above that of the femoral artery, 

 owing to the influence of gravity. In the feet-up posture there exists 

 no great vascular field like the splanchnic area to expand under the 

 hydrostatic pressure. The face and neck alone become injected, for the 

 brain is confined within the rigid wall of the cranium, and thus the 

 cerebral vessels cannot expand. 



In changes of posture, it is apparent, from the experiments related, 

 that the pressure in the carotid artery is maintained at practically a 

 constant height by the tone of the splanchnic arterioles and action of 

 the respiratory muscles. If the splanchnic arterioles are in an efficient 

 state of tone, and if the abdominal veins are supported by the tone of the 

 abdominal wall, then the splanchnic vessels are similar in nature to a 

 model of rigid tubes, and will not dilate under the hydrostatic stress of 

 gravity. The nervous mechanism involved is probably of the simplest 

 nature, for if the arterial pressure suddenly rise or fall at the moment of 

 change in posture, the bulbar centres are thereby either directly or 

 reflexly excited to decreased or increased activity. A sudden fall of arterial 

 tension always provokes acceleration of the heart, amplified respiration, 

 and increased vaso-constriction. A sudden rise of tension, on the other 

 hand, provokes a slow heart, shallow respiration, and vaso-dilatation. 



Fig. 64.— Aortic and vena cava 

 pressures. Morphinised and 

 cnrarised dog. — Hill and Bar- 

 nard. 



A, Artificial respiration withdrawn. 

 Vertical feet-down posture. 



A-B, Asphyxial rise of aortic pres- 

 sure. Traube-Hering curves. 



H and C, Effect of compression of 

 abdomen. At C this produced 

 paralytic dilatation of the heart. 



