78 CIRCULATION OF THE BLOOk. 



ence of not more than half an inch to an inch in the cardiometer. When the respiratory 

 movements are exaggerated, the oscillations are very much more marked. 



Interruption of respiration is followed by a very great increase in the arterial press- 

 ure. This is due, not to causes within the chest, but to obstruction to the circulation in 

 the capillaries. We are already aware of the influence which the flow of blood into the 

 capillaries is constantly exerting upon the arterial pressure. This tendency to diminish 

 the quantity of blood in the arteries, and consequently the pressure, is constantly coun- 

 teracted by the blood sent into the arteries by the contractions of the heart. With an in- 

 terruption of the respiratory function, the non-aerated blood passes into the arteries but 

 cannot flow readily through the capillaries ; and, as a consequence, the arteries are abnor- 

 mally distended and the pressure is greatly increased. If respiration be permanently 

 arrested, the arterial pressure becomes, after a time, diminished below the normal 

 standard, and is finally abolished, on account of the stoppage of the action of the heart. 

 If respiration be resumed before the heart has become arrested, the pressure soon returns 

 to its normal condition. 



Muscular effort considerably increases the arterial pressure. This is due to two causes. 

 In the first place, the chest is generally compressed, favoring the flow of blood into the 

 great vessels. In the second place, muscular exertion produces a certain amount of ob- 

 struction to the discharge of blood from the arteries into the capillaries. Numerous 

 experiments upon animals have shown a great increase in pressure in the struggles which 

 occur during severe operations. It has been shown that galvanization of the sympa- 

 thetic in the neck and irritation of certain of the cerebro-spinal nerves increase the arterial 

 pressure, probably from an influence on the muscular coats of some of the arteries, caus- 

 ing them to contract and thereby diminishing the total capacity of the arterial system. 



Effects of ffcemorrhage. Diminution in the quantity of blood has a remarkable ef- 

 fect upon the arterial pressure. If, in connecting the instrument with the arteries, we 

 allow even one or two jets of blood to escape, the pressure will be found diminished per- 

 haps one-half, or even more. It is hardly necessary to discuss the mechanism of the effect 

 of the loss of blood on the tension of the vessels, but it is wonderful how soon the press- 

 ure in the arteries regains its normal standard after it has been lowered by haemorrhage. 

 As the pressure depends largely upon the quantity of blood, as soon as the vessels absorb 

 the serosities in sufficient quantity to repair the loss, the pressure is increased. This 

 takes place in a very short time, if the loss of blood be not too great. 



Experiments on the arterial pressure with the cardiometer have verified the fact 

 stated in treating of the form of the pulse ; namely, that the pressure in the vessels bears 

 an inverse ratio to the distention produced by the contractions of the heart. In the car- 

 diometer, the mean height of the mercury indicates the constant, or arterial pressure ; 

 and the oscillations, the distention produced by the heart. It is found that when the 

 pressure is great, the extent of oscillation is small, and vice versa. It will be remembered 

 that the researches of Marey demonstrated that an increase of the arterial pressure 

 diminishes the amplitude of the pulsations, as indicated by the sphygmograph, and that 

 the amplitude is very great when the pressure is slight. It is also true, as a general rule, 

 that the force of the heart, as indicated by the cardiometer, bears an inverse ratio to the 

 frequency of its pulsations. 



Depressor Nerve of the Circulation Within the last few years, an important discovery 

 has been made by Cyon and Ludwig, of a nerve arising in the rabbit by two roots, one 

 from the main trunk of the pneumogastric and the other from the superior laryngeal 

 nerve, which joins the sympathetic filaments in the chest and passes to the heart. This 

 nerve has a reflex action, as was shown by the experiments of Cyon, its galvanization 

 reducing the arterial pressure by one-third or one-half. This action is known to be 

 reflex, for, when the nerve is divided, galvanization of the central end affects the arterial 



