ARTERIAL BLOOD PRESSURE. 665 



way, the pressure is shown by Ludwig, Volkmann, and others, to vary 

 in animals of different size, and, in the same animal, in arteries at 

 different distances from the heart, as well as according to different 

 states of the circulation, respiration, and nervous system. Thus, in 

 the horse the average blood pressure was nearly 11 inches ; in the dog, 

 nearly 6 inches; in the rabbit, as a mean, rather more than 1 inch; 

 and in the frog, rather less than 1 inch. Again, in the carotid of the 

 calf, the pressure was equal to 4J inches of mercury ; but in the 

 metatarsal artery to only 3J inches. Lastly, in medium-sized animals, 

 the blood pressure varies from th to f ths of an inch in the larger 

 vessels. According to other authorities, it differs much more than 

 this, even in the same artery. Moreover, there are slight fluctuations, 

 due to the state of the respiration, to hemorrhage, starvation, muscular 

 effort, and other causes, implying variations in the force of the heart, 

 either increase or diminution. The pressure is weaker in younger 

 animals. In the pulmonary arteries, the pressure is only equal to 

 from J to 1 inch of mercury ; but the abnormal disturbing effects of 

 opening the thorax cannot be accurately estimated (Ludwig). 



The force of the blood-current in the arteries, or the blood pressure, 

 not only varies between each ventricular systole, and according to the 

 strength of the heart's action in different circumstances; but it is 

 increased by an addition to the quantity of blood already contained in 

 the system, as when blood is artificially injected into the veins; whilst, 

 on the other hand, it is lessened by a diminution in the quantity of 

 blood in the body, as in cases of hemorrhage. 



The influence of the respiratory movements on the pressure of the 

 blood in the arteries, is very complex. Inspiration, or breathing in, 

 is usually said to produce a diminution in the arterial pressure, and 

 expiration, or breathing out, to cause an increase in that pressure. In 

 explanation of this view, it is stated, that, during the act of inspira- 

 tion, the blood enters the thorax more readily, and thus relieves the 

 whole vascular system of tension; whilst during expiration, the diffi- 

 culty offered to the entrance of blood into the chest, increases the 

 tension in the vessels, in the arteries as well as in the veins. Accord- 

 ing to Vierordt, however, in inspiration, the readier entrance of the 

 blood into the thorax, causes the right side of the heart, and soon, 

 also, the left side of that organ, to become more distended, and the 

 arterial pulse, accordingly, increases in fulness, owing to increased 

 arterial blood pressure, during the course of inspiration ; on the other 

 hand, in expiration, from the resistance offered to the flow of blood 

 into the chest, the right side of the heart, and soon, also, the left side, 

 receive less blood, so that the arterial pulse, owing to diminished 

 arterial pressure, becomes, in the further progress of expiration, 

 smaller. With regard to the blood pressure in the arteries, Vierordt, 

 referring to the effects of inspiration and expiration, in filling the 

 heart with blood, states that, in the former act, the blood pressure, 

 though at first lessened, afterwards increases, reaching its maximum 

 at the beginning of expiration, after which it diminishes. These views, 

 are further modified by the researches of Dr. Sanderson, who states 

 that the rise in the arterial pressure begins with the act of inspira- 



