THE MECHANISM OF THE CIRCULATION 445 



in the degree of contraction and relaxation of the muscles in the walls of 

 the vessels, and in the strength of the contraction of the heart The blood 

 pressure immediately rose when the animal strained its muscles to get loose 

 an effect that was due to the muscular contractions, especially those of 

 the abdominal muscles, forcing much blood towards the heart. In this cele- 

 brated experiment about a quart of blood was lost in making the several 

 trials, and Hales estimated that about 17 quarts were lost in all before the 

 animal died. Taking into account the blood that was obtained from the 

 vessels after death, he considered that 44 Ibs. was a low estimate of the 

 total quantity of blood in the horse. 



The cause of the blood pressure is twofold. On the one hand the heart 

 is always engaged in driving into the vessels, which are already distended, 

 or more than full, an additional quantity of blood; and on the other the 

 current of the blood experiences great resistance to its onward passage in 

 the smaller vessels, owing to their reduced diameter and the great friction 

 that it consequently experiences in traversing them. The blood pressure 

 would be much greater than it is were it not that, owing to the large 

 number of the capillary vessels, the channel is greatly increased, the 

 united area of the capillaries having been estimated to be eight hundred 

 times greater than that of the aorta. 



The rate of movement of the blood through the arteries is estimated to 

 be about 1 foot 8 inches per second. 



Prevention of Death by Hemorrhage. Several circumstances 

 concur to prevent the loss of life that would certainly occur, owing to 

 the fluidity of the blood, when even a small vessel is divided. The first 

 and most important of these is the almost complete closure of the opening 

 of the tube by the contraction of the muscular tissue forming its walls. 

 This tissue, as has been already stated, is most abundant in the small 

 vessels which are chiefly exposed to such injuries. The closure of the 

 vessel is of course aided by the elastic coat, which, being less and less 

 distended, recoils as the blood pressure falls with the escape of blood 

 from the system. Then the arteries are enclosed by a loose sheath, and 

 when cut their proper walls retract to a considerable distance within it, 

 leaving a narrow and tortuous passage, which impedes the exit of the 

 blood. Again, the innermost coat of the artery is highly elastic, and has 

 a tendency, when divided, to roll up within the artery, and thus to form a 

 kind of valve, which is an additional obstacle to the escape of the blood. 

 The coagulation of the blood of course plays an important part in arresting 

 hemorrhage; a clot speedily forms in the loose tissue near the arterial or 

 venous wound, which gradually stops up the opening and forms a plug for 

 some distance up the vessel. Moreover, when much blood has been lost 



