444 HEALTH AND DISEASE 



is completed, and the blood-supply is no longer required, the contractility 

 of the vascular walls comes into play again under the influence of another 

 set of nerves (constrictor), chiefly proceeding from the sympathetic system. 

 The vessels now contract, the mucous membrane becomes pallid, secretion 

 and absorption cease, and the organ resumes its normal condition in the 

 fasting state. A similar succession of events may be observed in every 

 organ of the body that undergoes variations in functional activity, as in 

 the lirain during mental effort, and in the muscles during exercise. 



Blood Pressure. — That when the skin is cut the blood spurts out is 

 a proof that it is subject to pressure within the vessels. The Rev. Stephen 

 Hales, the rector of Faringdon in Hampshire, was the first, at the beginning 

 of the eighteenth century, to estimate what the degree of that pressure is in 

 the living animal, and the animal he selected was the horse. He caused a 

 mare to be tied down on her back, opened the main artery of the thigh, 

 inserted into it a brass pipe the bore of which was ^ inch in diameter, and 

 to this, by means of another brass pipe, which was accurately adapted to it, 

 he fixed a glass tube, of nearly the same diameter, which was 9 feet in 

 length. On untying the ligature previously placed on the artery, he 

 observed the blood to rise in the tube 8 feet 3 inches perpendicular above 

 the level of the left ventricle of the heart. This experiment was an original 

 and a highly instructive one. It has often been repeated, not only in the 

 horse, but in many other animals. The result of many observations has 

 been to show that the pressure of the blood in the vessels is equal to that 

 of a column of mercury 150 to 200 millimetres, or from 6 to 8 inches in 

 height. But Hales pushed his experiment a step farther. He proceeded 

 to investigate the effects of loss of blood on the general blood pressure. 

 He measured the blood as it ran out of the artery, and after each quart of 

 blood had escaped he refixed the glass tube to the artery " to see how much 

 the force of the blood was abated ". This he repeated to the eighth quart, 

 and then, its force being much lowered, he applied the glass tube after each 

 pint had flowed out. He noted several remarkable circumstances. First, 

 that as each quart of blood was removed the blood pressure sank consider- 

 ably, but after the lapse of a minute, more or less, it again began to rise, 

 and although it did not rise to its original level, yet it ultimately attained, 

 on each occasion, a level higher than that to which it had previously fallen. 

 This, there can be no reasonable doubt, was mainly due to the vessels 

 accommodating themselves by virtue of their elasticity and their con- 

 tractility to the reduced volume of their contents. Again, it was found 

 that tho decrease in the blood pressure was not strictly proportionable to 

 the quantity of })lood withdrawn; indeed, it sometimes rose above the level 

 attained during the previous emission, which was probably due to variations 



