THE CIRCULATION OF THE BLOOD. 



337 



in the rabbit, from 90 to 100 mm.; in the sheep, 170 mm.; in the calf, from 

 133 to 165 mm. In two observations made on human beings previous 

 to the amputation of a limb, the pressure was found in the brachial 

 artery of one patient to vary from no mm. to 120 mm. Hg., and in the 

 anterior tibial artery of the other patient from no mm. to 160 mm. Hg. 



The investigations made in different parts of the arterial system indicate 

 that the mean pressure is remarkably constant and uniform and does not 

 show any noticeable falling off until near the arteriole region where the resist- 

 ance suddenly and rapidly increases. Thus Volkman found simultaneously 

 in the carotid artery and in the metatarsal artery of the sheep a mean pressure 

 of 165 and 146 mm. Hg. respectively and this for the reason that the resist- 

 ance throughout the arterial system does not markedly increase until the 

 arteriole region is reached. The careful investigations of Dawson show that 

 in the large blood-vessels of the dog the diastolic pressure is as constant 

 as the mean pressure though it undergoes slight variations in different 

 regions; but that the systolic pressure, as shown by taking the end pressure 

 in the thyroid and similar sized arteries in different parts of the arterial tree, 

 undergoes a considerable falling off, though it, too, remains high in large 

 arteries. 



The numerical expressions of these various pressures in different parts 

 of the arterial system are shown in the following table abstracted from the 

 more extensive tables of Dawson. The results were obtained from experi- 

 ments made on dogs. The figures represent in millimeters of mercury certain 

 average end pressures in the arteries named. 



The Capillary Pressure. The small size of the capillaries precludes 

 an investigation of their pressure by manometric methods. It may be stated, 

 however, to be approximately equal to the pressure required to obliterate 

 their lumina and to whiten the skin. The apparatus of v. Kries is based on 

 this theory. A small glass plate, from 2.5 to 5 sq. mm., is fastened to the 

 under surface of a support of suitable size carrying a small scale pan. The 

 glass plate is placed on the skin near the root of a finger-nail and the scale 

 pan gradually weighted until the vessels are obliterated, as shown by the 

 blanching of the skin. From results obtained with this apparatus v. Kries 

 estimated the pressure in the capillaries of the hand at 37 mm. Hg. and in the 

 ear at 20 mm. 



The Venous Pressure. In passing from the capillaries to the heart 

 the pressure continues to fall. The increasing size of the veins permits 

 again of manometric observations in different regions. In the crural vein 



