BLOOD-PRESSURE IN THE ARTERIES. 167 



* Carotid, Horse, 161 mm. 

 + , ,, 122-214 mm. 



,, Dog, 151 mm. 



130- 190 mm. (Ludwig). 



+ ,, Goat, 118-135 mm. 



+ Aorta of frog, 22-29 mm. 



+ Gill artery of Pike, 35-84 mm. 



Brachial artery of man during an 



operation, 110-120 mm. (Faivre). 



Perhaps too low owing to the 



injury. 



+ ,, Rabbit, 90 mm. 

 + Fowl, 88-171 mm. 



The pressure in the aorta of mammals varies from 200 to 250 mm. Hg. 



As a general rule, the blood-pressure in large animals is higher than 

 in small animals, because in the former the blood-channel is consider- 

 ably longer, and there is greater resistance to be overcome. In very 

 young and in very old animals the pressure is lower than in individuals 

 in the prime of life. 



(b.) Branching of the Blood-Vessels. Within the large arteries the 

 blood-pressure diminishes relatively little as we pass towards the 

 periphery, because the difference of the resistance in the different 

 sections of large tubes is very small. As soon, however, as the 

 arteries begin to divide frequently, and undergo a considerable diminu- 

 tion in their lumen, the blood-pressure in them rapidly diminishes, 

 because the propelling energy of the blood is much weakened owing 

 to the resistance which it has to overcome (p. 118). 



(c.) Amount of Blood. The blood-pressure is increased with greater 

 fitting of the arteries, and vice versd ; it 



Increases 



1. With increased and accelerated 



action of the heart; 



2. In plethoric persons; 



3. After increase of the quantity of 



blood by direct transfusion, or 

 after a copious meal. 



Decreases 



1. During diminished and enfeebled 



action of the heart; 



2. In anaemic persons; 



3. After haemorrhage or considerable ex- 



cretions from the blood by sweat- 

 ing, the urine, severe diarrhoea. 



The blood-pressure does not vary in the same proportion as the variations in the 

 amount of blood. The vascular system, in virtue of its muscular tissue, has the 

 property, within liberally wide limits, of accommodating itself to larger or smaller 

 quantities of blood (C. Ludwig and Worm Miiller, 102, d). Small and moderate 

 haemorrhages (in the dog to 2 '8 per cent of the body- weight) have no obvious effect 

 on the blood-pressure. After a slight loss of blood the pressure may even rise (Worm 

 Miiller). If a large amount of blood be withdrawn, it causes a great fall of the 

 blood-pressure (Hales, Magendie), and when haemorrhage occurs to 4-6 per cent. 

 of the body-weight, the blood-pressure = 0. The transfusion of a moderate amount 

 of blood does not raise the mean arterial blood-pressure. [There are important 

 practical deductions from these experiments, viz., that the blood-pressure cannot 

 be diminished directly by moderate blood-letting, and that the blood-pressure is 

 not necessarily high in plethoric persons.] 



Capacity of the Vessels. The arterial pressure rises when the 

 capacity of the arterial system is diminished, and conversely. The 

 plain circularly-disposed muscular fibres of the arteries are the chief 



