THE BLOODVESSELS 71 



tation or constriction of the splanchnic area. Under abnormal 

 conditions, such as ill-health, these compensations are absent or 

 imperfect, and anyone who has been in bed for a few days, and 

 then assumes the vertical position, knows the faintness which 

 follows from the general fall in blood-pressure due to dilatation 

 of the splanchnic area. If the ordinary hutch rabbit be held 

 in the vertical position for a period varying from one-quarter 

 to three-quarters of an hour, it will in all probability die. The 

 cause of death is cerebral anaemia ; under the influence of 

 gravity the blood collects in the splanchnic area and the blood- 

 pressure in the aorta falls. With a wild rabbit, dog, or cat, 

 this experiment fails, the explanation being that the tense 

 abdominal walls of the animal leading an active life compresses 

 the splanchnic area and prevents the blood collecting in the ab- 

 dominal cavity. In a dog or cat previously poisoned by chloro- 

 form or chloral the experiment succeeds, as the tone of the 

 abdominal walls is lost. When a horse rears up and falls back- 

 wards, it may be due to temporary cerebral anaemia resulting 

 from want of compensation in a vertical position, apart from 

 any question of loss of balance. 



The arterial pressure varies, as has been said, with each systole 

 of the ventricle, but besides this there are also certain larger and 

 longer undulations obtainable in graphic records of blood- 

 pressure which hitherto have not been regarded by physiologists 

 as connected with the heart, but caused by the movements of 

 respiration. It has been observed that inspiration causes a rise 

 and expiration a fall in pressure. This important action of the 

 respiratory pump has never been quite satisfactorily explained, 

 and we now have the view of T. Lewis, who finds that the rise of 

 pressure on inspiration is due to the lessened pressure in the 

 pericardium and consequent increased filling of the heart. If 

 the pericardial sac be opened to the air, no such curves are 

 produced. Lewis therefore concludes that the effect produced 

 is due entirely to the heart, brought about by changes in the 

 intrapleural pressure. The question will be again examined in 

 dealing with respiration. The character of the curve produced 

 is seen in Fig. 29. 



The Blood-Pressure in the Capillaries is very difficult to ascer- 

 tain. It is probably \ to \ of that in the large arteries or lies 

 between 20 to 40 mm. of mercury. 



Hill compares the capillary system to a sponge, squeezed and 

 filled continuously by the active motions of the body. The 

 contents of the capillary vessels are constantly being pressed 

 onwards by muscular movements, so that the pressure in them 

 must be continually varying. Hill has shown in the human 

 subject that in an arm held down, the fist being clenched, the 



