lo Sept., 1908.] Elements of Animal Physiology. 525 



the arteries, blood will enter the limb, but none will leave, 

 and, in consequence, the limb will swell up and become 

 dropsical. 

 2. If a vein is cut, blood of a dark colour will well up from the 

 wound in a continuous stream ; the bleeding can be stopped 

 by the application of pressure on the side away from the 

 heart. If an artery is cut, the blood will spurt out in 

 bright-red jets that keep time with the heart beats. In this 

 case the bleeding can be stopped by application of pressure 

 on the side near the heart. 



We have now to investigate the significance of the pressure in the 

 systemic arteries and the changes which it undergoes. That such pres- 

 sure should exist is due to the fact that the heart is continually pumping 

 blood into the aorta and the elastic arteries, whilst the arterioles, by 

 their constriction, hinder the escape of the blood into the capillaries. 

 It is evident that the arterial pressure will rise — 



(a) If the force and frequency of the heart beat is increased, the 



arterioles remaining the same ; 

 (h) If the arterioles constrict still further, the heart beat remaining 



the same; 

 (c) If the force and frequency of the heart is increased, the 

 arterioles at the same time constricting. 



The muscles of the arterioles are connected with the central nervous 

 system by two sets of nerves. In the first place fibres from the sympa- 

 thetic system called vaso-constrictor can produce constriction of the 

 arterioles ; in the second place there are nerves of uncertain origin which 

 produce temporary parahsis (inhibition) of the muscles and therefore 

 dilatation of the arterioles. These nerves are termed vaso-dilator. 

 Whenever an organ is called upon to act, more blood is required, and 

 this increaseu flow can be obtained by the simple device of dilating the 

 arterioles in the organ — the blood will flow in greater volume towards 

 the region of lessened resistance. A still greater flow can be obtained 

 if the arterioles in the organ are dilated, and the arterioles in other 

 regions of the body are constricted ; whilst an even greater flow can be 

 obtained by these changes in the arcerioles combined with an increase 

 in the force and frequency of the heart beat. The proper amount of 

 blood can therefore be regulated to a nicety. If the region requiring 

 the increased blood-flow were a large one, say the muscles of the limbs, 

 then if the arterioles of the muscles were simply dilated the resistance 

 to the blood escaping into the capillaries would be so greatly reduced that 

 the arterial blood-pressure would fall so much in amount that the blood- 

 flow through other regions, and particularly the brain, would almost 

 stop. This is obviated by other arterioles, and especially those in the 

 abdominal region constricting so that the lessened resistance in the 

 muscles is balanced by a greater resistance in the abdominal vessels. 

 This explains why, for instance, digestion of a liberal meal proceeds so 

 badly during violent exercise; the blood is flowing chiefly through the 

 muscles, and a much diminished supply through the digestive organs in 

 the abdominal cavity. 



Investigation has proved to what an extraordinary extent the blood 

 pressure is influenced by changes in the arteriolar calibre of the abdominal 

 region ; in fact the major changes seemed to be conditioned by its state 

 so that the abdominal system has been aptly called the " resistance box " 

 of the circulation. If the constriction of the abdominal arterioles were 



