526 Journal of Agriculture. [10 Sept., 1908. 



altogether abolished the blood pressure would fall to such a degree that 

 death would result, even though the arterioles in the muscles and skin 

 were constricted to their utmost. This explains the pallid skin pinched 

 leatures and general collapse when inflammatory trouble occurs in the 

 bowel. 



The actual pressure in the systemic arteries varies with each animal 

 and to a slight extent with each artery. If a vertical tube were connected 

 with one of the larger arteries of a horse the blood would rise in the 

 tube to the height of about 9 feet and, until clotting occurred, would 

 show oscillations at the same rate as the heart beat. It is the pressure 

 in the arteries which produces the quick jetting of blood which occurs 

 when an artery is cut and which is apt to give a false idea of the rate 

 of flow in tne intact artery. 



General constriction of the systemic arterioles can be brought about 

 by the following means : — ■ 



T. Salts of barium which act directly on the muscles of the arterioles. 



2. The drug, or hormone, adrenalin, which acts on the receptive sub- 



stances of the arteriolar muscles. 



3. The drug nicotine (in its first stage of action) which stimulates the 



nerve cells in the sympathetic ganglia. 



4. Venous blood, as occurs in asphyxia, which acts on the nerve cells 



in the medulla oblongata in the central nervous system, where 

 vaso-constrictor impulses arise. 



When general vaso-constriction occurs the resistance to the blood 

 escaping into the capillaries is greatly increased and, if the heart were to 

 beat at the ordinary rate, the arterial pressure would mount up to a 

 dangerous extent. In such circumstances however the vagus centre in 

 the medulla is stimulated and the heart is greatly slowed so that a 

 dangerously high pressure is avoided. Conversely, when the arterioles, 

 are dilated, either generally or only in the abdominal region, as occurs 

 when certain poisons enter the circulation, or if the blood pressure is 

 lowered bv extreme haemorrhage, the heart through the sympathetic system 

 is stimulated to greater activity whilst at the same time the vagus ceases 

 to act. 



The arterial pressure in the lesser or pulmonary circuit is low and, as 

 it varies but slightly, and as these variations are dependent on what is 

 happening in the systemic circulation, they possess but little importance. 



Pressure in the veins is always low, in fact in the great veins near 

 the heart a negative pressure may exist in which case if the vein wall be 

 cut, air may actually be sucked into the vessel. 



When the left ventricle forces its contents into the aorta a pressure 

 wave is started which rapidly spreads through the branching arteries 

 and dies down before it reaches the capillaries. Much the same sort of 

 wave is seen in a long rubber tube in which water is flowing under a 

 little pressure (as in a garden hose). When a blow is struck on the tube 

 near the tap, a wave can be seen running along and the jet from the 

 nozzle can be seen to jump a very short time after the blow is given. 

 This pressure wave in the arteries is called the pulse and can be felt well 

 whenever an artery is near the skin and has a bony background. The 

 pulse, as felt by the finger, gives one the rate of the heart and the presence 

 or absence of regularity in the heart beat. The initiated can also draw 

 conclusions from the feel of the pulse whether "full" or "thready," 

 &c., though it must be remembered that the veins which accompany the 

 aiterv have a share in giving rise to what is termed the "volume" of the 



