ARTERIES. 51 



ARTERIES. 



The Arteries are a series of branching tubes conveying blood to all 

 portions of the body. They are composed of three coats 



1. External, formed of areolar and elastic tissue. 



2. Middle, contains both elastic and muscular fibres, arranged trans- 



versely to the long axis of the artery. The elastic tissue is more 

 abundant in the larger vessels, the muscular in the smaller. 



3. Internal, composed of a thin homogeneous membrane, covered with 



a layer of elongated endothelial cells. 



The arteries possess both elasticity and contractility. 



The Property of Elasticity allows the arteries already full to accommo- 

 date themselves to the incoming amount of blood, and to convert the 

 intermittent acceleration of blood in the large vessels into a steady and 

 continuous stream in the capillaries. 



The Contractility of the smaller vessels equalizes the current of blood, 

 regulates the amount going to each part, and promotes the onward flow of 

 blood. 



Blood Pressure. Under the influence of the ventricular systole, the 

 recoil of the elastic walls of the arteries, and the resistance offered by the 

 capillaries, the blood is constantly being subjected to a certain amount of 

 pressure. If a large artery of an animal be divided, and a glass tube of 

 the same calibre be inserted into its orifice, the blood will rise to a height 

 of about nine feet ; or if it be connected with a mercurial manometer, the 

 mercury will rise to a height of six inches. This height will be a measure 

 of the pressure in the vessel. The absolute quantity of mercury sustained 

 by an artery can be arrived at by multiplying the height of the column by 

 the area of a transverse section of that artery. 



The pressure of the blood is greatest in the large arteries, but gradually 

 decreases toward the capillaries. 



The blood pressure is increased or diminished by influences acting upon 

 the heart or upon the peripheral resistance of the capillaries, viz. : 



If, while the force of the heart remains the same, the number of pulsa- 

 tions per minute increases, thus increasing the volume of blood in the 

 arteries, the pressure rises. If the rate remains the same, but the force 

 increases, the pressure again rises. Causes that increase the peripheral 

 resistance by contracting the arterioles, e. g., vasomotor nerves, cold, etc., 

 produce an increase of the pressure. 



On the other hand, influences which diminish either the volume of the 

 blood, or the number of pulsations, or the force of the heart, or the pe- 

 ripheral resistance, lower the pressure. 



