56 HUMAN PHYSIOLOGY. 



stroy the termination of the pneumogastric, stimulation of its trunk has no 

 effect upon the heart. The inhibitory fibres in the vagus are constantly 

 in action, for division of the nerve on both sides is always followed by an 

 increase in the frequency of the heart's pulsations. 



The Accelerator fibres arise in the medulla, pass down the cord, emerge 

 in the cervical region, pass to the last cervical and first dorsal ganglia of the 

 sympathetic, and thence to the heart. Stimulation of these fibres causes an 

 increased frequency of the heart's pulsations, but they are diminished in 

 force. 



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. 



