CIRCULATION OF THE BLOOD. 123 



The nerves which connect the heart with the central nerve system are the 

 sympathetic and the pneumogastric, or vagus. 



The sympathetic nerves are derived mainly from the ganglion stellatum. 

 The cells of this ganglion, however, are in relation with nerve fibers which 

 emerge from the spinal cord in the second and third thoracic nerves, and 

 which have their origin in cells located most probably in the medulla 

 oblongata. 



Stimulation of the sympathetic fibers beyond the ganglion stellatum, 

 is followed by an increase in the rate and sometimes by an increase in the 

 force of the heart beat. For this reason the sympathetic is said to exert an 

 accelerator and an augmentor influence on the heart beat. The center from 

 which the nerve impulses physiologically arise is known as the cardio acceler- 

 ator center. 



The pneumogastric, or vagus nerve, close to its connection with the medulla 

 oblongata, receives motor nerves from the spinal accessory. It also contains 

 efferent fibers, which come direct from the medulla. It then passes down the 

 neck and enters the thorax. Some of its fibers join the cardiac plexus and 

 by this route reach the heart. Experimental evidence indicates that the 

 terminal fibers of the vagus arborize around the nerve cells in the heart wall. 

 Feeble stimulation of the trunk of the vagus is followed by a diminution in the 

 rate of the beat; strong stimulation is followed by complete cessation or in- 

 hibition of the heart beat, the organ coming to rest in the condition of diastole. 

 Division of both vagi, in the dog, at a time when the heart is beating nor- 

 mally, is followed by a considerable increase in the frequency of the beat. 

 For these reasons the vagus nerve is said to have an inhibitor or restraining 

 influence on the rate of the heart beat. 



ARTERIES. 



The arteries are a series of branching tubes conveying blood to all por- 

 tions of the body. They are composed of three coats: 



1. External, formed of areolar and elastic tissue. 



2. Middle, contains both elastic and muscle fibers, arranged transversely 

 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 accommodate 

 themselves to the incoming amount of blood, and to convert the intermit- 



