CIRCULATION OF THE BLOOD. 137 



In the dog and the mammalian heart generally, the nerve cells 

 though present are not arranged in such definite groups, but are dis- 

 tributed in the terminations of the venae cavae, pulmonary veins, the 

 walls of the auricles and in the neighborhood of the base of the 

 ventricles. 



The nerves which connect the heart with the central nerve 

 system are the pneumogastric, or vagus, and the sympathetic. 



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

 medulla oblongata, receives motor nerves from the spinal accessory. 

 It also contains motor 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 stimu- 

 lation of the trunk of the vagus is followed by a diminution in the 

 rate of the beat ; strong stimulation is followed by complete cessa- 

 tion or inhibition 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 normally, 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. 



The sympathetic nerves are derived mainly from the ganglion stel- 

 latum. The cells of this ganglion, however, are in relation with 

 nerve fibers which emerge from the spinal cord in the second and 

 third dorsal nerves. 



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. 



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 muscle fibers, 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. 



