168 ANATOMY FOR NURSES [Chap. X 



Function. — These valves offer no resistance to the passage of 

 blood from the heart into the arteries, as the free borders project 

 into the arteries, but they form a complete barrier to the passage 

 of blood in the opposite direction. In this case each pocket 

 becomes filled with blood, and the free borders are floated out and 

 distended so that they meet in the centre of the vessel. The 

 corpora Arantii assist in the closure of these valves and help to 

 make the barrier perfect. 



The orifices of the heart which serve for openings into veins 

 are not protected by valves, with the possible exception of the 

 opening into the inferior vena cava which is partly covered by a 

 membrane known as the Eustachian valve. 



Blood Supply. — Just after the aorta leaves the left ventricle it 

 gives off two small branches, called the right and left coronary 

 arteries. They encircle the heart like a crown, hence their name. 

 They supply the substance of the heart with blood, as the blood 

 contained within the cavities of the heart only nourishes the 

 pericardium. 



Nerve supply. — The heart is supplied (1) by the pneumo-gastric 

 nerves from the central nervous system and (2) from the sympa- 

 thetic system. Stimulation of the pneumo-gastric fibres slows the 

 action of the heart. They are therefore known as cardiac inhibi- 

 tors. Stimulation of the sympathetic fibres increases the force of 

 the heart beat, therefore they are known as cardiac accelerators. 



ARTERIES 



These are hollow vessels that lead from the heart and are com- 

 posed of three coats : — 



1. A smooth endothelial lining. 



2. A middle coat of fibrous elastic tissue with muscle fibres 

 interlaced and circularly disposed around vessel. 



3. An outer, dense, fibrous coat with fibres arranged longitudi- 

 nally. 



1. The inner lining or endothelium is continuous with the 

 endocardium which lines the heart. It furnishes a smooth, slippery 

 surface over which the blood can fiow without any friction. 



2. By virtue of the structure of the middle coat, the arteries 

 are both contractile and elastic. It is thicker and contains a larger 

 proportion of elastic tissue in the large arteries. In the smaller 



