STRUCTURE OF ARTERIES. 



133 



sinuses of Valsalva or Morgagni behind them. The vessels sup- 

 plying the heart with blood are the right and left coronary arteries ;" ^^ 

 the veins which accompany them empty by a common trunk into the 

 right auricle. The nerves are derived from the cardiac plexuses 

 of the sympathetic. 



PERICARDIUM. 



The pericardium is a membranous sac, in which the heart is con- 

 tained. It consists of two layers, the external of which is fibrous, 

 dense and white, and the internal is serous. The serous lamina lines 

 the fibrous layer, and then is reflected over the heart and the roots 

 of its vessels. 



SECTION V. 



VESSELS. 



STRUCTURE OF ARTERIES. 



Arteries are composed of three coats, external, middle and internal. 

 ' The external is firm and strong, formed of condensed cellular 

 tissue ; it does not yield 



upon the application of a Fig. 115. 



ligature. 



Tlie 7niddle is formed 

 of elastic, fibrous tissue, 

 and was formerly called 

 the muscular coat. It is 

 thick, and its fibres are 

 arranged circularly ; in 

 which direction they readi- 

 ly yield to a ligature. 



77^e internal is a thin, 

 serous covering, which di- 

 minishes the friction of the 

 passage of the blood. It 

 is smooth and transpa- 

 rent, and readily torn. 



Arteries convey blood 

 from the heart, and veins 

 carry blood to the heart ; 

 thus, the pulmonary ar- 

 tery contains blue or 

 what is usually called 

 venous blood, aiid the 

 pulmonary veins contain 

 red or what is termed ar-< 

 terial blood. The nutria 



13 



