116 



THE VASCULAR SYSTEM 



all these groups varies, there is a gradation of the type of construction 

 all along the line, as can be understood best b}' studying the smallest 

 arteries first. 



Small Arteries.— The smallest branches adjacent to capillaries 

 have a small amount of connective tissue supporting isolated smooth 

 muscle cells outside the endothelium. These are pre-capillary arter- 

 ioles. They are branches of larger arterioles which have a definite 

 smooth muscle sheath surrounded by a sheath of fil^roelastic tissue. 

 The small arteries have thick walls in comparison with the size of 

 the lumen. The intima has a thin subendothelial laver of fibro- 



1 Capillary 



S [ Arterioles 

 4 



Fig. 66. — Diagram of small arterioles and a capillary. 



elastic connecti^'e tissue separated from the media by an internal 

 elastic lamina, a membrane of elastic fibers. The media is 

 composed mainly of smooth muscle cells interspersed with fibro- 

 elastic connective tissue and is separated from the adventitia 

 by an external elastic lamina which resembles the internal 

 membrane. (Fig. 07.) The adventitia is composed of fibroelastic 

 connective tissue with a very few smooth muscle cells. It is usually 

 not as thick as the media but much thicker than the intima. With 

 the addition of the muscle tissue the vessels take an acti^'e i)art in 

 distributing blood by rhythmic contractions or peristaltic action. 

 These small arteries grade into medium-sized arteries. 



Medium-sized or Muscular Arteries. These are larger than the 

 preceding vessels and the predominant tissue of the media is smooth 



