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MANUAL OF PHYSIOLOGY. 



FlG. 125. 



these unions are simple junctions of single branches, never being 

 worthy of the name of a network or plexus, such as those seen in 

 the capillaries or in the veins. The walls of the arteries are made 

 up of three coats : 1. An external tough fibrous layer which gives 

 strength to the vessels, like the webbing in the wall of rubber 



water- hose, and acts as a bond 

 of union between it and the neigh- 

 boring tissues. This coat (tunica 

 adventitia} carries the minute 

 vessels necessary for the nutri- 

 tion of the vessel wall, and nerves. 

 2. The middle coat (tunica media) 

 forms the more characteristic part 

 of the arterial structure, being a 

 mixture of pure elastic tissue and 

 unstriated muscle. It is much 

 thicker in the arteries than in the 

 veins, where its special functions 

 are not required. It differs essen- 

 tially in character in the larger 

 and smaller arteries, the change 

 occurring gradually on passing 

 along the diminishing branches. 

 In the large arteries and the ar- 

 terioles the middle coat differs 



much in structure, and in both it forms the most important part 

 for the due performance of their respective functions. In the 

 large vessels it is made up of elastic fibres of various shapes, 

 and sheets of elastic tissue woven into a dense feltwork, inter- 

 spersed with a few muscle cells. In the small arteries (ar- 

 terioles) the great mass of the middle coat is made up of 

 muscle cells, the elastic tissue being but sparsely represented. 

 Between the large arteries and the capillaries every grade of 

 transition between these two extremes may be found ; the elastic 

 tissue gradually becoming less abundant and the muscle elements 

 more important in proportion as the capillaries are approached. 

 3. The internal lining (tunica intima) of the arteries is composed 



Portion of Small Artery fixm Subtiiu- 

 cous Tissue of Mouse's Stomach, stained 

 with gold chloride, showing the nuclei 

 of the muscle cells (M) passing trans- 

 versely around the vessel to form the 

 middle coat, outside which is the fibrous 

 tissue of the outer coat (p). Around the 

 vessel several fine nerve fibrils form a 

 network (N). 



