280 



MANUAL OF PHYSIOLOGY. 



FIG. 125. 



Arterial twigs of considerable size here and there form con- 

 nections with those of a neighboring trunk (anastomoses), but 

 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 pa-sin- 

 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 



Portion of Small Artery from 

 Submucous Tissue of Mouse's Stom- 

 ach, stained with gold-chloride, 

 showing the nuclei of the muscle 

 cells (M) passing transversely 

 around the vessel to form the mid- 

 dle coat, outside which is the 

 fibrous tissue of the outer coat (F). 

 Around the vessel several line 

 nerve fibrils form a network (N). 



