180 



THE BLOOD VASCULAR SYSTEM 



tissue network. Small nutrient blood-vessels, both arteries and veins 

 (vasa vasorum), and minute nerve trunks with occasional ganglia, occur 

 in this coat. From these vasa et nervi vasorum capillaries and fine nerve 

 fibers, both sensory and autonomic vasomotor, are distributed to the mus- 

 cular coat. No blood- 

 vessels are found in the 

 tunica intima. In the 

 larger vessels the adven- 

 titia may contain also an 

 occasional lamellar cor- 

 puscle. The adventitia 

 contains abundant peri- 

 vascular lymphatics. 

 Nervi vasorum are said 

 to be lacking in the 

 blood-vessels of the brain 

 and spinal cord. 



General Characteris- 

 tics of the Arterial Wall. 

 The tunica media is 

 almost invariably the 

 thickest of the arterial 

 coats. In the medium- 

 sized vessels, e.g., the 

 iliac arteries, the adven- 

 titia is often of nearly 

 .Vasvasis equal thickness, but in 



the smaller vessels it is 

 much thinner. The ar- 

 terial wall as a whole, 

 also, is very thick as 

 compared with the lumen 

 of the vessel, and is much 

 thicker than that of a vein of corresponding size. 



The wall of the larger arteries is relatively thinner as compared with 

 the lumen than is the case with the arterioles; in the latter vessels 

 the thickness of the arterial wall often exceeds the diameter of their 

 lumen. In certain small arteries, e.g., those of the liver, even this ratio 

 may be exceeded. 



The arterial wall contracts firmly in rigor mortis, hence the arteries 



Elastica internet, 



tEndothelial lay&r 



Elastic J.bers 



Media 



f Bundles of smooth 

 | muscle cells 



lastica externa 



Adventitia 



FIG. 199. PART OF A CROSS-SECTION OF THE 

 FEMORAL ARTERY OF A DOG. X 150. (From 

 Szymonowicz-MacCallum, ' 'Histology and Mi- 

 croscopic Anatomy.") 



