362 



BLOOD-VESSELS. 



by the naked eye, it seems best to adhere to it as the basis of our description ; 

 although it will be seen, as we proceed, that some of these coats are found on 

 microscopic examination really to consist of two or more strata differing from 

 each other in texture, and therefore reckoned as so many distinct coats by some 

 authorities. 



Internal coat (Tunica intima) (fig. 417, #, 0). This may be raised from the 

 inner surface of the arteries as a fine transparent colourless membrane, elastic but 

 very easily broken, especially in the circular or transverse direction, so that it cannot 

 be stripped off in large pieces. It is A^ery commonly corrugated with fine and close 

 longitudinal wrinkles, caused most probably by a contracted state of the artery after 

 death. Such is the appearance presented by the internal coat to the naked eye, but 

 by the aid of the microscope, it is found to consist of three different structures, 

 namely : 



1. An epithelial layer (endothelium of the artery) (fig. 417, a, and fig. 418) form- 

 ing the innermost part or lining. This is a simple layer of thin elliptical or 

 irregularly polygonal cells, which are often lengthened into a lanceolate shape. The 

 cells have round or oval nuclei, with nucleoli : their outlines are often indistinct in 



Fig 418. EPITHELIAL LAYER LINING THE POSTERIOR TIBIAL ARTERY OF MAN. 250 DIAMETERS. 



(E. A. S.) 

 Nitrate of silver preparation. 



Fig 419 CELL-SPACES OF SUB-EPITHELIAL LAYER OF ARTERY (POSTERIOR TIBIAL). 250 DIAMETERS. 



(E. A. S.) 



The ground substance is stained by nitrate of silver, and the cell-spaces of the tissue are thus made 

 manifest as white patches, the contained cells not being seen. 



the fresh state, but may be brought into view by means of nitrate of silver. When 

 the vessels are empty and collapsed, the endothelium cells are less flattened, and the 

 part of each cell which contains the nucleus may project somewhat into the lumen of 

 the vessel. 



2. A sulepithelial layer (striated layer of Kolliker). This is composed of a 

 finely fibrillated connective tissue with a number of branched corpuscles lying in the 

 cell-spaces of the tissue (fig. 419). This layer is most developed in the larger 

 arteries : it exists however as a thin stratum in the medium-sized ones. In the 

 aorta it is very well marked and contains a large number of anastomosing cells and 

 cell-spaces lying in a finely fibrillated ground-substance. Longitudinal networks of 



