STEUCTUEE OF AETEEIES. 



167 



have been usually described as formed of three coats, named, from their 

 relative position, internal, middle, and external (fig. 107, in section) ; 

 and as this nomenclature is generally followed in 

 medical and surgical works, and also correctly ap- 

 plies to the structure of arteries so far as it is dis- 

 cernible 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 difiFering from each 

 other in texture, and therefore reckoned as so 

 many distinct coats by some authorities. 



Internal coat {fig. 107, a, h). 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 very commonly corragated 

 with very fine and close longitudinal wrinkles, 

 caused most probably by a contracted state of the 

 artery after death. Such is the appearance pre- 

 sented 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 eintlie- 

 lioid layer (fig. 107, a, and fig. 108) forming the 

 innermost part or lining. This is a simple layer 

 of thin elhptical or irregularly polygonal cells, 

 which are often lengthened into a lanceolate shape. 

 These epithelioid elements have round or oval 

 nuclei, with nucleoli: the outlines of the cells are often indistinct 

 in the fresh state, but may be brought into view by means of 

 nitrate of silver. 2. A su'be})itheUal layer (striated layers of Kolliker). 

 This is composed of a homogeneous connective tissue with a greater 

 or less number of branched corpuscles lying in the cell-spaces of 

 the tissue (fig. 109). In some instances the ground substance is 

 striated or even fibrillated, and pervaded by longitudinal elastic net- 



Fk'. 109. 



Fig. 108. — Epithe- 

 lioid Layer Liking 

 Posterior Tibial 

 Artery. 250 Dia- 

 meters. 



Nitrate of silver 

 preparation. 



Fig. 109. — Cell-Spaces of Sue-epithelial Layer op Artery (Posterior Tibial). 



250 Diameters. 

 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. 



