62 CIRCULATION OF THE BLOOD IN THE VESSELS. 



The largest arteries are very strong and elastic. Their external coat is 

 composed of ordinary fibrous tissue, with a few longitudinal and oblique 

 fasciculi of non-striated muscular fibres. This coat is no thicker in the 

 largest vessels than in some of the vessels of medium size ; and in some 

 medium-sized vessels it is actually thicker than in the aorta. This is the 

 only coat that is vascular. 



The middle coat, on which the thickness of the walls of the vessel de- 

 pends, is composed chiefly of yellow elastic tissue. This tissue is disposed in 

 a number of layers. Externally there is a thin layer of ramifying elastic 

 fibres, and then a number of layers of elastic membrane, with oval, longitudi- 

 nal openings, an arrangement which has given it the name of the " fenes- 

 trated membrane." Between the different layers of this membrane are found 

 a few non-striated muscular fibres. These muscular fibres, however, are not 

 abundant and have but little physiological importance. A small portion of 

 the aorta and pulmonary artery near the heart is entirely free from mus- 

 cular fibres. In the largest arteries the fibres are arranged in fasciculi, with 

 amorphous and fibrous connective tissue running in circular, longitudinal 

 and oblique directions. The longitudinal and oblique fibres exist chiefly in 

 the outer coat. 



The internal coat of the largest arteries does not differ materially from 

 the lining membrane of the rest of the arterial system. It is nearly identical 

 in structure with the endocardium and is continued throughout the vascular 

 system. It is a thin, homogeneous, elastic membrane, covered with a layer 

 of elongated cells of endothelium, with oval nuclei, the long diameter of the 

 cells and nuclei following the direction of the vessel. Between the endo- 

 thelial cells, is an amorphous cement-substance, which is rendered dark by a 

 solution of silver nitrate, so that this reagent clearly defines their borders. 



The arteries of medium size possess considerable strength, some elasticity 

 and very great contractility. In the outer and inner coats there is no great 

 difference between these and the largest arteries, even in thickness. The 

 essential difference in the anatomy of these vessels is found in the middle 

 coat. Here there is a continuation of the elastic elements found in the 

 largest vessels, but relatively diminished in thickness and mingled with the 

 fusiform, non-striated muscular fibres arranged nearly always at right angles 

 to the course of the vessel. These fibres are found chiefly in the inner layers 

 of the middle coat and only in arteries smaller than the carotids and primi- 

 tive iliacs. In arteries of medium size, like the femoral, profunda femoris, 

 radial or ulnar, the muscular fibres exist in several layers. There is no dis- 

 tinct division, as regards the middle coat, between the largest arteries and 

 those of medium size. As the arteries branch, muscular fibres make their 

 appearance between the elastic layers, progressively increasing in quantity, 

 while the elastic elements are diminished in their relative proportion. 



In the smallest arteries, the external coat is thin and disappears just be- 

 fore the vessels empty into the capillary system ; so that the very smallest 

 arterioles have only the inner coat and a layer of muscular fibres. Although 

 most of the muscular fibres in the middle coat of the arteries are arranged 



