498 



ANGIOLOGY 



I 



whence it is conveyed to the right atrium. From this it will be seen that the 

 blood contained in the portal vein passes through two sets of vessels: (1) the 

 capillaries in the spleen, pancreas, stomach, etc., and (2) the sinusoids in the liver. 

 The blood in the portal vein carries certain of the products of digestion : the carbo- 

 hydrates, which are mostly taken up by the liver cells and stored as glycogen, and 

 the protein products which remain in solution and are carried into the general 

 circulation to the various tissues and organs of the body. 



Speaking generally, the arteries may be said to contain pure and the veins 

 impure blood. This is true of the systemic, but not of the pulmonary vessels, 

 since it has been seen that the impure blood is conveyed from the heart to the lungs 

 by the pulmonary arteries, and the pure blood returned from the lungs to the heart 

 by the pulmonary veins. Arteries, therefore, must be defined as vessels which 

 convey blood from the heart, and veins as vessels which return blood to the heart. 



Structure of Arteries (Fig. 448). The arteries are composed of three coats: an internal or 

 endothelial coat (tunica intima of Kolliker); a middle or muscular coat (tunica media); and an 



external or connective-tissue coat (tunica adventitia). 

 The two inner coats together are very easily separated 

 from the external, as by the ordinary operation of 

 tying a ligature around an artery. If a fine string be 

 tied forcibly upon an artery and then taken off, the 

 external coat will be found undivided, but the two 

 inner coats are divided in the track of the ligature 

 and can easily be further dissected from the outer 

 coat. 



The inner coat (tunica intima) can be separated 

 from the middle by a little maceration, or it may be 

 stripped off in small pieces; but, on account of its 

 friability, it cannot be separated as a complete mem- 

 brane. It is a fine, transparent, colorless structure 

 which is highly elastic, and, after death, is commonly 

 corrugated into longitudinal wrinkles. The inner coat 

 consists of: (1) A layer of pavement endothelium, 

 the cells of which are polygonal, oval, or fusiform, 

 and have very distinct round or oval nuclei. This 

 endothelium is brought into view most distinctly by 

 staining with nitrate of silver. (2) A subendothelial 

 layer, consisting of delicate connective tissue with 

 branched cells lying in the interspaces of the tissue; 

 in arteries of less than 2 mm. in diameter the sub- 

 endothelial layer consists of a single stratum of stel- 

 late cells, and the connective tissue is only largely 

 developed in vessels of a considerable size. (3) An 

 elastic or fenestrated layer, which consists of a mem- 

 brane containing a net-work of elastic fibers, having 

 principally a longitudinal direction, and in which, 

 under the microscope, small elongated apertures or 

 perforations may be seen, giving it a fenestrated ap- 

 pearance. It was therefore called by Henle the fenes- 

 trated membrane. This membrane forms the chief 

 thickness of the inner coat, and can be separated into 

 several layers, some of which present the appearance 

 of a net-work of longitudinal elastic fibers, and others 

 a more membranous character, marked by pale lines 

 having a longitudinal direction. In minute arteries 

 the fenestrated membrane is a very thin layer; but in the larger arteries, and especially in the 

 aorta, it has a very considerable thickness. 



The middle coat (tunica media) is distinguished from the inner by its color and by the trans- 

 verse arrangement of its fibers. In the smaller arteries it consists principally of plain muscle 

 fibers in fine bundles, arranged in lamellae and disposed circularly around the vessel. These 

 lamellae vary in number according to the size of the vessel; the smallest arteries having only a 

 single layer (Fig. 449), and those slightly larger three or four layers. It is to this coat that the 

 thickness of the wall of the artery is mainly due (Fig. 448A, m). In the larger arteries, as the 



Fio. 448. Transverse section through a small 

 artery and vein of the mucous membrane of the 

 epiglottis of a child. X 350. (Klein and Noble 

 Smith.) A. Artery, showing the nucleated endo- 

 thelium, e, which lines it; the vessel being con- 

 tracted, the endothelial cells appear very thick. 

 Underneath the endothelium is the wavy elastic 

 lamina. The chief part of the wall of the vessel 

 is occupied by the circular muscle coat m; the 

 rod-shaped nuclei of the muscle cella are well seen. 

 Outside this is a, part of the adventitia. This is 

 composed of bundles of connective tissue fibers, 

 shown in section, with the nuclei of the connec- 

 tive tissue corpuscles. The adventitia gradually 

 merges into the surrounding connective tissue. 

 V. Vein showing a thin endothelial membrane, 

 e, raised accidentally from the intima, which on 

 account of its delicacy is seen as a mere line on the 

 media m. This latter is composed of a few circular 

 unstriped muscle cells a. The adventitia, similar 

 in structure to that of an artery. 



