294 STRUCTURE OF ARTERIES. 



are partly longitudinal and partly encircle the cylinder of the tube 

 in an oblique direction. Upon the surface the cellular tissue is loose, 

 to permit of the movements of the artery in distention and contraction. 



The middle coat is that upon which the thickness of the artery 

 depends ; it is yellowish in colour, and so brittle as to be cut through 

 by the thread in the ligature of a vessel. 



The internal coat is a thin serous membrane which lines the interior 

 of the artery, and gives it the smooth polish which that surface 

 presents. It is continuous with the lining membrane of the heart, and 

 through the medium of the capillaries with that of the venous system. 



In intimate structure an artery is more complicated than the above 

 description would imply. The internal coat, for example, is com- 

 posed of two layers, and' the middle of three, so that with the exter- 

 nal coat there are six layers entering into the composition of an 

 artery. The innermost coat is a tesselated epithelium analogous to 

 that of other serous membranes. The second coat from within is a 

 thin, rigid membrane, pierced with a number of round or oval-shaped 

 holes, and supporting a thin layer of flat, longitudinal fibres. From 

 these characters it has been denominated the fenestrated or striated 

 coat. The third layer, which is the innermost part of the middle 

 coat, is composed of flat, longitudinal fibres, analogous to those of 

 organic muscle. The fourth layer, the thickest of the whole, is com- 

 posed of muscular fibres of organic life, arranged in a circular direction 

 around the vessel. The fifth, or outermost part of the middle coat 

 is a thin layer of elastic tissue ; this is present only in the large 

 arteries. The sixth is the external or cellulo-fibrous coat. 



The arteries in their distribution through the body are included 

 in a loose cellular investment which separates them from the surround- 

 ing tissues, and is called a sheath. Around the principal vessels the 

 sheath is an important structure ; it is composed of cellulo-fibrous 

 tissue, intermingled with tendinous fibres, and is continuous with the 

 fasciae of the region in which the arteries are situated, as with the 

 thoracic and cervical fasciae in the neck, transversalis and iliac fasciaB, 

 and fascia lata in the thigh, &c. The sheath of the arteries contains 

 also their accompanying veins, and sometimes a nerve. 



The coats of arteries are supplied with blood like other organs of 

 the body, and the vessels which are distributed to them are named 

 vasa vasorum. They are also provided with nerves ; but the mode of 

 distribution of the nerves is at present unknown. 



In the consideration of the arteries, we shall first describe the aorta, 

 and the branches of that trunk, with their subdivisions, which toge- 

 ther constitute the efferent portion of the systemic circulation ; and 

 then the pulmonary artery as the efferent trunk of the pulmonary 

 circulation. 



