THE PERICARDIUM 525 



in bony grooves are termed (venous) sinuses ; examples of this are the sinuses of 

 the dura mater of the brain. 



Structure of Arteries. — The wall consists of three coats. The external coat 

 (Tunica externa) consists chiefly of fibrous connective tissue. In the deeper part 

 are some elastic fibers, and in some arteries there are also longitudinal unstriped 

 muscle-fibers. The middle coat (Tunica media) is composed of unstriped muscle 

 and elastic tissue in medium-sized arteries. In small vessels the^re is only the 

 muscular tissue, and in the largest trunks only elastic tissue. The internal coat 

 or intima (Tunica intima) consists of a layer of endothehal cells, resting on an elastic 

 membrane. The sheath (Vagina vasis) is a condensation of the surrounding con- 

 nective tis>u(\ and is attached more or less closely to the external coat. 



Structure of Veins. — The walls of veins are similar in structure to those of 

 the arteries, Init are very much thinner, so that veins collapse more or less com- 

 pletely when empty, while arteries do not. The middle coat is very thin and con- 

 sists to a large extent of ordinar}^ connective tissue. The intima is also less elastic 

 thiin in the arteries. In many veins this coat forms semilunar valves, the free 

 edges of which are directed toward the heart. They are most numerous in the 

 veins of the skin and the deep veins of the extremities (except the foot), while in 

 most veins of the body cavities and viscera they are absent or occur only where the 

 veins open into larger ones or where two veins join. 



The walls of the vessels are supplied with blood by numerous small arteries, 

 called vasa vasorum. These arise from branches of the artery which they supply 

 or from adjacent arteries, ramify in the external coat, and enter the middle coat 

 also. 



The nerves of the vessels consist of both medullated and non-medullated fibers. 

 They form plexuses around the vessels, from which fibers pass mainly to the muscu- 

 lar tissue of the middle coat. 



BLOOD-VASCULAR SYSTEM OF THE HORSE 

 THE PERICARDIUM' 



The pericardium is the fibro-serous sac which surrounds the heart, and also 

 the great vessels in connection with it to a greater or less extent. Its form is 

 in general similar to that of the heart. The fibrous layer is relatively thin, but 

 strong and inelastic. It is attached dorsally to the large vessels at the base of the 

 heart, and is continued in part up to the longus colli muscle. Ventrally it is firmly 

 attached to the middle of the posterior half of the thoracic surface of the sternum. 

 The serous layer is a closed sac, surrounded by the fibrous pericardium, and invag- 

 inated by the heart. It is smooth and glistening, and contains a small amount of 

 clear serous fluid, the liquor pericardii. Like other serous membranes, it may be 

 regarded as consisting of two parts, jiarietal and visceral. The parietal part lines 

 the fibrous layer, to which it is closely attached. The visceral layer covers the 

 heart and parts of the great vessels, and is therefore also termed the epicardium. 

 The serous pericardium is composed of a connective-tissue membrane, rich in 

 elastic fibers, and covered on its free surface b}' a layer of flat endothelial cells. 



The pericardium is covered by the mediastinal pleura (Pleura pericardica) 

 and is crossed laterally by the phrenic nerves. Its lateral surfaces are related 

 chiefly to the lungs, but the lower part is in partial contact with the chest-wall. 

 On the left side the area of contact is from the third to the fifth intercostal space 

 inclusive. On the right side the contact is smaller and is at the third and fourth 



' The pericardium is described before the heart since it must be examined first in the labor- 

 atory. 



