870 THE VASCULAE SYSTEM. 



the heart are prolonged into it at the terminations of the venae cavse. The external 

 tunic is frequently thicker than the middle tunic, and the two are not easily 

 separable from one another. 



Vascular and Nervous Supply of Arteries and Veins. Blood-vessels. The 

 walls of the blood-vessels are supplied by numerous small arteries, called vasa 

 vasorum, which are distributed to the outer and middle tunics. They arise either 

 from the vessels they supply or from adjacent arteries, and after a short course 

 enter the walls of the vessels in which they end. The blood is returned by small 

 venae vasorum. 



Lymphatics. Although the cell spaces in the middle and inner tunics may be 

 regarded as the commencement of lymphatics, definite lymphatic vessels are limited 

 to the outer tunic. 



Nerves. Arteries and veins are well supplied with nerves, which form a 

 coarse network in the outer tunic. Branches from this network enter the tunica 

 media, where they form a finer network which supplies twigs to the muscle fibres 

 and sends fine filaments into the inner coat. 



Divisions of the Blood- Vascular System. Blood-vessels convey blood to or 

 from the tissues of the body generally, or to and from the lungs. The former 

 constitute the systemic vessels or general system ; the latter form the pulmonary 

 system. The two systems are connected together by the heart. 



The venous trunks passing to the liver, and their tributaries, form a subsidiary 

 part of the general systemic group of vessels, which is known as the portal system. 



COR. 



The heart is a hollow muscular organ, which is enclosed in a fibro-serous 

 sac known as the pericardium. It receives blood from the veins, and propels it 

 into and along the arteries. The cavity of the fully developed heart is completely 

 separated into right and left halves by an obliquely placed longitudinal septum, and 

 each half is divided into a posterior receiving chamber, the atrium, and an anterior 

 ejecting chamber, the ventricle. The separation of the atria from the ventricles, 

 however, is not complete. Externally a comparatively shallow constriction, 

 running transversely to the long axis of the organ, indicates the distinction 

 between the atria and ventricles ; internally a wide aperture is left between 

 the atrium and ventricle of each side. Each atrio - ventricular aperture is pro- 

 vided with a valve which allows the free passage of blood from the atrium to the 

 ventricle, but effectually prevents its return. 



The delicate walls of the blood capillaries allow the fluid part of the blood blood plasma 

 to pass outwards to the tissues. In the tissues the plasma enters spaces, or intercellular channels, 

 in which the tissue elements lie ; thus the latter are directly bathed in blood plasma which 

 contains nutritive materials and oxygen. The intercellular spaces may be looked upon as the 

 commencement of the lymph -vascular system. They communicate together, and lymph plasma 

 passes from them into lymph vessels which carry it to the blood -vascular system. It must 

 be remembered, in addition, that materials also pass from the tissues into the blood capillaries. 



Lymph vessels, in other words, convey material from the tissues. Blood-vessels convey 

 material both to and from the tissues. 



The removal of waste products which have passed from the tissues to the blood is provided 

 for by special organs, some of which are simply interposed in the course of the general circulation 

 e.g. the liver, the kidneys, and the skin. The lungs, however, where the impure or venous 

 blood receives its main supply of oxygen and gives up most of its carbon dioxide, etc., do not 

 lie in the course of the general or systemic circulation ; for them a secondary or pulmonary 

 circulation is established, by which venous blood is conveyed from the heart to the lungs by the 

 pulmonary artery and its branches, and, after passing through the pulmonary capillaries, is 

 returned again to the heart, as oxygenated or arterial blood, by the pulmonary veins. 



The heart, anatomically a single organ, is correspondingly modified, and, as described above, 

 it is divided by a septum into a right and a left part. The right side receives the blood from 

 the systemic veins, and ejects it into the pulmonary artery ; whilst the left side receives blood 

 from the pulmonary veins, and ejects it into the main systemic artery the aorta. 



The shape of the heart is that of an irregular and somewhat flattened cone ; and 

 a base, an apex, two surfaces (inferior or diaphragmatic and antero-superior 01 

 sterno-costal), and three borders (right, left, and inferior) are distinguishable. 



