738 THE VASCULAR SYSTEM. 
regarded as the commencement of lymphatics, definite lymphatic vessels are limited 
to the outer coat. 
Nerves.—Arteries and veins are well suppled with both medullated and non- 
medullated nerve fibres. The fibres form dense plexuses on the outer surfaces of the 
vessels, from which filaments pass to the middle coat to be distributed almost 
entirely to its muscular fibres. 
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 con- 
stitute the systemic vessels or general system; the latter form the pulmonary system. 
These two systems are connected together by the heart. 
The venous trunks passing to the liver form a subsidiary part of the general 
systemic group of vessels, which is known as the portal system. 
THE HEART. 
The heart (cor) is a hollow muscular organ, and is enclosed in a fibro-serous sac 
known as the pericardium. It receives blood from the veins, and propels 1t into 
and along the arteries. The cavity of the fully-developed heart is completely 
separated into right and left halves by an ot liquely -placed longitudinal septum, and 
each half is divided into an upper receiving chamber, the. auricle, and a lower 
ejecting chamber, the ventricle. The separation of the auricle from the ventricle, 
however, is not complete. Externally a comparatively shallow constriction, 
running transversely to the long axis of the organ, indicates the distinction 
between the auricles and ventricles; internally a wide aperture is left between the 
auricle and ventricle of each side. Each auriculo-ventricular aperture is provided 
with a valve which allows the free passage of blood from the auricle to the 
ventricle, but effectually prevents its return. 
It has already been pointed out that the delicate walls of the blood capillaries allow of the free 
passage outwards of nutritive plasma from the blood. It passes into spaces, or intercellular 
channels, in which the tissue elements lie; thus the latter are directly bathed in blood plasma. The 
intercellular spaces form the commencement of the lymph-vascular system. They communicate 
together, and open into lymph-vessels which carry the used plasma ‘back to the blood-vascular 
system, but in addition they also convey new nutritive material, the product of digestive pro- 
cesses, from the alimentary canal. 
Lymph- -vessels, in other words, convey material from the t 
material both to and from the tissues. 
The removal of waste products from 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 carbonic oxide, etc., do not lie in the course of the general or sy: stemic 
circulation, and 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 pure arterial ’plood, by the 
pulmonary veins. 
The heart, anatomically a single organ, is correspondingly modified, and, as described above, 
it is divided ‘into a left part, the systemic heart, and a right part, the pulmonary heart. It 
receives on its right side the blood from the sy stemic veins, which it ejects into the pulmonary 
artery ; whilst on its left side it receives blood from the pulmonary veins, and ejects it into the 
main systemic artery - aorta. 
tissues. Blood-vessels convey 
The shape of the heart is that of an irregular and somewhat flattened cone ; and 
a base, an apex, two surfaces (inferior and ‘antero-superior), and two borders (right 
and left) are distinguishable. 
An oblique groove—the auriculo-ventricular groove (sulcus coronarius)—runs 
transversely to the long axis of the organ, and separates the upper auricular 
portion from the lower ventricular part. The separation of the auricular portion 
into right and left chambers is only marked externally at the base of the heart, 
where an indistinct interauricular groove exists. The division of the lower part 
into right and left ventricles is more definitely marked on the surface by anterior 
and inferior interventricular sulci (sulci longitudinales). 
Enclosed in the pericardium, which accordingly intervenes between it and the 
