748 THE VASCULAR SYSTEM. 
Weight.—The average weight of the heart in the male adult is 11 ounces (810 grms.), and in 
the female adult 9 ounces (255 ‘grms.) ; but the weight varies greatly, always, however, in definite 
relation to the weight of the body, the relative pr oportions changing at different periods of life. 
Thus at birth the heart w eighs 135 drachms (24 grms.), and its relation to the body weight 
is as 1 to 130, whilst in the adult the relative proportion is as 1 to 205. The heart is said to 
rapidly increase in weight up to the seventh year, then more slowly up to the age of puberty, 
when a second acceleration sets in; but after the attainment of adult life the increase, which 
continues till the seventieth year, is very gradual. 
The above changes affect the whole heart, but the several parts also vary in their relation to 
each other at different periods of life. During fetal life the vight auricle is heavier than the left ; 
in the first month after birth the two become equal, and at the second year the right again begins 
to preponderate, and it is heavier than the left during the remainder of life. In the latter part 
of foetal life the two ventricles are equal ; after birth the left grows more rapidly than the right, 
until, at the end of the second year, a position of stability is gained, when the right is to the left 
as | to 2, and this proportion is maintained until death. 
Capacity.— During life the capacity of the ventricles is probably the same, and each is capable 
of containing about four ounces of blood, whilst the auricles are a little less capacious. After 
death the cavity of the right ventricle appears larger than that of the left. 
Vascular Supply of the Heart.—The walls of the heart are supplied by the coronary arteries 
(p. 755), the branches of which pass through the interstitial tissue to all parts of the muscular 
substance and to the sub-endocardial and stib-epicardial tissues ; the endocardium and the valves 
are devoid of vessels. The capillaries, which are numerous, form a close-meshed network around 
the muscular fibres. Sometimes the valves contain a few muscular fibres, and in these cases they 
also receive some minute vessels. The majority of the veins of the heart end in the coronary 
sinus, Which opens into the lower part of the right auricle ; some few very small veins, how- 
ever, open directly into the right auricle, and others are said to end in the left auricle, and 
in the cavities of the ventricles. 
Lymphatics of the Heart.— Lymphatic vessels are freely distributed throughout the whole 
substance of the heart, but they are most numerous in the sub-endocardial and the sub-pericardial 
tissues, and the vessels which he in the latter situation communicate through stomata with the 
pericardial cavity. The smaller lymphatic vessels accompany the blood-vessels ; ultimately 
they converge to two main trunks—an anterior and an inferior—which lhe respectively in the 
anterior and the inferior interventricular sulci. Having collected lymph from the ventricles, 
these vessels pass to the base of the heart, where they receive additional tributaries from the 
auricles ; after passing by the roots of the great arteries, they terminate in the glands which lie 
round the bifurcation of the trachea. 
Nerves of the Heart.—The heart receives its nerves from the superficial and deep cardiac 
plexuses which lhe beneath the arch of the aorta, and through them it is connected with the 
vagus, the spinal accessory (through the vagus), and the sympathetic nerves. After leaving the 
plexuses many of the nerve fibres enter the walls of the auricles, and anastomose together in the 
sub-epicardial tissue, forming a plexus in which many ganglion cells are embedded, especially 
near the terminations of the inferior vena cava and the pulmonary veins. From the sub-epi- 
cardial auricular plexus, nerve filaments, on which nerve ganglion cells have been found, pass into 
the substance of the auricular walls. 
Other fibres from the cardiac plexuses accompany the coronary arteries to the ventricles, and 
upon these also ganglion cells are found in the region immediately below the auriculo-ventricular 
suleus. 
The nerve fibres which issue from the ganglonated plexuses of the heart are non-medullated. 
They form fine plexuses round the muscle fibres, and they terminate either in fine fibrils on the 
surtaces of the muscle fibres, or in nodulated ends which le in contact with the muscle cells. 
THE PERICARDIUM. 
The pericardium is a fibro-serous sac which surrounds the heart. It lies in the 
middle mediastinum, and is attached below to the diaphragm, and above and 
behind to the roots of the great vessels. Anteriorly and posteriorly it is to a great 
extent free; laterally it is in close apposition with the pleural sacs. 
The fibrous pericardium is a strong fibrous sac of conical form; its base is 
attached to the central tendon and to a part of the muscular substance of the 
diaphragm, and it is pierced by the inferior vena cava. At its apex it 1s gradually 
lost upon the great vessels which enter and emerge from the heart, giving sheaths 
to the aorta, the two branches of the pulmonary artery, the superior vena cava, the 
four pulmonary veins, and the ligamentum arteriosum. Its anterior surface forms 
the posterior boundary of the anterior mediastinum, and it gives attachment, above 
and below, to the superior and inferior sterno-pericardial ligaments. In the greater 
part of its extent it is separated from the anterior wall of the thorax by the 
anterior margins of the lungs and pleural sacs, but it is in direct relation 
with the left half of the lower portion of the body of the sternum and, in many 
