880 THE VASCULAK SYSTEM. 



Size of the Heart. The heart is about 125 mm. (five inches) long, 87 mm. (three and a half 

 inches) broad ; its greatest depth from its sterno-costal to its diaphragmatic surface is 62 mm. (two 

 and a half inches), and it is roughly estimated as being about the same size as the closed fist. The 

 size, however, is variable, the volume increasing at first rapidly, and then gradually, with increasing 

 age, from 22 cc. at birth to 155 cc. at the fifteenth year, and to 250 cc. by the twentieth year. 

 From that period to the fiftieth year, when the maximum volume (280 cc.) is attained, the in- 

 crease is much more gradual, and after fifty a slight decrease sets in. The volume is the same 

 in both sexes up to the period of puberty, but thereafter it preponderates in the male. 



Weight. The average weight of the heart in the male adult is 310 grms. (11 ounces), and in 

 the female adult 255 grms. (9 ounces) ; but the weight varies greatly, always, however, in definite 

 relation to the weight of the body, the relative proportions changing at different periods of life. 

 Thus at birth the heart weighs 24 grms. (13|- drachms), 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 

 increase rapidly 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 

 one another at different periods of life. During foetal life the right atrium is heavier than the left ; 

 in the first month after birth the two become equal ; 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 1 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 atria 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. 887), the branches of which pass through the interstitial tissue to all parts of the muscular 

 substance and to the subendocardial and subepicardial 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 those 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 atrium ; some few very small veins, how- 

 ever, open directly into the right atrium, and others are said to end in the left atrium, and 

 in the cavities of the ventricles. 



Lymphatics of the Heart. Lymphatic vessels are freely distributed throughout the whole 

 substance of the heart. They all communicate with the superficial network which lies beneath 

 the epicardium. The efferent vessels from the subepicardial network accompany the coronary 

 arteries to the coronary sulcus and pass thence to the anterior mediastinal glands (p. 1011). 



Nerves of the Heart. The heart receives its nerves from the superficial and deep cardiac 

 plexuses. The former lies beneath the aortic arch and the latter between the arch and the bifurca- 

 tion of the trachea. Through the plexuses it is connected with the vagus, the accessory 

 (through the vagus), and the sympathetic nerves. After leaving the cardiac plexuses many oi 

 the nerve -fibres enter the walls of the atria and anastomose together in the subepicardial 

 tissue, forming a plexus in which many ganglion cells are embedded, especially near th( 

 terminations of the inferior vena cava and the pulmonary veins. From the subepicardia 

 atrial plexus, nerve filaments, on which nerve ganglion cells have been found, pass into the 

 substance of the atrial walls. 



Other fibres from the cardiac plexuses accompany the coronary arteries to the ventricles, anc 

 upon those also ganglion cells are found in the region immediately below the coronary sulcus. 



The nerve-fibres which issue from the ganglionated plexuses of the heart are non-medullated 

 They form fine plexuses round the muscle fibres, and they terminate either in fine fibrils on th 

 surfaces of the muscle fibres, or in nodulated ends which lie in contact with the muscle cells. 



PEKICAKDIUM. 



The pericardium is a fibro-serous sac which surrounds the heart. It lies i: 

 the middle mediastinum, and is attached below to the diaphragm, and above an 

 posteriorly to the roots of the great vessels. Anteriorly and posteriorly it is i 

 relation with the structures in the corresponding mediastina and laterally it is i 

 close apposition with the pleural sacs. 



The fibrous pericardium is a strong fibrous sac of conical form ; its base 

 attached to the central tendon and to the adjacent part of the muscular sul 

 stance of the diaphragm, and it is pierced by the inferior vena cava. At its ape 

 and posteriorly it is gradually lost upon the great vessels which enter and enier^ 

 from the heart, giving sheaths to the aorta, the two branches of the pulmonai 

 artery, the superior vena cava, the four pulmonary veins, and the ligamentu 

 arteriosum. Its anterior surface forms the posterior boundary of the anteri 



