— er ae 
STRUCTURE OF THE HEART. 747 
section, and after special methods of preparation, it is possible to recognise many layers and 
bundles, some of which are, however, probably artificially produced. iG Oa 
In the auricles the muscular fasciculi fall naturally into two groups, those special to each 
auricle, and those common to both auricles; the former are situated deeply under cover of 
the latter. 
The deep special fibres are—(a) Looped fibres which pass over the auricles from before back- 
wards or from side to side ; their extremities are attached to the fibrous rings which surround the 
auriculo - ventricular orifices. (6) Annular fibres which surround (1) the extremities of the 
large vessels which open into the auricle, (2) the auricular appendices, and (3) the fossa ovalis. 
The superficial fibres, which are common to both auricles, for the most part run transversely 
across the auricles, but a few of them turn into the interauricular septum. They are most 
numerous on the anterior aspect. 
In the ventricles, also, two main groups of fasciculi, a superficial and a deep, have been 
described, but it is inthis region especially that there is doubt regarding the individuality of 
many of the muscular bundles which have been noted, for it appears probable that many of 
them are artificial products due to the method adopted by the dissector. There is no doubt that 
in the middle of the thickness of the ventricular walls the arrangement of the fibres is mainly 
circular, some surrounding one and some both ventricles. Near the surfaces the fasciculi 
assume an oblique direction, and it is not improbable that many of the bundles are arranged in 
figure of 8 loops, whose upper extremities are attached to the fibrous rings round the auriculo- 
ventricular orifices. 
The superficial fibres of the ventricles are attached above to the fibrous rings at the base, and 
from this attachment they pass obliquely downwards to the apex, those on the anterior surface 
trending towards the left, and those on the inferior surface towards the right. On the inferior 
surface almost all the fasciculi appear to pass across the septum, but on the anterior surface the 
middle fasciculi dip into it, and only those near the base and apex cross from right to left. All 
the superficial fibres which reach the apex are coiled there into a whorl or vortex, through which 
they pass upwards into the substance of the left ventricle, those descending from the front and 
left side entering the base of the posterior papillary muscle, whilst those from the back and right 
side terminate in the anterior papillary muscle. The muscular fasciculi which enter the papillary 
muscles are continued, by means of the chord tendine, to the flaps of the mitral valve and 
so to the fibrous ring round the mitral orifice; obviously, therefore, many of the superficial 
fasciculi of the ventricles form simple oblique loops which commence externally at the fibrous 
rings round the right and left auriculo-ventricular orifices, and terminate internally by gaining 
attachment to the ring round the left of these orifices (mitral). 
The deep fascicult of the ventricles may be subdivided into two main groups—(1) Those 
common to both ventricles, and (2) those special to each ventricle. 
The fasciculi common to both ventricles include—(a) Fibres which commence above from the 
posterior sections of the fibrous ring at the base of the right ventricle; either directly or by 
means of the chordae tendineew of the posterior papillary muscle they pass obliquely downwards 
to the septum, traverse it, and ascend to the front of the fibrous ring at the base of the left 
ventricle. (b) Fibres from the anterior portions of the fibrous ring at the base of the right 
ventricle, which pass obliquely downwards and assume a transverse course in the posterior wall 
of the left ventricle. (¢) Annular fibres which encircle both ventricles. 
The deep special fibres of the left ventricle are (4) V-shaped loops which commence at the 
fibrous ring at the base, and descend to the apex, where they turn upwards in the septum, and 
terminate by joining the central fibro-cartilage ; (b) fibres which descend from the base, enter 
the lower and front part of the septum, and, passing through it, assume an annular course in the 
posterior wall. 
The deep special fibres of the right ventricle are (7) looped fibres which pass downwards in 
the external wall from the fibrous rings to the apex, where they enter the septum and ascend to 
the central fibro-cartilage ; (b) circular fibres round the pulmonary orifice ; and (c) radiating fasci- 
culi from the base of the anterior papillary muscle to the front part of the pulmonary orifice. 
The epicardium, or visceral portion of the pericardium, consists of white connective and 
of elastic tissue, the latter forming a distinct reticulum in the deeper part. The surface which 
looks towards the pericardial cavity is covered with flat polygonal endothelial plates, which are 
partially separated here and there by stomata through which the pericardial cavity communicates 
with the lymphatics of the epicardium. 
The endocardium lines the cardiac cavities and is continuous with the inner coats of the 
vessels which enter and leave the heart. It consists, like the epicardium, of white connective 
tissue and elastic fibres, but it is much thinner than the epicardium, and its elastic fibres are im 
some places blended into a fenestrated membrane. Its inner surface is covered with endothelial 
cells, and it rests externally upon the sub-endocardial tissue, in which there are blood-vessels and 
nerves; the endocardium itself is entirely devoid of vessels. 
Size of the Heart.—The heart is about five inches (125 mm.) long, three and a-half inches (87 
mm.) broad; its greatest depth from its antero-superior to its inferior surface is two and a-half inches 
(62 mm.), 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 ce. at birth to 155 ce. at the fifteenth year, and to 250 cc. by the twentieth year. 
From this period to the fiftieth year, when the maximum volume (280 cc.) is attained, the in- 
erease 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. 
