510 THE CIRCULATORY APPARATUS. 



ventricles ; those of the left face, by directing their course from above to 

 below, and before to behind ; and the posterior, in rolling themselves from 

 left to right around the left ventricle. On arriving near the point of the 

 heart, they turn from left to right, and before to behind, in forming a twisted 

 spire; then they are reflected from below upwards, to enter the inferior 

 extremity of the ventricles, on the internal face of whose proper fibres they 

 spread and ascend to the fibrous zones at the base of the heart, where they 

 terminate. Some of these reflected fibres are disposed in relief to constitute 

 the columnae carneae, and reach the auriculo-ventricular zones through the 

 medium of the tendinous cords which directly connect these fibrous rings 

 with the summits of the muscular pillars. 



Such is the general disposition of the unitive fibres of the ventricles; 

 and it will be seen that they form a superficial and a deep or reflected plane, 

 between which are comprised the fasciculi proper to each ventricular pouch. 



The unitive fibres of the ventricles, therefore, form collectively a kind 

 of figure 8, the smallest loop of which is at fche point of the heart ; there 

 the fibres are heaped together, leaving in the centre of the loop a very small 

 space, through which it is possible to pass a probe into the ventricle, without 

 piercing anything but the external and internal serous membranes of the 

 organ. 



2. FIBRES OF THE AURICLES. The fibres of the auricles are either 

 common to the two cavities, or proper to each. The unitive fibres con- 

 stitute tWo thin bands a right and left, carried from one auricle to the 

 other. 



The proper fibres are divided into several fasciculi, some of which are 

 arranged in rings around the auriculo-ventricular opening ; others in inter- 

 woven loops, and others, again, in sphincters, which surround the embouchures 

 of the veins. These fibres are arranged in such a manner that, in contracting, 

 they diminish the auricles by their superior and lateral planes and ex- 

 tremities, and propel the blood towards the auriculo-ventricular openings. 



(The arrangement of the muscular fibres constitutes the most remarkable 

 feature in the anatomy of the heart. We have seen that the auricles, as well 

 as the ventricles, possess not only fibres proper to each compartment, but 

 also unitive or common fibres which assure the simultaneousness in action 

 of the similar or homologous cavities. Besides, the fibres of the auricles 

 and those of the ventricles are distinct, and not continuous ; so that, from 

 their independence of each other, it results that these two sections of the heart 

 may act separately, and contract, not simultaneously, but alternately, a con- 

 dition indispensable to the free course of the blood. The extremely fine 

 and close connective tissue uniting the muscular fibres, is another peculiarity 

 of structure that must be favourable to the solidarity of their action, 

 which ought to be simultaneous. Adipose tissue is only found in the 

 grooves on the surface of the organ, around the vessels lodged in them, and 

 particularly at its base, between the large arterial trunks.) 



C. VESSELS AND NERVES OF THE HEART. Blood is carried to the muscular 

 tissue of the heart by two large vessels, the coronary arteries. They emanate 

 from the trunk of the aorta, at the sigmoid valves, and each divides into 

 two principal branches : one passing along the horizontal, the other in the 

 vertical furrow of the heart. Collectively, these arteries form two circles, 

 which surround the heart in intersecting it at a right angle in the auriculo- 

 ventricular groove. 



The blood is carried from the walls of the heart by a single but impor- 

 tant vein, which empties itself into the right auricle. 



