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Updated: June 25, 2025
The wall of the right ventricle may now be raised and the cavity exposed. Now cut through the middle of the loosened ventricular wall from the apex to the middle of the right auricle, laying it open for observation. Observe the openings into the auricle, there being one each for the vena cava superior, the vena cava inferior, and the coronary vein.
The left ventricle with its enormous strain is perhaps the first part to dilate, thus enlarging the opening of the defective mitral valve. The left auricle is then unable to cope with the increased amount of regurgitant blood, and there is in consequence congestion in the lungs, and the right ventricle finds the pressure ahead in the lungs greater than it can well overcome.
These two great venous trunks are the inferior vena cava, bringing the blood from the trunk and the lower limbs, and the superior vena cava, bringing the blood from the head and the upper limbs. These two large trunks meet as they enter the right auricle. The four pulmonary veins, as we have learned, carry the arterial blood from the lungs to the left auricle.
On the other hand, some animals have an auricle without any ventricle; or, at all events, they have a sac analogous to an auricle; or the vein itself, dilated at a particular part, performs pulsations, as is seen in hornets, bees, and other insects, which certain experiments of my own enable me to demonstrate, have not only a pulse, but a respiration in that part which is called the tail, whence it is that this part is elongated and contracted now more rarely, now more frequently, as the creature appears to be blown and to require a large quantity of air.
Distention of any hollow muscular organ, if the distention is not to the point of paralysis, means a greater inherent or reflex attempt of that organ to evacuate itself; the muscular tissue begins to grow, and a hypertrophy of the left auricle with the above-named lesion develops. The muscular tissue of the auricle, however, is not sufficient to allow any great hypertrophy.
From here the blood returns, as oxydised or arterial blood, through the pulmonary veins to the left auricle, and is forced by the left ventricle into the arteries of the body. Between the pulmonary arteries and veins is the capillary system of the small or pulmonary circulation. Between the body-arteries and veins is the capillary system of the large or body-circulation.
The first contact and union of the vena cava with the pulmonary veins, which occurs before the cava opens properly into the right ventricle of the heart, or gives off the coronary vein, a little above its escape from the liver, is by a lateral anastomosis; this is an ample foramen, of an oval form, communicating between the cava and the pulmonary vein, so that the blood is free to flow in the greatest abundance by that foramen from the vena cava into the pulmonary vein, and left auricle, and from thence into the left ventricle.
The branches of the pulmonary artery lie alongside of, and divide similarly to, the bronchial tubes. From these capillaries the blood is conveyed by the pulmonary veins to the left auricle.
Thus the blood brought to the lungs by the pulmonary arteries leaves the lungs entirely different in character and appearance. This part of the circulation is often called the lesser or pulmonic circulation. The four pulmonary veins bring back bright, scarlet blood, and pour it into the left auricle of the heart, whence it passes through the mitral valve into the left ventricle.
It has recently been shown that the permanently irregular pulse is due to fibrillary contraction, or really auricular fibrillation in other words, irregular stimuli proceeding from the auricle and that such an irregular pulse is not due to disturbance at the auriculoventricular node, as believed a short time ago.
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