Monary Circulation

Foetal Circulation.

The foetus has no independent respira tion or digestion and therefore depends entirely on the oxygen and nutritive substances which diffuse through the placenta from the mother's blood. The arterial blood flowing from the placenta along the umbilical vein is partly conveyed to the vena cava ascendens by means of the ductus venosus and partly flows through two trunks which unite with the portal vein returning the blood from the intestine into the liver, thence to be carried back to the vena cava by the hepatic veins. The arterial blood flowing from the placenta becomes mixed in the vena cava with the venous blood which has returned from the trunk and the lower extremities. In the right auricle it would also become freely mixed with the venous blood returning from the head and upper extremities were it not for a special arrangement which impedes but does not entirely prevent this mixture.

On account of the presence of special valves (Eustachian valves), the upward arterial stream is directed into the left side of the heart through the foramen ovale—an opening in the intra auricular septum—whilst the venous current from the superior vena cava is directed into the right ventricle. When the ven tricles contract, the arterial blood contained in the left ventricle is expelled into the ascending aorta and thence to the head and upper extremities, while the venous blood of the right ventricle is expelled into the pulmonary artery and then through the ductus arteriosus (which branches off from the pulmonary artery before it passes into the lungs) into the descending aorta. This separa tion of the two streams is not perfect, but it is responsible for the fact that the upper parts of the foetus receive blood which contains more oxygen than that received by the lower parts. This is possibly the reason why the head and the arms of the foetus are always considerably more developed than the pelvis and legs. A portion of the blood flowing through the descending aorta enters the two umbilical arteries and is conveyed to the placenta, where it is re-oxygenated.

At birth, the course of circulation undergoes changes. As soon as the lungs become distended by the first respiration, a portion of blood is diverted from the pulmonary artery into the lungs. As the pulmonary circulation increases, the ductus arteriosus becomes gradually obliterated, and finally disappears. The foramen ovale between the auricles also becomes closed. The circulation, which was carried out in the foetus upon the plan of that of the higher reptiles, becomes that of the warm-blooded animal, and the venous blood becomes separated from the arterial blood. After birth, the

umbilical arteries and vein shrink and close up and form the liga ments of the bladder, and the ligamentumteres of the liver. The ductus venosus disappears.

Physiological Properties of the Heart.

The cardiac muscle must be regarded as endowed with four fundamental properties, viz., Rhythmicity, the function of originating its impulse; Conduc tivity, the function of transmitting the impulse; Irritability, thy function of responding to stimuli ; and Contractility, the function of developing tension which may be utilized for performing the work of circulating the blood.

Rhythmicity.

The cause of the heart beat has naturally been one of the most continued subjects of inquiry. H. Allen in 1757 was the first to show that the activity of the heart is not dependent on its connections with the nervous system. The heart is controlled and influenced by the nervous system, but this control is not essential for life. The excised heart of a frog continues to beat rhythmically for days, provided that it is sup plied with oxygen and prevented from drying. In the case of the warm-blooded animal, the heart is similarly capable of continu ing its rhythmic contractions for some time after excision.

(I) The Amphibian and Reptilian Heart.—The frog's heart con sists of a sinus venosus which receives the venous blood from the body, two auricles, the ventricle and the bulbus arteriosus, which divides into the two aortae. The frog's heart in the body, or after excision from the body, beats regularly, the contractions starting in the sinus, then travelling to the auricles, ventricles and bulbus.

At one time the rhythmic contraction of the heart was attributed to the action of groups of nerve cells in the sinus that were dis covered by Robert Remak in 1848. Some experimental support for this theory, which is known as the "neurogenic" theory of the heart beat was given by Stannius, who demonstrated that the auricle and ventricle of the frog's heart cease to beat for some time if the sinus is tied off with a ligature, while the sinus goes on beating regularly. Further experiments have shown however that the beat of the heart cannot be ascribed to the rhythmic activity of these ganglion cells, for every strip of the heart muscle is capable of rhythmic action, whether it contains nerve cells or not. In the developing chick, the heart contracts by the 24-28th hour of incu bation, while the nerves do not grow into the heart before the 6th day. The inherent power of rhythmic contraction belongs to the cardiac muscle itself (the "myogenic" theory of the heart beat).