4th. That on opening one of the larger arteries, blood issues in a jet from the end next to the heart at the time of every contraction of that organ, and that in general no blood flows from the orifice of the remote part of the artery: arid that on opening a vein the converse is ob served, the blood issuing freely in a continued stream from the remote part, but none proceed ing from the part of the vein adjoining the heart.
5th. That the passage of the blood from the arteries to the veins in the small or Capillary vessels has been observed by means of the microscope in transparent parts of animals, and, though it has not been seen in man, we are entitled from the general analogy in the struc ture of the organs of circulation to infer that the same passage occurs in the human body.
6th. That, by mechanical arrangements, fluids may easily be made to pass in the dead body through the whole course of the double circulation, but not in a direction different from that which the blood has been stated to pursue.
7th. That by the operation of transfusion, the blood of one animal may be made to circu late through the heart and vessels of another, by connecting together the bloodvessels (whe ther arteries or veins) of the two animals, in such a manner that the course in which the blood is directed by the action of the heart of the animal from which the blood is derived is that of the natural circulation in the animal into which it is introduced.
8th. The phenomena presented by the circu lation of the blood in various diseased condi tions of the heart and bloodvessels may be ad duced as affording additional illustration of the natural course of the blood, by pointing out the effect of morbid obstructions and other varieties in different parts of the circulatory organs.
Course qf the blood in thefalus belore birth. —The double circulation just described is the course performed by the blood from the time of birth during the whole of life.
The circulation of the blood, however, begins at a very early period of fcetal life; but the difference in the mode in which respiration is effected in tbe child so long as it is contained in the uterus, induces a modification in the course of the blood to which we shall now advert.
There being no inhalation of air into the lungs of the fcetus, the blood is sent only in small quantity to these organs, and does not undergo in them any change of properties. A considerable portion of the blood of' the fcetus passes out of its body through the umbilical cord (fig. 313, U, u) into_the placenta of the uterus. The minutely divided fcetal ves'sels are bathed by the blood of the mother contained in the placental sinuses, and, though no direct continuity of tube exists between the maternal and fcetal vessels, the blood of the child seems to undergo a respiratory alteration, or a certain degree of arterialization, in being brought into near proximity with the maternal blood.
The blood of the fcetus, after passing., through the minute ramifications of the umbilical arte ries (1/1, U) in the placenta, returns by the umbilical vein (u) into its body.
The umbilical vein carries part of its blood directly by the ductus venosus (d) to the vena cava inferior, and part is distributed by the bmnches of the vena portm ( L), with which the umbilical vein unites, through the sub stance of the liver, and is then conveyed by means of the hepatic veins (/) into the general current of the returning blood.
The right auricle of the heart (hi), therefore, receives not only the blood which has circu lated through the body of the fcetus, but also that which has passed through the placenta, consequently a mixture of venous and arterial blood ;—the blood in the superior vena cava (0) being entirely venous, that in the inferior vena cava ( V) being mixed. The blood which is brought to the right auricle is in much greater quantity in the fcetus before birth than in the child which has breathed air; a part of this blood passes from the right into the left auricle (h) by the foramen ovale (f) in the sep ttitti auricularum, and it would appear that it is chiefly the blood from the inferior vena cava which takes that course.
The rest of the blood entering the right auricle takes the same route as in the adult, viz. into the right ventricle (.111), and thence into the pulmonary artery, but, as very little blood is sent to the collapsed longs, a passage of communication is established in the foetus from the pulmonary artery into the descending aorta through the ductus arteriosus (D) and thus the greater mass of the blood, whi'ch in the adult would have proceeded to the lungs, is in the fcetus immediately transmitted to the aorta (A).
From the disposition of the Eustachian valve, it is believed that nearly the whole of the blood of the inferior vena cava passes from the right to the left auricle through the foramen ovale, while the blood brought from the head and superior extremities (parts which are compara tively large in the fcetal condition) passes through the right side of the heart. The as cending aorta, rising from the left ventricle, delivers almost all the blood expelled by the contraction of that cavity into the carotid and subclavian arteries, while the ductus arteriosus passing between the trunk of the pulmonary artery and the descending aorta directs the blood which passes through the right ventricle to the lower regions of the body. In this manner the upper regions of the body are sup plied with the most arterialized part of the blood from the left side of the heart and aorta, while the purely venous blood is propelled from the right ventricle through the pulmonary artery and ductus arteriosus into the descend-. ing aorta, and consequently into the lower part of the body, and by the umbilical vessels to the placenta.
The foramen ovale in the septum of the au ricles, the ductus arteriosus passing from the pulmonary artery to the aorta, the cluctus ve nosus leading from the umbilical vein to the vena cava inferior, and the umbilical vein and arteries are the structural peculiarities of the fcetal circulating organs. These passages are all closed up, and the umbilical vessels obliterated at the navel after aerial or pulmonic respiration is established at birth.*