By the changes thus produced in its com position, &c., the venous blood which returns to the heart is rendered unfit for nutrition, until it has been acted upon by the atmos pheric air in the lungs, which restores to it its bright red colour and arterial composition and properties.
The great systemic veins are therefore con nected with the right side of the heart (Hp, and the stream of VCDOUS blood brought by them to the right auricle (hi), next issues from the heart by the pulmonary artery (P), into which it is propelled by the contraction of the right ventricle CH') as it passes through that cavity. The minute branches of the pulmo • nary arteries and veins (P,p), and the capil lary vessels by which they communicate with one another, are wholly distributed on the membrane lining the air-cells of the lungs. In passing through these vessels then, the venous blood is exposed to the action of the at mospheric air contained in the pulmonary cells ; and, after having acquired arterial properties, is returned to the centre of the circulation by tbe pulmonary veins (p). The left auriele (h) receives the newly arterialized blood from the pulinonary veins, and transmits it to the left ventricle (H), from which it is ready to Start again, when the ventricle contracts, on the same course as has just been described.
In this double circulation, the path which the blood traverses in passin„0- from the left to the right side of the heart through the aortic arteries and the corresponding veins, has been called the greater or systemic circulation : and the route of the blood from the right tO the left side of the heart through the pulmonary arte ries and veins has been termed the lesser or pulmonic circulation. The names of pulmonic and systemic, indicating the parts of the body in which each of these circulations respectively occurs, are on the whole preferable to the cor responding terms of lesser and greater.
There is still one part of the course of the blood to be mentioned, viz. that of the Venous blood of the principal abdominal viscera through the liver, or what has been terined the system of the vena port.
The blood supplied by the cceliac and me senteric arteries (/, i) to the abdominal viscera is not returned directly to the heart by theit corresponding veins, as occurs in other parts of the body. The veins of the stomach and
intestinal canal, of the spleen pancreas, ine sentery, omenta and gall-bladder, unite to gether below the liver into one large vessel (L), the trunk of the vena portx, which branches out again and distributes to the liver by its ramifications the whole of the venous blood corning from the above-mentioned organs. The blood of the vena portw, being joined iri the minute branches by that of the hepatia artery (/*), passes into the smallest ramifica, tions of the hepatic veins, by the principal trunks of which (/), the venous and arterial blood circulated through the liver is carried to the inferior vena cava, and thus reaches at last the right side of the heart.
Proffs qf the circulation.—After this brief outline of the course which the blood takes through the circulatory organs in man and warm-blooded animals, it may be proper to introduce an enumeration of those circum stances which are generally adduced as af fording the most satisfactory " proofs of the circulation" or evidence that the blood pursueS the paths above detailed.
As proofs of the circulation, besides those derived from the connection of the different orders of great vessels with the cavities of the heart to which they are respectively attached, may be mentioned 1st. The structure and disposition of the auriculo-ventricular valves of the heart, and semilunar valves of the aorta and pulmonary artery, which admit of the passage of blood from the auricles to the ventricles, and from the latter cavities to the great arteries, but not in a revkse direction.
2nd. The mechanism of the valves of the systemic veins which allow of the motion of fluid only in the direction towards the heart.
3. rd. The fact that when a ligature is applied.
to an artery, or any other impediment opposed to the free passage of blood through it, the vessel becomeS dilated on the side next the heart, while the application of a. ligature to the trunk of a vein is followed by a. turgescence of the veSSel beyond the place where the obstrue tion occdrs.