and is forced by the contraction of their muscular walls through the auriculo-veutricular openings, the valves of which open outwards, into the ventricles. When the ven tricles are distended their muscular walls contract and force the blood into the arteries the right ventricle into the pulmonary artery, the left into the aorta the valves at the mouth of each artery opening outwards to allow of the free passage of the fluid. To prevent, during the ventricular contraction, the regurgitation of blood into the auricles, the auriculo-ventricular valves are floated away from the sides of the ventricle across their respective openings, and by the apposition and slight overlapping of their edges temporarily close the openings. The tilting upwards of the valves into the auricles is prevented by the contraction of the musculi papillares, and their connection with the cusps of the valve through the chordae tendinese. Pettigrew has shown that casts of the ventricular cavities, more especially of the left, have the form of a double cone, spirally twisted from right to left, and has described the blood as forced in spiral streams against the under surface of the segments of the . valve, which are twisted and wedged into each other so as to prevent regurgitation. The propulsion of the blood into the arteries distends the elastic walls of those tubes ; but when the ventricular contraction has ceased, the elastic wall recoils, and the blood is propelled onwards in the circula tion. The regurgitation of the blood into the ventricles is prevented by the closure of the semilunar valves, the seg ments of which are thrown across the arterial orifices through the pressure exercised on the column of blood in the lumen of the artery and in the sinuses of Valsalva. s The Arteries. These vessels were named arteries by the older anatomists, on the supposition, now known to be erroneous, that they contained air. The term is now employed to express a blood-vessel, which, arising either directly or indirectly from the heart, conveys blood away from that organ. Arteries divide and subdivide into smaller vessels in their course, and to the individual branches descriptive names are applied. Some of these names express the position of an artery, as subclavian, axillary ; Bothers, the organ in which it is distributed, as pulmonary,
- hepatic ; others a peculiarity in its course, as circumflex,
[coronary. The branches of arteries may be either collateral or terminal. The collateral branches arise from the sides of the parent artery either at an acute, a right, or an obtuse angle. Terminal branches arise at an acute angle by the bifurcation of the parent artery, which is the most common form, or by the breaking up of the artery into a cluster fof branches. Branches which arise either from the same | artery or from different arteries may be distributed in a | common locality, may there unite together, and form what is called an inosculation or anastomosis, so that the blood from one artery may thus flow from it into another. The most common anastomosis is by the formation of loops between adjacent branches, but sometimes, as when the **vo .vertebral arteries join to form the basilar, a convergence of two almost straight arteries takes place ; and in other cases, as where the two anterior cerebral arteries are joined together by the anterior communicating, a connecting branch passes transversely across the mesial plane. A more complex form of anastomosis is when an artery (and a similar arrangement is sometimes found in veins) rapidly subdivides into numerous branches, which may again join to form a trunk either with or without the formation of a plexus. This is called a rete mirabile, an arrangement not uncommon in the cetacea, in the internal carotid arteries of ruminants, in the mesenteric arteries of the pig, in the arteries of the limbs of the sloths and lemurs, and in the arterial system of fishes. The only examples of a rete in the human body are the convoluted Malpighian tufts of the kidney and the arterial distribution in the coccygeal body. [VASCULAK SYSTEM The distribution of the pulmonary artery will be con sidered in the anatomy of the lungs. That of the aorta will now be briefly described. The Aorta (Plate XX. figs. 2, 3, A) lies in the cavities of the Aorta, thorax and abdomen,and arises from the base of the left ven tricle. It ascends forwards, upwards, and to the right as far as the level of the second right costal cartilage, then runs backwards and to the left to reach the left side of the body of the 4th dorsal vertebra, and then descends almost verti cally to reach the left side of the body of the 5th dorsal ver tebra. It forms, therefore, an arch, well known as the arch of the aorta, which arches over the root of the left lung, and which has attached to its concave surface a fibrous cord, known as the obliterated ductus arteriosus, which connects it with the left branch of the pulmonary artery. The aorta continues its course downwards in close relation to the bodies of the lower dorsal vertebras, then passes through an opening in the diaphragm, enters the abdomen, and descends in front of the bodies of the lumbar vertebrae as low as the 4th, where it is usually described as dividing into the two terminal branches, the common iliac arteries. At the angle of bifurcation, however, a long slender artery, called the middle sacral, is prolonged downwards in front of the sacrum to the end of the coccyx. In animals with long tails this artery can be recognised as a direct continuation of the aorta, prolonging it downwards in front of the caudal vertebras, whilst the iliacs are seen to be collateral branches ; but in man, where the coccyx is rudi mentary, and the lower limbs largely developed, the iliac arteries which supply those limbs are so big as to obscure the true signification of the middle sacral artery, and appear themselves to be the terminal branches of the aorta. The branches which arise directly from the aorta may be arranged in four groups. 1st, Branches for the supply of the viscera of the thorax and abdomen proper ; 2d, branches for the walls of the thorax, abdomen, and pelvis; 3d, branches for the head, neck, and upper limbs ; 4th, branches for the lower limbs, pelvic walls, and viscera. The branches of the aorta which supply the viscera of visceral the thorax are the coronary, the oasophageal, the bronchial, brancliei and the pericardial. The coronary arteries, two in num ber, are the first branches of the aorta, and arise opposite the right and left segments of the semilunar valve, from the wall of the aorta, where it dilates into the sinuses of Valsalva. The mouths of these arteries are closed by the opening outwards of the aortic valves during the ventricular contraction. The elastic recoil of the aorta following that contraction not only closes the aortic valves, but drives the blood into the coronary arteries. These arteries break up into branches in the muscular walls of the heart, and the sudden turgescence of its walls, which results from the fill ing of these vessels, is, according to Briicke and Garrod, the cause of the dilatation of the ventricular cavities. The bronchial arteries are two in number; one accom panies each bronchial tube, and supplies the tissues of the lung. The cesophageal arteries, three or four in number, supply the coats of the oesophagus. The pericardial branches are very small arteries which supply the back of the bag of the pericardium. The branches of the aorta which supply the viscera of the abdomen arise either singly or in pairs. The single arteries are the creliac axis, the superior mesenteric, and the inferior mesenteric, which arise from the front of the aorta ; the pairs are the capsular, the two renal, and the two spermatic or ovarian, which arise from its sides. The single arteries supply viscera which are either completely or almost completely invested by the peritoneum, and the veins corresponding to them are the roots of the vena
portse. The pairs of arteries supply viscera developed
