204 



THE CAT. 



[CHAP, vn. 



from the lungs to the heart carry "bright scarlet or arterial blood, 

 although they (inasmuch as they are vessels conveying hlood towards 

 the heart) are called " veins " the pulmonary veins. Similarly, 

 the vessels which carry the blood from the heart to the lungs contain 

 dark purple or venous blood, although they (inasmuch as they are 

 vessels conveying blood from the heart) are called " arteries " the 

 pulmonary arteries. 



12. The current of the blood through the heart is kept up by 

 the alternate contraction and dilatation of its four chambers, which 

 is brought about by the successive contraction and relaxation of its 

 various sets of muscular fibres. Its course through the heart is, 

 however, determined by the action of certain VALVES which guard its 

 most important apertures, and which, by acting like the valves of 

 the veins, readily allow the blood to pass in one direction, while 

 they oppose an effectual barrier to its passage in the opposite direc- 

 tion. The valves which most resemble those of the veins are the 

 valves which are placed at the root of the aorta and pulmonary 

 artery. These are called semilunar valves, and there are three in 

 each vessel. They consist of processes of fibrous tissue (invested 

 with endothelium), and each is continuous by one curved edge with 

 the wall of the artery, while the other edge hangs freely so far out 

 from the wall of the vessel that the whole three valves, when so 

 hanging, meet together (by a tri-radiate suture), and form a complete 

 partition across the vessel. The margin of each valve is strengthened 

 by a tendinous band, and at its middle is a small fibre-cartilaginous 

 nodule called a corpus Aurantii. 



While the blood is flowing from the contracting ventricles, these 

 valves lie back against the inner walls of the arteries, but when the 

 ventricles dilate and the elasticity of the arterial walls tends to drive 

 the blood back into the ventricles, these valves immediately descend, 

 and the greater the pressure from above, the more completely and 

 accurately do they close together. 



At the roots of the pulmonary veins there are no valves, and there 

 is only a rudimentary one at the root of the venae cavae. At each 

 of the auriculo-ventricular openings, however, there is a valve of 

 complex structure, consisting of membranous flaps, with delicate 

 tendinous cords (chordce tendinece) attaching them to papillary pro- 

 longations inwards columnce carnece of the muscular ventricular 

 walls. 



The valve of the right auriculo-ventricular aperture is called the 

 tri-cuspid valve, because it consists of three segments. The delicate 

 tendinous cords above named proceed partly from the walls of the 

 ventricle (especially the septum), but mainly from little muscular 

 prominences the columnae carneae. The tendinous cords are so 

 distributed that some of those from each origin proceed to the edge 

 of one valve, while the others proceed to the adjacent edge of another 

 valve thus diverging as they advance (Fig. 102, B). 



The valve of the left auriculo-ventricular aperture is called the 

 U-cuspid or mitral valve, and consists but of two segments. It is 



