206 THE CAT. [chap. vii. 



valves) whicli leads into tlic pulmonary artery is remote from the 

 auriculo-ventricular opening. 



The LEFT VENTRICLE, by far the thickest-walled portion of the 

 heart, extends backwards to its apex mainly on the posterior aspect 

 of the organ. The side which is formed by the septum is concave 

 instead of convex — the transverse section of the cavity of this 

 ventricle being oval. The opening (with its semilunar valves) 

 which loads into the aorta is close beside the auriculo-ventricular 

 opening, but to the right, and somewhat vontrally to the latter. 



The coronary arteries, which convey the blood for the nutrition of 

 the substance of the heart, do not lead from the cavity of the 

 ventricle itself, but from the aorta (one opening on each side) 

 immediately in front of the attachment to it of its semi-lunar valves. 



§ 14. All the ARTERIES of the body, with the exception of the 

 pulmonary artery and its branches, arc larger or smaller ramifica- 

 tions of that great artery, called the aorta ; and all without exception, 

 therefore, are ramifications of vessels which proceed directly from 

 one or other of the ventricles of the heart. Those which proceed 

 from its right side, are the arteries of the pulmonic circulation, 

 and carry venous blood ; those which proceed from its left side, are 

 arteries of the systemic circulation, and carry arterial blood. 



The PULMONARY ARTERY is the most capacious vascular trunk in 

 the body, but is very short. Arising from the conus arteriosus, it 

 passes dorsally and to the left, and then bifurcates. One branch 

 passes to the right, above the arch of the aorta. It then divides 

 and sub-divides within the right lung. The other branch passes 

 to the left, beneath the dorsally and posteriorly-extending part 

 of the aortic arch, and then divides and sub-divides within the 

 left lung. Each pulmonary artery, at the root of the lung, lies 

 dorsally to the pulmonary veins, and vcntrally to the air-tube or 

 division of tlie windpipe, which there enters the lungs. A little to 

 the left of its bifurcation, the pulmonary artery is connected by a 

 fibrous band with the concavity of the aortic arch. This band is 

 the relic of a foetal tubular structure, which in the embryo places 

 the cavities of these two great arteries in direct communication, and 

 is then called the ductus arteriosus (Fig. 10-i, da). 



§ 15. The AORTA springs from the base of the heart, rather 

 dorsally to the pulmonary artery and between the anterior ends of the 

 two auriculo-ventricular apertures. It arches to the left, over the 

 right branch of the pulmonary arterj^, and over the root of the left 

 lung, till it reaches the front — i.e., the ventral side of the vertebral 

 column. Thence it passes backwards through the hinder margin of 

 the diaphragm, and ends by dividing into the iliac arteries, which 

 go to the pelvic viscera and pelvic Ihnbs. 



The aorta is, as it were, the axis of the systemic arterial system ; 

 and (apart from small branches and large vessels given otf' to the 

 abdominal viscera) may be said to give off eight branches or sub- 

 divisions, four being anterior and four posterior. Of the four 

 anterior branches, one goes to each pectoral limb and one to each 



