450 ZOOLOGY SECT, xin 



is the fossa ovalis (/. ov.)\ it marks the position of an aperture, 

 the foramen ovale, in the foetus. The crescentic anterior rim of the 

 aperture is known as the annulus ovalis. The cavity of the right 

 auricle communicates with that of the right ventricle by the wide 

 right auriculo-ventricular opening. This is guarded by a valve, 

 the tricuspid (tri. v.), composed of three membranous lobes or cusps, 

 so arranged and attached that while they flap back against the 

 walls of the ventricle to allow the passage of blood from the auricle 

 to the ventricle, they meet together across the aperture so as to 

 close the passage when the ventricle contracts. The lobes of the 

 valve are attached to muscular processes of the wall of the ventricle, 

 the musculi papillares (m. pap.), by means of tendinous threads 

 called the chordce tendinece. The right ventricle, much thicker 

 than the auricle, forms the right side of the conical apical portion, 

 but does not extend quite to the' apex. Its walls are raised up 

 into muscular ridges called columnce carnece. It gives off in front, 

 at its left anterior angle, the pulmonary artery, the entrance to 

 which is guarded by three pouch-like semilunar valves (sem. v.). 



The left auricle, like the right, is provided with an auricular 

 appendix. Into its cavity on its dorsal aspect open together the 

 right and left pulmonary veins. A large left auriculo-ventricular 

 opening leads from the cavity of the left auricle into that of the 

 left ventricle : this is guardea by a valve, the mitral, consisting of 

 two membranous lobes or cusps with chordae tendineae and musculi 

 papillares. In the walls of the ventricle are columnse carnsee, rather 

 more strongly developed than in the right. At the basal (anterior) 

 end of the left ventricle is the opening of the aorta, guarded by 

 three semilunar valves similar to those at the entrance of the 

 pulmonary artery. The coronary arteries, which supply the 

 muscular substance of the heart, are given off from the aorta just 

 beyond the semilunar valves. The corresponding vein opens into 

 the terminal part of the left precaval. The pulmonary artery divides 

 into two, a right and a left, each going to the corresponding lung. 



The aorta gives origin to a system of arterial trunks by which 

 the arterial blood is conveyed throughout the body. It first runs 

 forwards from the base of the left ventricle, then bends round the 

 left bronchus, forming the arch of the aorta (Fig. 1102), to run 

 backwards through the thorax and abdomen, in close contact 

 with the spinal column, as the dorsal aorta (d. ao.). From the arch 

 of the aorta are given off two large arteries, the innominate (in.) 

 and the left subclavian. The innominate divides to form the right 

 subclavian (s. d. a.) and the right (r. c. c.) and left (I. c. c.) common 

 carotid arteries. The right subclavian passes to the fore-limb as 

 the brachial artery, giving origin first to the vertebral artery, which, 

 after passing up through the vertebrarterial canal, enters the cranial 

 cavity, having first supplied branches to the spinal cord ; and then 

 to the anterior epigastric or internal mammary, which supplies the 



