3i6 AN INTRODUCTION TO ZOOLOGY 



series of interlacing muscular ridges, the musculi pectinati. It is 

 these appendages that show prominently externally when the atria 

 are deflated. 



Into the right atrium open two large veins, the anterior and 

 posterior caval veins, whose openings are separated by a ridge, the 

 tuberculum intervenosum, and a smaller vessel, the coronany vein, 

 bringing back the blood from the heart itself. A sinus venosus, 

 such as we find in the frog, is not present, it having been absorbed 

 into the atrium in the course of the evolution of the mammal. Into 

 the left atrium open the two pulmonary veins. The septum 

 atriorum is a fairly thin smooth-walled partition that completely 

 separates the two atrial cavities, and in it is an oval area much 



FIG. 1 08. The base of the heart. From Quain. 



The auricles h'ave been cut away, and the valves are closed. The pericardium has also been 

 removed to expose the muscular fibres. 



i and i', right ventricle ; 2, left ventricle ; 3, wall of right auricle ; 4, wall of left auricle ; 

 5, 5', and 5", the tricuspid valve ; 6 and 6', the mitral valve ; 7, pulmonary artery ; 8, aorta ; 

 9 and 9', coronary arteries. 



thinner than the rest and semi-transparent. This is termed the 

 fossa ovalis, and in the embryo it was an aperture, leading directly 

 from one cavity to the other. Each atrium opens into its corre- 

 sponding ventricle by an atrio-ventricular orifice guarded by strong 

 valves. On the right the valve is composed of three large, tough, 

 membranous flaps, and so is termed the tricuspid valve, while on the 

 left there are but two similar flaps constituting the bicuspid or mitral 

 valve, from its somewhat fanciful resemblance to a bishop's mitre. 



The ventricles stand in marked contrast with the auricles 

 on account of their very thick, strongly muscular walls. Their 

 cavities are completely separated by the septum ventriculorum, 

 which is also thick and muscular. A transverse section through the 



