60 AN INTRODUCTION TO ZOOLOGY 



The two auricles together form a large dark hemispherical sac, 

 with thin walls, often termed the atrium, lying immediately in front 

 of the ventricle from which it is separated by a deep furrow, the 

 coronary sulcus. This sac is completely divided by a vertical 

 partition, the inter-auricular septum, into two chambers, a large right 

 and a smaller left auricle. The right communicates with the sinus, 

 as already noted, and into the left opens the common pulmonary 

 vein formed by the union of the two pulmonary veins, one from each 

 lung. The two auricles open into the ventricle by a single aperture, 

 the auriculo-ventricular aperture, which is partly divided by the 

 posterior free edge of the septum. The opening is guarded by two 

 auriculo-ventricular valves, one dorsal and one ventral, which stop 

 the blood flowing from ventricle to auricle. These are two mem- 

 branous flaps whose free edges are tied to the wall of the ventricle 

 by a number of tough tendinous cords, the chordae tendinse, and are 

 so enabled to withstand the pressure of the blood during the con- 

 traction of the ventricle. 



The ventricle is a moderately elongated pinkish structure bluntly 

 pointed at its posterior end. A horizontal cut shows it to have a 

 comparatively small cavity surrounded by very thick muscular 

 walls. The walls appear spongy, owing to the presence of a number 

 of interlacing projecting muscular ridges between which the ventri- 

 cular cavity dips deeply. This sponginess of the ventricular wall 

 plays a part in the separation of the two different sorts of blood 

 brought to the heart. 



The Conus arteriosus is a fairly stout tube, with walls composed 

 of cardiac muscle, which arises from the anterior ventral corner of 

 the ventricle on the right and runs forward obliquely towards the 

 left on the ventral surface of the atrium. Its exit from the ventricle 

 is guarded by three semi-lunar valves, i.e. valves shaped something 

 like small watch pockets, whose free edges can meet in the lumen 

 and stop the blood returning to the ventricle. Within it is a spiral 

 fold of membrane, and its anterior end, marked by another set of 

 three semi-lunar valves ; it is continued into a small terminal portion, 

 the truncus arteriosus, forming a common chamber from which two 

 branches are given off. Each of these is divided internally into 

 three ; the carotid in front, the systemic in the middle, and the 

 pulmo-cutaneous behind, which soon manifest themselves externally 

 as three separate arteries. These three arteries on each side 

 constitute the aortic arches. By some authors the conus is termed 

 the pylangium, and the truncus the synangium, quite superfluous 

 terms.* 



* Considerable confusion exists in the way these terms are used in text- 

 books. The term conus arteriosus (bulbus cordis or pylangium) should be 



