262 ANIMAL BIOLOGY 



whitish color of the coated or furred tongue is caused mainly 

 by these outwandering cells which are especially abundant 

 under certain abnormal conditions. 



The organ that keeps the blood in constant circulation 

 is the heart, which is situated near the middle of the chest 

 with its pointed lower end lying on the left side between 

 the fifth and sixth ribs, where we can plainly feel its 

 beating. To understand how the beating of the heart 

 causes the flow of blood we must study its inner mechanism. 

 The heart is composed mainly of muscular fibers whose 



periodic contraction affords the 

 energy for propelling the blood. 

 It is divided into four chambers, 

 the two auricles above and the 

 two ventricles below. The auricle 

 and ventricle of one side are sepa- 

 rated from the corresponding 

 chambers on the other side by a 

 median partition which com- 

 pletely shuts off all communica- 

 tion between the two sides. Each 

 auricle communicates with the 

 FIG. i 9 4.-The heart seen vent ricle of the same side by a 



from in front. AO, aorta; J 



LA, left auricle; LV, left ven- valve which allows blood to paSS 



tricle; PA, pulmonary artery; ,. . -i i A ji 



PV, pulmonary veins; RA, * m the auricle into the VCn- 



right auricle; Rv, right ven- tr j c i e but prevents its flow in the 



tncle; VC, superior vena cava. 



reverse direction. The ventricles 



are connected with the outgoing blood vessels or arteries, 

 while the auricles receive the incoming vessels or veins. 

 The muscular walls of the ventricles are considerably 

 thicker than those of the auricles as they have to force 

 the blood through the organs of the body, whereas the 

 auricles simply propel the blood they receive into the 

 ventricles. The valves which are present where the arte- 



