418 



Our Surroundings 



Clotting is of importance in stopping the flow of blood from 

 a wound. Sometimes, in disease or sickness, a clot forms in the 

 body with unfavorable results. 



Blood Temperature. The normal temperature of the human 

 blood is about ninety-eight and six-tenths degrees Fahrenheit. 

 Under ordinary circumstances it varies little if at all. Any 

 considerable rise above or drop below the normal is an indication 

 of serious illness. Man and other mammals and birds are warm- 

 blooded and as such are not able to adapt themselves readily to 

 extreme ranges of temperature. In this respect they differ from 



Jj cold-blooded animals, such as 



I jfl, cU. frogs, fish and snakes, whose 



blood has no fixed temperature 

 but may vary by several degrees 

 with the temperature of the me- 

 dium in which they live. 



The Heart. The heart is a 

 hollow muscular organ, conical 

 in shape, and, in an adult, about 

 the size of the fist. It lies in the 

 breast, its side resting upon the 

 diaphragm. It is enclosed in a 

 membrane called the pericar- 

 dium. Its front surface is nearly 

 covered with the lungs. It con- 

 sists of four chambers, two 

 upper chambers called auricles, 

 and two lower chambers called 

 'ventricles. 



The work is done largely by 

 the ventricles, which have thick 

 muscular walls. The left ven- 

 tricle, which has the stronger 



muscle, forces the blood through the greater part of the body. 



The right ventricle forces the blood through the lungs. 



Pumping Action of the Heart. The heart may be compared to 



a double pump with the strong muscles in the ventricles supplying 



THE HEART A WONDERFUL 



PUMP 



FORCE 



Artery to the Head. 

 Right Bronchial Tube. 

 Aorta. 



Pulmonary Artery. 

 Left Bronchial Tube. 

 Right Ventricle. 

 Left Ventricle. 

 Right Auricle. 

 Left Auricle. 



