230 FIRST YEAR SCIENCE 



the right ventricle, which is a chamber with very thick 

 muscular walls. From the right ventricle, the blood is 

 driven out through the arteries, capillaries and veins of 

 the lungs, where carbon dioxide is given 

 off and oxygen absorbed by the red cor- 

 puscles. 



Returning from the lungs, the blood 

 enters the left auricle and when this be- 

 comes full, passes through a valve into 

 the left ventricle. This has such power- 

 fully muscular walls that it is able to 



force the blood throughout the body and 

 CROSS SECTION OF . . J 



THE HUMAN HEART, back again to the right auricle. As the 



Showing auricle, blood leaves either ventricle, there are 



ventricle and ven- V alves that close and prevent its return. 



If the hand is placed a little to the left 



of the breastbone, the strong contraction of the ventricle 



can be felt. 



108. The Senses. In order that the brain may com- 

 municate with the outside world and so be able to protect 

 the animal from destruction and to provide for its well- 

 being, animals have become provided with a number of 

 sense organs which communicate with the brain by the 

 nerves. The most conspicuous sensations of the human 

 body are sight, hearing, taste, smell and touch. 



The organ of sight, the eye, is an exceedingly sensitive, 

 automatically adjustable camera that records through the 

 nerves. The camera box is the hard bony socket in which 

 it is placed, the eyelid is the shutter, and the iris, the dia- 

 phragm. The iris is the membrane in the front of the eye 

 which opens or contracts to let in more or less light. In 

 the center of it is a hole, the pupil. 



Back of the shutter, or iris, is a small adjustable lens 

 and beyond this the sensitive plate, the retina. Between 



