12 



SCIENCE PRIMERS, 



[§n. 



I 



in the chest there seems to be a large empty space. 

 But as we shall see further on, the lungs did quite fill 

 the chest before you opened it, but shrark up very 

 much directly you cut into it, and so left thr great 

 bpace you sec. 



9. The trunk then is really a great chamber contain- 

 ing what are callea the viscera, and divided into an 

 upper and lo\/er half, the upper half being filled with 

 the heart and lungs, the lower with the liver, stomach, 

 bowels, and some other organs. In front the abdomen 

 is covered by skin and Muscle only. But if all the 

 sides of the trunk were made of such soft material 

 it would be then a mei'e bag which could never keep 

 its shape unless it were stuffed quite full. Some part 

 of it must be strengthened and stiffened. And indeed 

 the trunk is not a bag with soft yielding sides, but a 

 box with walls which are in part firm and hard. You 

 noticed that when you were cutting through the front of 

 the chest you had to cut through several hard places. 

 These were the ribs (Fig. i, a)^ made either of hard 

 bone or of a softer gristly substance called cartilage. 

 And if you take away all the viscera from the cavity 

 of the trunk and pass your finger along the back of 

 the cavity, you will feel all the way down from the 

 neck to the legs a hard part. This is the backbone 

 or vertebral column. When you want to make a 

 straw man stand upright you mn a pole right through 

 him to give him support. Such a support is the back- 

 bone to your own body, keeping the trunk from 

 falling together. 



In the abdomen nothing more is wanted than this 

 backbone, the sides and front of the cavity being 

 covered in with skin and muscle only. In the chest 



