CHAPTER III. 



THE STRUCTURE, COMPOSITION, AND HYGIENE OF THE 

 BONY SKELETON. 



1. The Parts of the Humerus. Though bones differ in 

 shape and size, we may get a pretty good idea of the way 

 they are all built by studying the humerus, Fig. 7. This 

 presents a central rounded portion, or shaft, bearing at 

 each end an enlargement, the articular extremity. The 

 shaft lies between the dotted lines x and z. One use of 

 these large ends is to give more room for the fastening 

 on of muscles. 



2. Internal Structure. If the humerus be sawed in two 

 lengthwise (Fig. 8) we find that its shaft is hollow; the 

 space is the marrow cavity, a, and during life is filled 

 with a kind of fat. We also see that there are two kinds 

 of bony substance; one is hard and close, the other loose 

 and spongy. The hard bone, b, lies on the outside, and 

 is thick in the shaft; it forms only a thin layer in the 

 extremities, which are filled with spongy bone, c. The 

 large marrow-cavity does not extend into the extremi- 

 ties. 



3. Why Bones are Hollow. All bones either contain a 

 marrow-cavity or are filled up with loose spongy tissue. 



1. Describe the humerus as viewed on its outside. For what are 

 its large ends useful ? 



2. What would we find inside the shaft ? The extremities ? 



3. What do all bones contain ? Why are they not filled with hard 

 bone ? Why are the iron pillars used in building made hollow ? 



