THE BONES. 29 



adapted for locomotion, and for sustaining the weight of the body. 

 They are never exactly cylindrical, being smallest about the 

 middle, and enlarged at each end. The broad or flat bones are 

 generally somewhat arched, and fitted to protect delicate organs ; 

 we find the best specimens of them in the cranium. The short 

 bones are of irregular figures, but all somewhat roundish ; they are 

 found in the wrist and the instep of the foot. The mixed or irre- 

 gular bones are usually classed with the short, but it is better to 

 separate them ; the bones of the spine are the best examples of 

 these. The ribs and bones of the pelvis may also be ranged with 

 them, combining the characters of the two preceding classes. 



If we prepare bones by careful maceration and drying, so as to 

 remove all the grease from them, and then saw them up, we 

 observe the density of the osseous tissue to differ very much in 

 different parts. The outer part is much harder and denser than the 

 internal part, and is called the compact substance. The internal 

 part is of a looser texture, and is called the cancellated or spongy 

 substance. These tissues are arranged differently. in the different 

 orders of bones. 



Appearance of a bone sawed longitudinally. 



In the flat bones, the compact substance is arranged into two 

 layers, separated by a thin stratum of cancellated structure, in 

 which the blood-vessels of the bone run. Where the bone is very 

 thin, the two outer layers are in contact, or appear compressed into 

 one, and the intermediate layer has disappeared. In the round 

 bones, there is a very thin layer of compact tissue on the outside, 

 while the internal part is composed of spongy tissue. In conse- 

 quence of this predominance of spongy tissue, and consequently of 

 blood-vessels, the round bones are much more liable to inflamma- 

 tion than any of the others. The long bones consist of three parts, 

 a shaft, and two extremities ; the shaft consists of very dense 

 compact tissue externally, becoming looser internally, and having 

 a canal running through them, nearly from end to end ; while the 

 extremities are of the same structure as the short bones. The canal 

 which runs through the long bones is lined with a delicate mem- 



