144 HUMAN BIOLOGY 



II. THE SKELETON 



209. Necessity for the skeleton. Most of the common ani- 

 mals with which we are familiar have some kind of skeleton that 

 serves as a means of protection, of support, or of locomotion. In 

 some animals, e.g. clams and lobsters, the skeleton is on the outside ; 

 in the vertebrates, on the other hand, the skeleton is internal. A 

 study of Figure 44 will make clear the general arrangement of the 

 skeleton of man. The position and general shape of the bones may 

 be determined by the pupil from a study of his own body. For con- 

 venience, the two hundred bones of the skeleton may be divided into 

 three groups, namely, (1) the bones of the neck and trunk, (2) the 

 bones of the arms and legs, and (3) the bones of the head. 



210. The skeleton of the neck and trunk. The erect position of 

 the adult human body is maintained by a column of bones called 

 vertebrce. The spinal column may be felt through the skin behind the 

 neck and down the middle of the back. The human spinal column 

 is a wonderful piece of mechanism, which by its structure is adapted 

 to perform at the same time three distinct functions. In the first 

 place, the vertebrae, piled one on the other, form a column strong 

 enough to support the weight of the body. Again, the structure of 

 the spinal column shows marvelous provisions for securing elasticity 

 and freedom of motion. Elasticity is secured by a succession of four 

 curves which are best seen in a side view of the body. By means of 

 these curves the head and the upper part of the trunk are saved from 

 sudden shocks that would result from running or jumping, for the 

 curves act like a series of springs. Pads of cartilage between the 

 vertebras serve as cushions to prevent jarring. This general ar- 

 rangement of the spinal column permits a considerable range of 

 movement. 



A third adaptation that is evident in the structure of the spinal 

 column is the protection it affords to the delicate spinal cord (231) 

 which is inclosed by it in a continuous tube. One would search 

 far before finding a more perfect means of securing strength, elas- 

 ticity, and flexibility than that provided in the structure of the 

 human spinal column. 



