STRUCTURE OF THE HUMAN MECHANISM 17 



Cervical 



Thoracic 

 (or dorsal) 



10 



the body. Almost every joint is the theater of such plays 

 of antagonistic muscles, which serve the double function of 

 keeping the bones in proper position with regard to one an- 

 other and of producing movement at the joint, the amount 

 of this movement being limited by 

 the slack but inextensible connective- 

 tissue ligaments which bind the bones 

 together. In Fig. 11 both the short- 

 ening of the muscle and the slack- 

 ness of the ligaments are purposely 

 exaggerated, in order to represent 

 more clearly the functions of these 

 tissues. Ligaments may also guide 

 the movement of bones by pre- 

 venting motion in one direction 

 or another. 



10. The ribs. Each rib consists 

 of a bony and a cartilaginous por- 

 tion. The former articulates (that 

 is, forms a joint with) the vertebral 

 column, while the latter continues 

 this bony portion to the ventral 

 median breastbone, to which it is 

 directly joined. The ribs form the 

 framework for the thorax and may 

 be lifted or lowered by muscles 

 which connect them with the verte- 

 bral column and other parts of the 

 skeleton (see Fig. 12). 



11. The skeleton and the central 



nervous system. The skull consists of the bones of the 

 face and those of the cranium, the latter holding the brain. 

 It is supported on the spinal, or vertebral, column, whose 

 ringlike vertebrae inclose a bony canal continuous with the 

 cranial cavity. This is known as the spinal, or vertebral. 



Lumbar 



Sacral 



FIG. 10. The vertebral 

 umn 



Seen from the side 



