20 GENERAL STRUCTURE OF THE BODY. 



Short bones are spongy throughout. They are used 

 for strength and where little motion is required. 



Flat bones are composed of two thin layers of compact 

 tissue with a varying amount of cancellous tissue be- 

 tween, and are for protection and muscular attachment. 

 The cancellous material between the two layers or tablets 

 of the skull is called the diploe. 



Eminences and depressions occur on bones and when 

 they are not articular are for the attachment of ligaments 

 and muscles. If they are articular, they help to form 

 joints. 



As a whole the bony framework serves to keep the 

 soft parts in place, to support and protect them, and to 

 aid in locomotion. The bones of the head and trunk 

 support and protect organs; those of the arms are for 

 tact and prehension; those of the lower extremities are 

 for support and locomotion. 



Normally bones have little sensibility, but when in- 

 flamed they are extremely sensitive and painful. 



Joints. The bones are connected with and move upon 

 one another by means of joints. These joints are of 

 three kinds: 1. Immovable, where the adjacent margins 

 of the bones are closely applied, with little fibrous tissue 

 between, as in the sutures of the head; 2. those with 

 limited motion, which are very strong, the parts being 

 connected with tough fibre-cartilage; and 3. freely mov- 

 able. In this last group the articulating surfaces are 

 covered with cartilage, which again is lined with a deli- 

 cate synovial membrane which secretes a small amount 

 of lubricating fluid, the synovial fluid, to reduce friction. 

 Their surfaces are also sometimes deepened by the pres- 

 ence of interarticular fibro-cartilages. Bursse or sacs of 

 synovial membrane occur outside the joints under ten- 

 dons and ligaments to reduce friction. 



The nature and extent of the motion of a joint is 

 defined and the bones are held together by strong bands 

 of fibrous tissue or ligaments, these ligaments being more 

 fully developed in joints where there is great freedom of 



