50 TEXT-BOOK OF PHYSIOLOGY 



united and held in position largely by the intervention of a firm, elastic 

 disc of fibro-cartilage. Each joint is also strengthened by ligaments. 

 C. Synarthroses. In this division are included all those joints in which the 

 opposing surfaces of the bones are immovably united, and hence do not 

 permit of any movement e.g., the joints between the bones of the skull. 

 Levers. In the animal machine, as in physical machines generally, 

 work is accomplished by the intermediation of levers. The bones col- 

 lectively constitute a system of levers the fulcra of which lie in the joints. 

 The long bones more especially, are the levers which are employed by the 

 muscles to overcome the opposing forces or resistances. The structure and 

 the chemic composition of the bones, consisting as they do of inorganic 

 matter 67 per cent, and of organic matter 33 per cent, endow them with both 

 rigidity and elasticity, physical properties which admirably adapt them to 

 the character of the work necessitated by the environment and the organiza- 

 tion of the animal. The rigidity of bone is considerable as compared- with 

 other hard and rigid materials. The breaking limit, in terms of the weight 

 in kilos required to tear across a rod i square millimeter in cross-section 

 of various materials is as follows: Cast iron 13; bone 12; oak 6.5; granite 1.9. 

 The elasticity is about one-sixth that of wrought iron and twice that of oak 

 parallel to the grain (MacAlister) . In youth bones are quite elastic; in 

 old age they are fragile because of a diminution of osseous tissue and an 

 increased porosity, and, therefore, at both periods less capable of function- 

 ating as effectively as in the middle period of life. The animal body presents 

 many illustrations of the three orders of levers, their advantages and dis- 

 advantages from the mechanical point of view. 



The nature of the opposing forces, however, is of such a character that 

 the animal and especially man, is but to a slight degree capable of over- 

 coming them with the natural anatomic and physiologic levers. With the 

 invention of tools (physical levers) of all kinds and their utilization by 

 man the effectiveness of the anatomic levers in the performance of work 

 has been enormously increased. Through their cooperation the progress 

 of man in the arts of civilization has been made possible. 



The axial portion of the skeleton possesses largely, joints of the amphi- 

 arthrodial character which endow the vertebral column with certain forms 

 of movement which are necessary to the performance of many body activities. 

 While the range of movement between any two vertebrae is slight, the sum 

 total of movement of the entire series of vertebrae is considerable. In 

 different regions of the column the character, as well as the range of move- 

 ment, varies in accordance with the form of the vertebrae and the inclination 

 of their articular processes. In the cervical and lumbar regions extension 

 and flexion are freely permitted, though the former is greater in the cervical, 

 the latter in the lumbar region, especially between the fourth and fifth 

 vertebrae. Lateral flexion takes place in all portions of the column, but is 

 particularly marked in the cervical region. A rotatory movement of the 

 column as a whole takes place through an angle of about twenty-eight 

 degrees. This is most evident in the lower cervical and dorsal regions. 



The appendicular portion possesses largely, joints of the diarthrodial 

 character which permit of free movement. The character of the 

 movement depends mainly on the shape and adjustment of the bones at 

 their points of union. 



