LOCOMOTIVE ORGANS. THE JOINTS. 153 



over, the cancelli and larger hollows of the bones, serve to expand 

 their surfaces, and so present broader points for contact with each 

 other at the joints, and broader surfaces for the attachment of mus- 

 cles. The compact bone is found in the shafts of the long bones, and 

 in other parts where strength is needed ; whilst the cancellated struc- 

 ture prevails at the ends of the long bones, and in the parts of the 

 vertebrae that rest over one another. The cavities in bone also secure 

 a greater relative amount of lightness in reference to bulk, for the 

 cancelli and the larger medullary cavities, are filled with a vascular 

 fatty tissue, much lighter than bone ; and in certain cases as, for 

 example, in the frontal, sphenoidal, and maxillary bones there are 

 spaces occupied by air. 



The bones of Fishes, the bodies of which are supported in water, are per- 

 fectly solid, whether their skeleton be osseous or cartilaginous. In Reptiles, 

 there are comparatively few medullary cavities or cancelli. These exist in all 

 the Mammalia, but are less marked in the aquatic cetacea. In Birds, many 

 of the bones, which in the Mammalia are occupied by fat, are filled only with 

 warm air. 



Other mechanical arrangements in the skeleton also deserve atten- 

 tion ; for example, the broad expanded shape of the skull-bones and 

 pelvic bones, for the purpose of protection ; the length of some bones, 

 as those of the limbs, where they are destined to act as long levers 

 with unequal arms ; the shortness of other bones, as those of the ver- 

 tebral column and foot, where shock has to be lessened, without any 

 sacrifice of strength, and with the preservation of flexibility and elas- 

 ticity; the projecting points, or the so-called "processes" of many 

 bones, which serve for the attachment of muscles, and, as we shall see, 

 also increase their leverage ; the formation of grooves for the play of 

 tendons; and, lastly, the presence of holes called foramina, for the 

 passage of nerves and vessels. Special adaptations of the forms of 

 different bones to their several purposes, will be exemplified in describ- 

 ing the habitual posture, and the various movements of man and the 

 lower animals. 



The Joints. 



The joints, or articulations, permit the various movements of the 

 animal frame ; they likewise serve to deaden the internal concussion 

 or shock produced by contact of the body with external objects, and, 

 moreover, they contribute to the strength of the skeleton, especially 

 to that of the back and lower limbs ; for it has been shown mechani- 

 cally that a rod or pillar, of a given height and thickness, has less 

 power of resistance to vertical pressure or crush than a number of 

 shorter rods or pillars built up one above the other to an equal height. 

 Whilst the muscles are the active agents in the movements of the 

 body, and whilst the bones give effect and precision to those move- 

 ments, it is the form of the surfaces of the bones at the joints which 

 finally determines their exact character and extent. 



The joints, in man and the higher animals, are first divided system- 

 atically, according to the degree of movement permitted by them, into 



