ARTICULATIONS OF THE BONES. 119 



a hinge or ginglymus (yiyfavpos cardo.) This kind of 

 articulation may be either simple or compound. It is 

 simple when the convex surface of the one bone is re- 

 ceived into the concave surface of the other. The under jaw 

 of quadrupeds is articulated in this manner with the head. 

 In the badger, this hinge-joint is so perfect, that the jaw 

 remains in its place, even after all the ligaments and cartila- 

 ges have been removed, and cannot be separated from the 

 head without breaking the margin of the joint. In the com- 

 pound ginglymus, both articulating surfaces mutually re- 

 ceive and are received. In this kind of joint, motion can 

 only be performed by flexion or extension. 



In attending to these different articulations in the animal 

 system, it will readily be perceived, that they pass into one 

 another by insensible gradations, according to the kind or 

 the extent of the motion which is required to be performed. 

 Hence, the same bone is not similarly articulated at the 

 same part in different species, nor at its different extremi- 

 ties in the same species. 



The different bones which occur in the body of an ani- 

 mal, when united, are termed its skeleton. They serve 

 as a support and protection to the soft parts, and to de- 

 termine, to a certain degree, the forms, magnitudes, and 

 motions of the individual. The bones of a skeleton are usu- 

 ally divided into those of the head, the trunk, and the ex- 

 tremities. 



The bones of the head are present in every animal fur- 

 nished with a skeleton. They consist of those various plates 

 which form the cranium, for the reception of the brain ; 

 those of the face, which, including the orbits of the eyes, 

 and the cavities of the nose ; and those of the mouth, in- 

 cluding the jaws. All these bones vary according to the 

 species, in number, form, articulation, and size. That 

 part of the body where the head is placed, is denominated 

 the anterior extremity. 



