SKELETON OF THE HORSE. 19 



positions, or articulated by wires or screws, in which case all the 

 soft tissues are first removed, and the preparation is termed an 

 artificial skeleton ; but if those soft structures, which, in the 

 living animal, join the bones together, are allowed to remain, 

 dried, and preserved, then the preparation is a natural skeleton. 



As the skeleton has two sides or halves, the majority of the 

 bones exist in pairs. There is, however, a central chain of single 

 bones, formed by the back bones or vertebrae and the median 

 segments of the head, extending the whole length of the frame- 

 work, and uniting its lateral halves. But even these single bones 

 present the same shape on either side of their central line ; hence, 

 if a skeleton were divided longitudinally, exactly in the median 

 plane, the two halves would correspond. This " right and left," 

 or double arrangement of parts, is termed bilateral symmetry. 



Anatomists differ as to the number of bones composing the 

 skeleton, some enumerating all ossific bodies, including the teeth 

 and sesamoids, which others eliminate ; some, again, regard certain 

 cranial bones as single, other authorities as double. It is of little 

 practical importance which view be taken ; it will be found that 

 there are about 216 separate bones ; or, including the teeth, 256 

 pieces in the skeleton of the horse. 



In speaking of the anatomical position and relation of bones 

 as of other structures, continual reference is made to imaginary 

 lines or planes. The skeleton of the quadruped, as well as that 

 of man, is, in an anatomical sense, viewed from before ; but man, 

 being erect, with the long axis of his body in a vertical position, 

 whilst that of the quadruped is horizontal, corresponding organs 

 do not always present themselves to observation from the same 

 point of view. With reference to quadrupeds, imaginary planes 

 are supposed to lie as follows : A longitudinal median vertical 

 plane descends through the centre of the head, vertebral chain, 

 and trunk, midway between the right and left extremities, to 

 the ground, dividing the body into two exact halves; right and 

 left lateral vertical planes are placed parallel to the former, but 

 external to the body ; at right-angles to the above an anterior 

 vertical plane is placed in front, and a posterior vertical one 

 behind ; a superior horizontal plane lies above the body, between 

 the anterior and posterior vertical planes ; while parallel with 

 the superior is an inferior horizontal plane, placed under the feet 

 The external surface of an organ or region is that which faces the 

 lateral plane on the side where the organ is situated. The 



