SKELETON OF THE HORSE. 19 



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, esactlj- in the median 

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

 or double arrangement of parts, is termed bilcderal symmetiy. 



Anatomists differ as to the number of bones composing the 

 skeleton, some enumerating all ossific bodies, inchiding 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 weU as that 

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

 being erect, Tvith 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, 

 nnd 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 fe-ces the 

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

 internal surface faces the median plane of the body, the anterior 

 surface the anterior vertical, the posterior surface the posterior 

 vertical plane, while the superior and inferior surfaces are those 

 facing their respective superior and inferior planes. This imagin- 

 ary index may be applied to any region, as well as to the entire 

 body. 



Modifications of these terms are used when it is requisite to 

 point out the precise situation of a structure. For example, take 



