78 THE ANATOMY OF THE HORSE. 



to be thought to have eight neck bones, but that which was 

 once considered the eighth bone of the neck is now re- 

 garded as the first bone of the back, only varied in forma- 

 tion, to enable the animal, which passes its life hanging 

 pendulous from trees, to freely turn its head so as to see 

 what takes place below it, or upon the earth. The number 

 of bones which form the other compartments of the spine 

 are not fixed ; indeed bones, as a general rule, are subject 

 to constant variations, as the instances of born-deformities 

 sufficiently testify. 



The chest is composed of part of the spine, of the back, 

 of the ribs, and the breast bone. 



The breast bone is composed of sev^eral pieces of bone 

 embedded in the cartilage ; the latter material, however, 

 preponderating. To it the lowermost parts of most of the 

 ribs are attached, and in form it is something like the keel 

 of a slftp, being rounded in front, or turned upwards, and 

 having a ridge projecting along its centre the entire length 

 {Skel. V, v). Its posterior portion is flattened and expanded 

 like to the tail of a fish. This expanded portion serves to 

 receive several of the abdominal muscles, and is well shown 

 in (Plate y III. I). 



The ribs are narrow and semicircular pieces of bone, at- 

 tached above to the spine of the back, and below to the breast 

 bone. They are, however, not composed of bone through- 

 out ; but the lower portion is formed of cartilage. The eight 

 first ribs are termed costse or " true ribs " {Skel. w), because 

 each has a distinct insertion into the breast bone ; the re- 

 mainder are called false ribs {Skel. y), because they only are 

 connected with the breast bone through one another. The 

 central rib is the longest, but the last is gifted with the 

 freest motion. The posterior edge on the inner surface of 

 each has a groove within which the vessels and nerves are 

 protected. The bottoms of the ribs are attached to the 

 breast bone by means of ligament, excepting the first rib, 

 which on account of its possessing more motion than 

 any other of the true ribs has a synovial articulation. 

 All the ribs form synovial joints with the spine of the back 

 {Skel. z, z, z, z) , and each is gifted with motion which takes 

 place forward and outward ; the true ribs being more imme- 



