28 THE HORSE IN MOTION. 



bar are in the same relation with the abdomen. Their broad and 

 long transverse processes afford a protecting roof to the abdominal 

 viscera, and give attachment to important muscles of locomotion on 

 the under surface. There is very little movement of these bones 

 upon each other, even less than in the dorsal series, so little that 

 bony union takes place between them in old age ; and the elastic 

 cartilages that, at an earlier period of life, were interposed between 

 each of the vertebra become degenerated into bony matter, and that 

 condition obtains technically known as ankylosis. 



The next series, and fourth in order, is the Sacral. Though in 

 the embryotic stage the sacrum is developed from several centres as dis- 

 tinct vertebra, yet before birth they are united into one broad triangular 

 bone, which, uniting with the iliac bones on each side, and the pubic 

 bones in front, forms the ring known as the pelvis. It is in the lower 

 or pubic portion of this pelvis that the cuplike cavities are formed 

 into which the heads of the hip bones are lodged, and where the force 

 of the levers of the posterior extremities is applied. The difficulty 

 in locomotion that would be experienced from the want of flexibility 

 of the spine, especially in old age, is obviated by the freedom of 

 motion that is secured in the articulation of the last of the dorsal 

 vertebra with the sacrum. This is what is known as the "coupling," 

 as it unites the two distinct systems of locomotive organs, the anterior 

 and posterior extremities. In the skeleton the connection seems very 

 slight; but the ligamentous connections are very strong, and the long 

 muscle of the back (longissimus dorsi or ilio spinalis), reaching out 

 from its spinal attachments, lays hold of the hip bone (crest of the 

 ilium) on each side as far as possible from the centre of motion at the 

 coupling, the more effectually to limit the flexion at that point. 



The last group of vertebral bones is known to anatomists as the 

 coccyx, from its resemblance in man to the beak of the cuckoo; but as 

 the resemblance totally fails in the Mammalia and all other vertebrates, 

 we shall call them by the more general name of Caudal bones. They 

 have no function in locomotion ; but " thereby hangeth a tale." 



Between all the vertebral bones is interposed a layer of clastic carti- 

 lage, of the same nature as that which covers the opposing surfaces 



