36 THE HORSE IN MOTION. 



mechanism of the locomotive organs, and the manner in which the 

 muscles act upon their bony levers to produce the movements in 

 progressive motion. 



The long muscle of the back holds the same relation to the loco- 

 motive muscles that the vertebra does to the bones ; it is a very com- 

 plex muscle or system of muscles ; it is called by Chauveau the ilio 

 spinalis, so named from its attachments. It fills the angular space 

 on each side of the spinous processes, giving roundness to the back. 

 It is very broad and thick over the loins, and is attached to the 

 whole anterior border of the ilium and strongly to its crest, or the 

 hip bone, as seen in Plate III., q; it is attached anteriorly to all 

 the spines of the vertebra, as far as the neck, and a strong mem- 

 brane, tendon-like in its construction, that is firmly fastened to the 

 same bones. This tendinous membrane, called aponeurosis, has 

 not been mentioned thus far, but it is tissue very important in 

 its relation to the muscles; it differs from fascia in several respects, 

 but specially in thickness and strength. It covers nearly all the 

 superficial muscles, and its strength is so great that the muscular 

 fibres are attached to its inner face as to a bone, and it serves them 

 often the same purpose as fixed attachments.* 



If one takes an elevated seat with the driver on a coach, and 

 looks down upon the wheel horse nearest him, he can see the action 

 of this muscle, and to the best advantage if the horse is trotting. 

 It will be noticed that the spine is flexed in a serpentine manner 

 as the diagonal legs move alternately. This movement is caused 

 by the impulses given to the pelvis by the heads of the femurs 

 alternately, which would seriously strain the articulation of the pel- 

 vis with the lumbar vertebra called the coupling, but for the action 

 of the ilio spinalis, which contracts simultaneously with the impulse 

 communicated to the opposite side of the pelvis, acting as a brace 

 checking the wrenching violence of the action and preventing 

 injury to the coupling. This is the function of the iliac wings, as 



* A familiar example may be seen in a porter-house steak of beef. The part known as 

 the tenderloin is a section of the psoas ; that above is a section of the ilio spinalis over- 

 laid by its aponeurosis. 



